THYSANOESSA EUPHAUSIIDS, COMPARATIVE MORPHOLOGY, ALLOMORPHOSIS AND ECOLOGYTAKAHISA NEMOTO
The genus Thysanoessa was established by Brandt for Thysanoessa longipes. These
diagnoses were again well examined by Hansen in 1911, who gave the refined genericdescription (Hansen, 1911).
There have been nine or ten species of Thysanoessa euphausiids considered up
to these days, and the morphology and ecology on those species have been contained
fragmentaly in many previous papers on euphausiids (Einarsson, 1945 : Boden,
Johnson & Brinton, 1955: Brinton, 1962b: Ponomareva, 1962: Nemoto, 1959:
1962: etc).
This study is mainly concerned with the comparative morphology, relative
growth (Allomorphosis or interspecific relative growth allometry) and discussions
on the characteristic in distributions and systematic relationships of each species
among the genus and other euphausiids.
In the morphological study, general external points considered as important
for taxonomies and other studies are examined, and some other internal characteristics such as structures of the stomach,. are also investigated. The allomorphosis
among different species has been examined to consider the ecological and phylogenetic characters of each species too.
MATERIALS AND METHODS
MATERIALS

The materials of Thysanoessa have been collected through the following expeditionsin the Antarctic and the North Pacific as shown in Table 1. They are mainlycomposed of two different lines of works, namely plankton net samplings and collecTABLE I.

EXPEDITION AND COLLECTIONS OF PLANKTONS FROM WHICH
THTSANOESSA EUPHAUSIID SAMPLES ARE DERIVED

CruiseNorth Pacific whaling
expeditions
N ort;h Pacific whale
marking
KT-64-17*
KT-65-12*
Antarctic whaling
expeditions
Atlantic st. M
OthersArea sampled
Bering and adjacent seasYear

No. of sample

1952-1964

Ca990**

Remarks
Stomach contents
whales.

Ca500**

45 cm vertical haul.

South of Izu.
Off Wakayama

1955, 1958,
1959, 1960,
1964
1965

Antarctic

1954-1965

"

9**
5**
Cal500**2
30**

North Atlantic
North Pacific

1.6 m oblique haul.
80 cm vertical haul.
Stomach contents ofwhales.
By courtesy of Dr. Jones.
]EDS-Cruise etc.

* Cooperative research cruise by Tansei-maru, Ocean Research Institute, University of Tokyo.
** Not all samples contained Thysanoessa euphausiids.
109

of

110

T. NEMOTO

tions of euphausiids from the stomach contents of baleen whales.
MEASUREMENT OF .BODY PARTS

The measuring positions are illustrated in Fig. 1, in which each indication ismeasured with built-in micrometer in binocular microscope. As for the length ofcarapace the cephalothorax length is considered as the common measureing position. But it is rather convenient to measure the edge along the lower margin of thecarapace, to compare the ratios of length between anterior top and carapace denticle
with the length of the carapace, and without dissection of the body. It may becalled ' carapace margin length ' in this report.

B

H

Fig. I. Measuring positions of Thysanoessa euphausiid. A-Body length: B-Carapace length,
carapace margin length and denticle length. C-Leg: D-First antenna: E-Eye: F-Sixth andfifth abdominal segments and telson: G-Sixth abdominal segment: H-Male copulatory organ.

The mature specimens are measured for the relative growth study. Those
are males having fully developed external copulatory organs and distinct spermatophores in the reproductive organs or ducts and females with spermatophores intheir thelycums or having fully developed thelycums (Einarsson, 1945).
From 5 to 15 specimens of males and females have been measured in each
body length group for the allomorphosis study. About 100 specimens are selectedfrom the mature or copulated samples, for the measurement of body length andgrowth, and the geographical dine examination in T. inermis.
KEY TO THE ADULT THYSANOESSA EUPHAUSIIDS
The following simple key may be applied as expressing the taxonomical characters
for Tlz:ysanoessa euphausiids.

THYSANOESSA EUPHAUSIID

Ia

Ill

Eye has clear transverse constriction above the middle. Second pair oflegs elongated.
2a Lateral margin of carapace has denticle.
3a Lateral denticle in the margin of carapace situated approximately at
the middle of the carapace.
4a Strong dorsal keel and spines present at third and fourth abdominal
segments.
T. longipes
3b Denticles situated considerably posterior part of the lateral margin.
5a Keels absent in the dorsal segments.
6a Sixth abdominal segment is much shorter than the sum of preceding two segments.
7a Setae on carpus and propodus of the first to third thoracic
legs are as long as those of dactylus.
T. gregaria
7b Setae on carpus and propodus of the first to third legs areshort and fine compared with those of dactylus.
T. parva
6b Sixth abdominal segment is longer than or as long as the sum of
the two preceding abdominal segments.
8a The end of terminal process of male copulatory organ isserrated. Antennulae with the upper flagellum somewhat
or considerably longer than the sum of distal peduncular
joints.
T. vicina
8b The end of terminal process of male copulatory organs is
not serrated. Antennulae with the upper flagellum somewhat or considerably shorter than the sum of distal two joints.
T. macrura
5b Keels present on third to fifth segments.
T. inspinata
2b Lateral margin of the carapace lacks the denticle.
T. longicaudata
I b Eye has only slight transverse constriction or without clear constriction.
Second pair of legs are not so elongated or not elongated.
9a Lateral margin of carapace with lateral denticle.
T. raschii
9b Lateral margin of carapace without lateral denticle.
1Oa Dorsal keels present from first to fifth abdominal
segments.
T. spinifera
I Ob Dorsal keel absent, only spines on fifth and sixth or
sixth abdominal segments.
T. inermis

T. NEMOTO

112

MORPHOLOGY AND ALLOMORPHOSISFIRST ANTENNAE

The first antenna, the peduncle and flagellum are often considered as sexual andtaxonomical characteristics. In Thysanoessa euphausiids, they are rather simple
TABLE 2.

RATIO AND NUMBER OF ARTICULATION OF ANTENNULAR
FLAGELLUM IN THTSANOESSA EUPHAUSIIDS

Longer groupT. spinifera
T. raschii
T. inermis
T. longicaudata
T. vicina
T. macrura
T. inspinata
T. longipes
T. gregaria

Intermediate group
Shorter group

A

Example ofRatio for peduncle (%)
Ca 90
Ca 120
Ca 93
Ca 89
Ca 40
Ca 52
Ca 56
Ca 56
Ca 48

B

c

D

E

G

No. of articulation
39-50
50-54
27-28
22-30
9-14
9-13
16-18
16-19
10-10

H

Fig. 2. Male first antenna of Thysanoessa euphausiids. A-T. longipes,B-T. inspinata, C-T.
vicina, D-T. macrura, E-T. gregaria, F-T. longicaudata, G-T. inermis, H-T. raschii, 1-T.spinifera, xlO.

THYSANOESSA EUPHAUSIID

113

and Hansen (1911) used the length of antennule with the upper flagellum as thetaxonomic character for the closely related Tf?ysaneossa macrura and T. vicina. Threetypes of flagellum, longer, intermediate and shorter types are observed as shown in
Fig. 2. The number of the articulations in upper and lower flagella are also
divided into two or three groups.
The flagellum of longer group is rather tough in preserving, but those of
shorter group are fragil especially in the intermediate species T. longicaudata. In thelonger groups, the articulations are ranging from 20 to some 60 and lengths oflonger flagellum are almost the same or longer than the peduncles. On the other
hand, the shorter group has less than 20 articulations and the flagella are shorter
than the peduncles. The distal fringes of flagellum are almost the same among
specimens of Thysanoessa euphausiids. They have usually four small setae in the
distal ends of the upper and lower flagella. One of the four setae has broad andtough base which sometimes two storied structure.
5.0

/

A

4.0
E

.5 3.0

B

/l

/~

E

=>

~ 2.0

l
·/

0

~0

~

1.0

6.0
5.0
4.o "E

.5
3.0 E
=>
(J)

2.0 g'

+

/
1.0

I

Male

2.0

3.0

4.0 5.0 6.0

longer llogellum· (mm 1

(j;
Ol

c

_g

/

3.0

5.Q

1.0
7.0 9.0

Carapace leng1h (mm lFig. 3. Allomorphosis in Thysanoessa euphausiids. Relation between longer and shorter
flagella, and carapace length and longer flagellum in males. Open circles-T. longipes,
Closed circles-T. gregaria, Crosses-T. inspinata, Oblique crosses-T. spinijera, Open trianglesT. raschii, Closed triangles-T. vicina, Open lozenge-T. inermis, Closed lozenge-T. longicaudata,
Circles with dot- T. macrura. Symbols are the same in following figures.

The sexual dimorphism in the peduncles of Thysanoessa euphausiids is typicalin T. inermis and T. spinifera. Einarsson ( 1945) considered the possible relationshipsof T. inermis to the Pacific species T. spinifera which has the antennule armature ofwhich bears close resemblance to that of T. inermis. The male T. inermis has arounded lobe on its inner side, in the first antennular segment, bearing slightly curved
spines (Einarsson, 1945).
The relation between the longer flagellum and the carapace length as allomorphosis gives three allometric tribes as shown in Fig. 3. The longer group,
namely T. inermis, T. spinifera and T. raschii form one tribe, and T. longicaudata
which has intermediate number of articulation between shorter and longer groups,
forms another point alone. The shorter group of other species runs the same re-T. NEMOTO

114

gression both in males and females. T. longicaudata ecologically bears the wider
distribution from temperate and boreal Atlantic Ocean to the Arctic, from theatlantic by 80°N (Einarsson, 1945), than other Thysanoessa euphausiids belonging
to the shorter group. On the other hand, it also covers the neritic and othershallow waters like the longer group.
CARAPACE

The carapace of the Thysanoessa group is various, but the rostrum is always welldeveloped in every species, which is one of the generic character of the Thysanoessa(Hansen, 1911). All species have keels in the center of rostrum part of the carapace,longer or short, and seven species out of ten have denticles in the lateral margin of
the carapace. The general outlines of each species are illustrated in Fig. 4 which
do not differ from those of other euphausiids.

A

F·

B

c

G

D

E

H

Fig. 4. Carapace of Thysanoessa euphausiids. A-T. inspinata.
D-T. gregaria. E-T. longicaudata. F-T. longipes. G-T. raschii.

B-T. vicina. C-T. macrura
H-T. spinifera. l-Tinermis.

Rostrum and keel
All Thysanoessa species have well developed spear pointed rostrums, but T.
gregaria, T. raschii and T. inermis have rather broad oblong tops. Other Thysanoessa
have acute distal ends when they are pressed on the deck glass for observations.
Thysanoessa spinifera have the most acute triangular, and longest rostrum, whichis narrower than other Thysanoessa euphausiids. Three species, T. longipes, T. vicina
and T. macrura have broader tryangular rostrums. The tip of the rostrum of T.
longipes is somewhat slender and differes from other two species. The general shapeof the rostrum of T. macrura is closely related to that of T. vicina. And it is also interesting that the rostrum of T. raschii is similer to T. gregaria, which is different cons-THYSANOESSA EUPHAUSIID

115

siderably in other external points. T. gregaria has oblong and broader rostrum, andthe shape of the keel in rostrum is different from T. raschii. The vertical sections of
the keel in the rostrum and the carapace show also two types. The keels in the
rostrum of T. inermis and T. raschii are rather gentle crest ridges, but T. longipes, T.
gregaria, T. vicina and T. macrura have the sharp ridges as shown in Fig. 5. The
geographical variation in rostrum of carapace in Euphausia diomediae is reported byHansen ( 1911). Some taxonomical importance in the strech of rostrum in Thysanopoda, T. aequalis and T. subaequalis, is also obtained (Boden & Brinton, 1957). No
geographical difference is observed in Thysanoessa euphausiids in rostrums in thisstudies.

A

)

c

~A

)
""

E

D

AA
G

J

Fig. 5. Rostrums of male Thysanoessa euphausiids. A-T. spinifera: B-T. inermis: C-T. longipes:
D- T. inspinata: E- T. vicina: F-T. raschii: G- T. gregaria: l- T. longicaudata: .J- T. macrura.

The edge in the cove between rostrum and the anterior top of the margin of
the carapace swells out in the larger Thysanoessa, T. macrura, T. inspinata, T. raschii
and T. inermis. But it is not so clear in T. gregaria and T. vicina. T. spinifera has aprominent supraorbital spine in the top of the swell. The sexual dimorphism is
not observed in all species but T. raschii, the male of which has broader rostrum
than females. In the younger stages of Thysanoessa, the rostrums are also welldeveloped. But their tip ends are not so pointed and they have rather blunt ends.

The anterior top of lateral margin of the carapace
Thysanoessa euphausiids have pointed ends except T. spinifera which has twopoints in the anterior top of the carapace along the supraorbital cove. As illustrated

116

T. NEMOTO

in Fig. 6, T. gregaria, T. longipes and T. raschii have all pointed ends, the groupalso includes other species but T. spinifera.
T. spinifera has one keel like spine
slightly upper positions of the end of the margin of the carapace. The end of themargin of the carapace also makes point, from which two streaks run along the edgeof the lateral margin of the carapace.
Lateral denticle

Seven species out of ten in Thysanoessa have lateral denticles in the lower marginof the carapace, and other three species completely lack the denticles.
Lateral denticle present Thysanoessa raschii
T. longipes, T. gregaria, T. macrura, T. vicina,
T. inspinata, T. parva.
Lateral denticle absent
T. inermis, T. spinifera, T. longicaudata.
All species are bearing only one denticle in one side margin of the carapace.

Fig. 6. The anterior top of the lateral margin of the carapace in Thysanoessa euphausiids.
A-T. gregaria, B-T. longipes C-T. raschii, D- T. spinifilra.

0.2MM

Fig. 7. Carapace denticles in Thysanoessa euphausiids. A-Thysanoessa longipes. B-Thysanoessa raschii deformed denticle. C-Th_ysanoessa raschii.

There are two types of lateral denticles of carapace. One is Thysanoessa raschii
bearing a acute spine in the anterior part of which is also rather completely freefrom the margin of the carapace. On the other hand, T. longipes has the acute but
narrower base spines which directly rise up from the margin of the carapace in theposterior pos1t10n. T. gregaria, T. macrura, T. vicina and T. inspinata all show lattercharacteristic and T. parva also possibly belong to this group.
In one specimens of T. raschii a deformed denticle which has two character ofthem is observed as illustrated in Fig. 7.
T. gregaria, T. vicina and T. inspinata have sometimes very incomplete lateral
denticles. Especially, T. gregaria sometimes completely lacks them. Samples of T.
gregaria collected in KT-64-17 and T. macrura collected in the Antarctic also lackthe lateral denticle in the lower margin of the carapace.
In other species of euphausiids, the local difference of the presence of thisdenticle is reported on Nematoscelis tenella. The male Nematoscelis has this denticle
in the Atlantic, but it is absent in Pacific specimens. (Boden, Johnson & Brinton,

THYSANOESSA EUPHAUSIID

1171955). Recently Mclaughlin (1965) described two forms, denticled and originalforms of Nematoscelis difficilis. Mclaughlin described that the prominent lateral
denticle, varying somewhat in size, occurres on the posterior third of the inferior
margin of the carapace about Nematoscelis difficilis denticle form, (Mclaughlin, 1965),
but it has another ' no-denticle ' form.

Position of lateral denticle
The position of lateral denticle is a very important taxonomic characteristic
especially on Thysanoessa longipes and T. inspinata (Nemoto, 1963). Hansen also used
the position of the lateral denticle for the taxonomic considerations on Thysanoessa
groups. He (1911) stated that lateral margins of the carapace with a denticle atthe middle on Thysanoessa longipes and denticles situated before the middle of thelateral margin of the carapace on T. raschii. As for Thysanoessa gregaria group, thatthe denticles is conspicuously posterior the middle of the lateral margin, is also
noted as the taxonomic character by him.
,_.,,,_... if
T.roschii
1-eEH

1-

T. longipes

T.gregoria

T. mocruro

T. vic!na

T. inspinofo

30

40

50

60

70

BO

%

Ralios. Denticle-L/ Carapace Morgio-l.

Fig. 8. Ratios, denticle length divided by carapacemarginlengthofthe carapaceof Tkysanoessa euphausiids in the world ocean. Range, mean and each standard
deviations are illustrated.

The positions of lateral denticles expressed as ratios, denticle length / carapace
margin length are shown in Fig. 8. There are fairly good expressions of the species
range, which also confirmes the taxonomic importance of them. And there is few
difference between males and females in all species.
The denticles of T. longipes is situated at the middle of the lateral margin, but
T. inspinata, the closely related species of the T. longipes, has its denticle in far posterior position as illustrated in Fig. 8. The position of the denticles in T. macrura
and T. vicina are rather similer in their positions, in which T. vicina shows slightly
posterior position of denticles. In other closely related euphausiids species, Euphausia paci.fica and E. nana established by Brinton (Brinton, 1962a) also show such tendency. The larger E. paci.fica has the comparatively anterior lateral denticles as shownT. NEMOTO

118

in Fig. 9, and E. nana from the East China sea has posterior lateral denticles. Thereis very clear difference between typical E. pacifica in the northern waters and E. nanain the Eastern China sea, however, some vague samples have been collected fromthe intermediate regions in the southern waters of the sea of Japan. These samples
collected from Tsushima waters are now under examination.

E. pocifico!Aloskon gulf]!Okhotsk I!OH SonrikuJ

E. nano

40

50

45

Ratios. Denlicle length/ Coro pace margin length

Fig. 9. Ratios, denticle length/carapace margin length of Euphausia
pacifica and E. nana. Ranges, means and standard deviations areillustrated.

8

9

10

Carapace length !mm!

Fig. 10. Allomorphosis in carapace of Thysanoessa euphausiids.
tion between denticle length and carapace margin length.

Rela-

Allomorphosis in the position of denticles
The allomorphosis in the relation between denticle length and carapace lengthfor Thysanoessa group is given in Fig. 10. There is no difference between males andTH'YSANOESSA EUPHAUSIID

119

females in all species. In a previous report (Nemoto, 1963), I noticed the clear
different allometry lines in T. longipes and T. inspinata which is one of the criticalpoint for dividing those two species. The relative growth coefficient for T. longipes
is 0.973 and T. inspinata 1.19. Thysanoessa macrura and T. vicina draw the same allometry line, the growth coefficient of which is 1.000.
Thysanoessa gregaria shows growth coefficient 0.925 which differs from thoseof other Thysanoessa. Thysanoessa raschii draw the line apart from other species asshown in Fig. 10. Those growth coefficients are as follows.
Thysanoessa raschii
1. 034longipes (adult)
0.973
macrura & vicina
I.OOO
gregarza
0.925
EYE

Shape and arrangement
The eye of Tfrysanoessa euphausiids are usually higher than broad with transverse constrictions or sometimes almost circular (Hansen, 1911 : Bodon, Johnson &Brinton, 1955). The ratio, lateral length/vertical length and the general shape of

50

60

70

80

90

100%

Fig. 11. Profiles and ratios of the width of the eye divided by the vertical length ofbilobate eye in Thysanoessa euphausiids in the world ocean.

eyes are illustrated in Fig. 11. Tfrysanoessa raschii has almost completely circular andbroad eye, and the ratio of lateral and vertical length is about 95 %. But from theside view, the upper part is rather compressed as compared with Euphausia whichis almost circular. T. spinifera agrees with T. raschii in this point showing the ratio
95 %, and the upper half of the eye is also compressed. T. inermis has a little higher
eyes showing the ratio about 87%, but no heavy constriction is observed in the fully
developed specimens. The compression in the upper part in T. inermis is heavier thanT. raschii and T. spinifera.
As it has been discussed by Hansen (1911), and Einarsson (1942), the younger

120

T. NEMOTO

specimens of T. inermis shows more higher than broad eyes. Many examples of theyounger juvenile stage after the furcilia possess more typical Thysanoessa eyes also in
T. inermis, but T. longipes of the same vertical size shows the narrower upper partof the eye and constriction. The ratio, lateral/vertical length is about 81 % in
T. inermis and 78% in T. longipes, latter value of which is not so different as thosein adults.
TABLE 3. ARRANGEMENT OF EYES IN EUPHAUSIIDS
Species and genus

Eye

Bentheuphausia
Thysanopoda
Nyctiphanes
Meganyctiphanes
Pseudeuphausia
Euphausia
Thysanoessa raschii
inermisspinifera
longipes
inspinatagregaria
parva
macruravicinalongicaudata
Tessarabrachion
Nematoscelis
Nematobrachion
Stylocheiron

(+)
+
+

+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+

Constriction

+
+
+
+
+
+
+
+
+
+
+

Size
Upper Lower

Crystalincones visible

Upper<Lower
" <
<
<
" <
" <
<
<

;;e:

>
" <

"

+

Depth ofliving range
500-4000
0-40000- 3000- 3000- 1500- 3000- 2000- 3000- 3000- 5000- 5000-1000500-2000
0- 3000- 3000-10000-10000-1000100-1000
0-1000

The general arrangement of the eyes m euphausiids are shown in Table 3.
Thysanoessa euphausiids are divided into two groups. One is having the constricted eyes group and the other is round eyes group. T. raschii having roundedeyes is living in the shallow waters, and T. spinifera also is mainly living in theshallow waters although it is sometimes found in the deeper waters along the edgeof coastal regions (Banner, 1954).
Other euphausiids bearing constricted eyes live in broader range in vertical
distribution in the sea. Thysanoessa, Tessarabrachion, Nematoscelis, Nematobrachion,and Stylocheiron have the constricted eyes. Those species have rather bigger eyes
and the range of the vertical distributions attain deeper waters in the sea except someAntarctic and Arctic species of Thysanoessa. The upper and lower part of the constricted eyes are sometimes about same size and sometimes different. In Thysanoessa,
the lower part is generally .bigger than the upper part. In Stylocheiron and Tessarabrachion, the lower parts are also larger than the upper parts. The lower partsof some species are larger than the upper but smaller in other Nematoscelis. The
upper parts are generally larger in Nematobrachion euphausiids.

THYSANOESSA EUPHAUSIID121

The size of the eye
The male has larger eyes than females in general comparing with the bodysize. The series of the relative size indicates that the smaller the body length is,
the larger the eye is among the gregaria group, which has constricted eyes in adults.
And this is not the same in T. spinifera, T. inermis and T. raschii.

22
Verti~ol le_nQth fmm lFig. 12. Allomorphosis in eye of Thysanoessa euphausiids in the world ocean.
Relations between lateral and vertical length. A-longipes, macrura: B-gregaria,
vicina, longicaudata: C-raschii, spinifera : D-inermis

2.5

female

Male

/
/,1
f.

E

.§ 2

Di
c:
~

+

1.5

0

,<,!

>"

I
II • f

A

I

/7),/

{

B

3

:!.

/

'G

//D
//

H
4

5

6 7 8

0·/e
~~
F

A
3

4

5678910

Carapace length Imm I

Fig. 13. Allomorphosis in eye of Thysanoessa euphausiids.
vertical length of eye and carapace margin length.

Relation between

Allomorphosis in eyes
The relation between vertical and lateral lengths of eyes in adult specimens
are plotted in chart as shown in Fig. 12. There are three or four allometry groups
both in males and females group.
T. macrura and T. longipes form the same line, the coefficient of relative allomorphosis of which is about 1.2. But T. gregaria and T. vicina shift the line, although122

T. NEMOTO

the constriction in the upper part of eyes is positive, and the growth coefficient of
the allometry is ranging 1.33-1.57. T. spinifera, T. inermis and T. raschii also form
one allometry group, but T. raschii shifts to a little high initial growth index in females
as illustrated in Fig. 12.
The relative growth between the vertical length of the eye and carapace length
is very various as illustrated in Fig. 13. There are many allometry lines in males.
Only T. macrura and T. longipes take the same line, but T. gregaria, T. vicina, T. raschii, T. inermis and T. spinifera occupy different allometry lines respectively. That
T. gregaria, T. vicina, and T. macrura differ considerably in the relative growth indicesbetween carapace length and the vertical length of eyes is possibly due to two reasons.
The relative growth for the carapace in T. longipes is the same as T. macrura. The
body length of T. longipes is about the same as T. macrura, and they are both living
in cold pelagic waters. They are considered to be surface living euphausiids shallower than 500 meter and the ecological position in the sea may be related with eachother both in northern and southern hemisphere.
TABLE 4. RELATIVE GROWTH COEFFICIENT IN ALLOMORPHOSIS BETWEENVERTICAL AND LATERAL LENGTH IN EYES OF THYSANOESSAEUPHAUSIIDS
CoefficientSpecies

T. macrnra, T. vicina
T. gregaria
T. spinifera, T. inermis
T. raschii

Male

Female

1.16
1.33
l.17

l.19
1.57
l.17

(1.17). (1.17)On the other hand, T. gregaria group, T. gregaria, T. macrura T. vicina are different considerably in males. in Thysanoessa euphausiids, the sexual difference inthe body length of males and females is very clear in T. gregaria and other species
which have bilobate eyes. The type of eyes of females in those species are that ofyounger stages of the same species eyes of males especially in T. gregaria and T. vicina.
The upper part ofbilobate eye of males swells like horse bean but guiter-shapedin females. This character is not so clear in T. macrura, T. longipes and T. longicaudata. In the younger stages of some Thysanoessa euphausiids, eyes are rather
related to each other even in the T. inermis. The eye of T. inermis in about 5 mm
specimen is nearly the same with that of T. longipes in the same body length as illustrated in Fig. 14 and former discussions. This shape is that of females in adults
in T. gregaria group as illustrated also in Fig. 14. The female type is becoming to
the younger type is already pointed out as for many other species as the evolution
in specrnt1ons. This sexual difference may be firstly connected to the step of thedevelopment of variations in the body parts (Eimer, 1890).
Brinton ( l 962b) considered the evolution and local forms of Stylocheiron affine
and S. longicome in the Pacific areas according to the combinations of the variationsin the ratios, length of sixth abdominal segment divided by depth and width oflower
section of eye divided by width of upper section. This combination is also checked

THYSANOESSA EUPHAUSIID

123

for Thysanoessa euphausiids in which possible relationship among species is also suggested. The inter specific differences in the ratios in those points nearly coincidewith other characters.

OOOO<D<D
(j)

c

B

A

D

Fig. 14. Eyes ofThysanoessa euphausiids. A-Male T. gregaria, B-Female T. gregaria, C-Female
T. vicina, D-Male T. vicina, E-Young T. inermis in juvenile stage 5 mm, F-Young T. longipes
in juvenile stage 5 mm.

Among Thysanoessa euphausiids, T. inermis, T. raschii and T. inermis make groupapart from other species, although species contained show sexual dimorphism. Except T. gregaria, other four species form group of high ratio in the upper and lower partsof eye and fifth and sixth abdominal segments. The former group scarcely show
the differences in two points, but T. gregaria and T. vicina show the sexual dimorphismin the ratio of abdominal segments. T. longicaudata and T. longipes show little difference in the ratio of eyes. On the other hand, T. gregaria shows heavy sexual
dimorphism in the ratio of eyes.
ABDOMINAL SEGMENTS

Keel and abdominal spines

The arrangement of keels and abdominal spines are given m Table 5.
Among nine species of Tlrysanoessa euphausiids treated here, three species have
TABLE 5. ARRANGEMENT OF KEELS AND ABDOMINAL SPINES ON THE
ABDOMINAL SEGMENTS IN THYSANOESSA EUPHAUSIIDS
Abdominal segments
SpeciesT. raschii
T. inermis
T. spinifera
T. longipes
T. inspinata
T. longicaudata
T. gregaria
T.parva
T. vicina
T. macrura

K

2

3

4

5

6

K

K
KSP
K

(ksp)
KSP
KSP
K
(k)

(k)sp
KSP
KSP
K
(k)

SP
SP
SP
SP
(sp)

K : Well developed dorsal keel. SP : Well developed dorsal spine. Smalls show the presence ofkeel or spines, but they are not well developed. Extraordinary ones are in bracket.

124

T. NEMOTO

dorsal keels and four species have abdominal spines. Dorsal keels are well developedin T. spinifera, T. longipes and T. inspinata. Thysanoessa raschii, T. gregaria, T. parva,
T. vicina and T. macrura have no developed keel and no spine. T. inermis has only
abdominal spines on fifth and sixth segments or sixth segment only, but some extraordinary specimens have also keels and spines on fourth segment. Two closely related species, T. longipes and T. inspinata have keels on third, fourth, fifth segments, and spines on sixth segments. But well developed abdominal spines arepresent on third, fourth and fifth segments of T. longipes alone. Thysanoessa
longicaudata has no clear keel and spine but it is considered to belong to T. longipesgroup. In some younger furcilia stage 5, slight keels are described on third andfourth abdominal segments by Einarsson (1945) and some present specimens fromthe station M also bear keels on fourth and fifth segments.
Thysanoessa gregaria and three other gregaria type euphausiids are without any
keels and abdominal spines.
Thysanoessa spinifera has keels on the first, second, third, fourth and fifth segments
and abdominal spines of fourth, fifth and sixth segments. The presence of clear keels
on first and second segments is only found in T. spinifera.

Two and one spine forms ef Thysanoessa inermis
In a previous ,report, (Nemoto, 1959), I described the possible difference beween one and two spine forms of Thysanoessa inermis in the Atlantic and the Pacific.

Fig. 15. Three spine, two spine and one spine forms of Thysanoessa inermis. A-Three spine
form T. inermis: B-Two spine form: C-One spine form: D-10.5mm larva: E-9.4mm
larva: F-8.2 mm larva. a-fifth dorsal segment in one spine form: b-fifth dorsal segment in
two spine form: c-sixth dorsal segment in two spine form.

According to Hansen ( 1911), the majority of specimens taken in the Pacific has adorsal spine which semetimes proportionaly long, sometimes rather short, and sometimes nearly or completely rudimentary, in the fifth abdominal segment and in rarecase wanting. On the other hand, Einarsson (1945) stated none of the numerous
specimens examined by him had shown even the slightest sign of a spine on the fifth
segment in the Atlantic. Hansen ( 1911) also saw some specimens, taken off Cape
Cod, with a conspicuous spine, while in another it was rudimentary I found noneof the two spine form in the sample of Okhotsk Sea in my previous investigation,(Nemoto, 1959). I have examined many specimens of Thysanoessa inermis in the
Okhotsk Sea after then, and the average values 16. 7 % in females and 11.1 % in males

THYSANOESSA EUPHAUSIID

125

are obtained by present examination as the percentage occurrence of two spine
form. Further the occurrences of two and one spine forms of Thysanoessa inermis are
examined on the materials collected from the Bering Sea to the waters of the Gulf
of Alaska in the North Pacific. The occurrences of one and two spine form of Thysanoessa inermis are shown in Fig. 16. The two spine form is about 50 to 60 percentof the total in the waters off Kamtchatka, and about 60 to 70 percent in the easternwaters of the Bering Sea. The two spine form occupies about 75 or more percent
in the waters of the Gulf of Alaska. This geographical dine is more appreciable
when we consider the lower occurrences of two spine form in the Okhotsk Sea.
1w·

16')"

170'

Q One spine form
Two spines form

II'"

, .
'

itO'

170'

1eo·

50•

!70'

160"

150•

Fig. 16. The occurrences of one and two spine forms of Th_ysanoessa inermis in the northernpart of the North Pacific. Black-Percentage occurrence of two spine form. White-One
spine form.

This type of geographical dine, specific dine (Huxley, 1940) is also found in a plaice
Platichys stellatus (Pallas) in the North Pacific. Platichys stellatus has the eyes in theright side of the body. But it has eyes in the left side from the specimens in thewestern Pacific and Japan. The percentage occurrence of left side eyes is about
70 % from the specimens collected from Alaskan Peninsula and about 50 % in the
California. The occurrence of the two spine form of T. inermis is somewhat or considerably lower than those reported in the former report, (Nemoto, 1957), and thisis attributable to the number of specimens examined and the selection of samples.
The younger T. inermis lacks completely abdominal spines of fifth and sixth
segments. As a example, small size specimens collected at 53-40N, 165-20W,
are examined. The average length is 8.2 mm in which both abdominal spines are
undeveloped. Slight protrusion is observed on sixth abdominal segment at the average
body length of 9.4 mm. The well developed abdominal spine is observed in samplesof 10.5 mm long. So, it is desirable to treat the adults or well developed specimens
to compare the occurrences of two and one spine forms of T. inermis as it is done here.

126

T. NEMOTO

Three spine form of T. inermis
The three spine form of Thysanoessa inermis is reported in 1962 (Nemoto, 1962).
Two new female specimens are found among samples collected at 52-53N and 16212E, which also bear the abdominal denticles on the fourth segments. One specimen has strong spines on fourth, fifth and sixth abdominal segments with welldeveloped keel on fourth and fifth abdominal segments. The another one is without
keels but has spines on fourth, fifth and sixth segments too. The body length are
21 and 18 mm respectively and copulated.
Einarsson (1945) stated that " I will here confine myself to pointing out thepossible relationship of this species to the Pacific species Tl!Jsanoessa spinifera Holmes,
which has dorsal spines on the three last abdominal segments and the antenna!
armature of which bears close resemblance to that of T. inermis ". It is very interesting to note that one keel on the fourth abdominal segment is higher in the middle
position and resembles rather to that of T. spinifera than to T. longipes.
Allomorphosis in length of abdominal segments
The sixth abdominal segment has been generally used as a taxonomical character to some extent for the species of euphausiids. It has been used as the combined

06B

Male

0

c:

Eo E
-o.§.

3.0

c

_Q

0 ~ 2.0...cc:
-

/

0

--'

.//

3.0

5.0

7.0 9.0

. •/

x/

/D

/'s•/

7·/

3.0

.--

+

:I-/

/
p/
5.0

4.0
3.0

£A
/

I

~·

/./\
•

/

'
I ,./E/.E<00·~ ?()·;/~

.../+

/.y

<lJ

·~ ~
l.5
<lJ
-

Female

xo/
o"

2.0

7.0 9.0

Carapace margin length Imm)

Fig. 17. Allomorphosis in abdominal segment of Tfiysanoessa euphausiids. Relation
between carapace margin length and length of sixth abdominal segment.

or direct comparison with fifth or fifth and fourth abdominal segments. Here thelength of sixth abdominal segments is plotted against the carapace margin length inlogarithmic chart in Fig. 17. T. macrura and T. vicina form one allometric tribe
and T. longipes, T. inspinata and T. raschii form another tribe. T. longicaudata belongs to T. macrura tribe in males but it shifts in females slightly.
T. spinifera and T. inermis occupy the same allometric tribe in females but theytake different lines in males respectively.
T. gregaria is one species which forms allometric tribe by itself. The growthconstant values are about the same for every groups except male T. spinifera.
It is very interesting to see that the T. raschii which differs consideraly from T.
longipes group in other external characters form the same allometric tribe with T.
longipes in sixth abdominal segment for carapace.

127

THYSANOESSA EUPHAUSIIDAs the next step, the length of sixth abdominal segments are plotted against
the length of fifth abdominal segments as shown in Fig. 18. Like above case, T.
macrura and T. vicina form the same allometric tribe, but T. longipes, T. inspinata andT. longicaudata make another tribe. Other T. gregaria, T. inermis, T. raschii and T.
spinifera draw respective allometric tribe by themselves. There is no difference between males and females of each species.
0·~ -3.0
o E

ci E
...00

-

....c:::

~ 2.0

-

E

.~ t»

"' ~ 1.5

/
3.0

2.0

1.0

2.0

1.0

3.0

length of fifth abdominal segment {mml

Fig. 18. Allomorphosis in abdominal segments of Thysanoessa euphausiids. Relation
between length of fifth abdominal segment and length of sixth abdominal segment.

6.0
E

Mole

5.0

.§.
c:

~

2

4.0
3.0

0,

~ 2.0
2.0

3.0

4.0

1.0

2.0

3.0

length of sixth abdominal segment (mm J

Fig. 19. Allomorphosis in telson of Thysanoessa euphausiids. Relation between lengthof telson and length of sixth abdominal segment.

The telson attached to the sixth abdominal segment is considered as one swimming apparatus. The length of telson against sixth abdominal segment in logarithmic chart is given in Fig. 19.
T. macrura, T. vicina and T. longicaudata form the same allometric tribe andgrowth constant. T. longipes and T. inspinata make the same tribe and T. inermis
and T. raschii make another one. T. gregaria and T. spinifera make respective allometric tribe by themselves.
PREANAL SPINESThe preanal spines of euphausiids sometimes have been considered as a taxonomic
character. The illustrations of them in Thysanoessa have been drawn for Thy-

T. NEMOTO

128

sanoessa longipes and T. gregaria. (Boden, Johnson & Brinton, 1955) and T. macrura(Nemoto & Nasu, 1958). The latter is also drawn for the sexual dimorphism fromthe young stages. The preanal spines of available species of Thysanoessa group are~ \( ~>t ~~D\(>{~ ~\(~:{~\{ ~\i\i~N~~~~~~~ ~\(\i~\(Fig. 20. Preanal spines of Thysanoessa euphausiids. A-T. longipes: B-T. inspinata: C-T.
spinifera: D-T. raschii: E-T. inermis: F-T. gregaria: G-T. vicina: H-T. macrura. Males areillustrated with dots.

Fig. 21. Preanal spines of Thysanoessa euphausiids. A-Thysanoessa longipes:
B-T. inspinata: C-T. longicaudata: D-T. spinifera. Males are illustrated withdots.

illustrated in Fig. 20.

These spines are divided into following three groups.

Typical sexual dimorphism present
Slight sexual dimorphism present
Sexual dimorphism nearly absent

T. macrura, T. gregaria, T. vicina
T. inermis, T. raschii, T. spinifera
T. longipes, T. inspinata, T. longicaudata

THYSANOESSA EUPHAUSIID

129

In a strict sence, the sexual dimorphism is existing also in the third group. As
it is illustrated in Fig. 21, the male Thysanoessa longipes has rather acute and thickspine. On the other hand, females have rather long and curved spines.
In all species in which the sexual dimorphism is observed, females have thedeep denticulations in the concave part of the edge of preanal spines. Males haverather simple spines with a few small denticles along the same part.
This group includes T. gregaria, T. macrura and T. vicina possibly including T.

parva.
Thysanoessa raschii and T. inermis show slight sexual dimorphism, but T. inermis
shows more distinct differences.
In other euphausiids, these sexual dimorphism is also observed. Euphausia vallentini in the southern hemisphere has heavy denticulated preanal spines in females
and small denticles along the inner margin of the preanal spines in males. Anotherexample is given by Boden, Johnson & Brinton ( 1955) on Nematobrachion flexipes.
They stated the preanal spine is simple in males and bifid in females, and the malepreanal spine is also simple in Nematoscelis difficilis (Mclaughlin, 1965).
THORACIC LEGSThe general arrangement of Thysanoessa is illustrated in Table 6. Three speciesof Thysanoessa, have non-elongated second legs and thoracic legs are similar in strucTABLE 6. ARRANGEMENT OF THORACIC LEGS IN THYSANOESSASpecies

Thysanoessa raschii
inermisspinifera
longi,pes
inspinatagregaria
parva
macruravicinalongicaudata

Number offunctional
legs

even

6
6

+
+

6
6
6
6
6
6
6
6

Similarity
uneven

(+)
+
+
+
+
+
+
+

Elongated
legs

Reduced legsMale
7, 8thFemale
8th*

(2nd)
2nd
2nd
2nd
2nd
2nd
2nd
2nd

* Distal three or four segments are lacking in 7th leg.

ture of setae and spine as the skimming apparatus for foods. Other seven species
have elongated second legs, and rather degenerated fifth and sixth thoracic legs.
The former group may be connected to genus Euphausia and the latter may resemble
to the other groups in the thoracic legs, such as Tessarabrachion, Nematoscelis,
Nematobrachion and Stylocheiron species. Of course, there is not enough data for thefeeding mechanism of euphausiids, it seems that euphausiids which have elongatedthoracic legs are carnivorous more or less or polyhagous as Thysanoessa species.
Especially Nematobrachion species are considered as typical carnivorous euphausiids
mainly feeding on copepods in the deeper layer of the sea. The possible relation
between the eyes and feeding apparatus for the living environment is now under130

T. NEMOTO

examination, and here the allomorphosis in Thysanoessa euphausiids in thoracic legs
and morphology of dactylus of first and second legs which have special feeding character are investigated.
Allomorphosis in the total length of legs
The first legs show three allometric tribes when they are plotted against carapace margin length. T. macrura, T. longipes, T. inspinata, T. vicina, T. longicaudata

8.0

FemalesMales"E 6.o
£
Dl

~ 4.0

2.0
3.0

5.0

7.0 9.0

3.0

5.0

7.0 9.0

Carapace margine length Imm)

Fig. 22. Allomorphosis in leg of Thysanoessa euphausiids.
total first leg and carapace margin length.

Relation between length of

Male

8.0

~ 6.0

I

OJ

"
-g 4.0JN~~"E 2.0

/
2.0

3.0

5.0

7.0 9.0

i'
3.0

a
x

/
5.0

7.0

Carapace margin length Imm I

Fig. 23. Allomorphosis in merus in second leg of Thysanoessa euphausiids.
between length of merus of second leg and carapace margin length.

Relationand T. gregaria are making one allometric tribe, which closely corresponds to theshorter spine group in the dactylus of the first logs.
The next group is T. spinifera which form the tribe by itself both in males andfemales. T. raschii and T. inermis are considered to form the same allometric tribe
although T. raschii shows some vague points in females.

THYSANOESSA EUPHAUSIID

131

10.0
8.0
6.0

4.0

E

2.0

E
tJ)

_<I!

3.0

5.0

7.0 9.0

3.0

5.0

7.0 9.0

3.0

5.0

7.0 9.0

3.0

5.0

7.0 9.0

-;:
tJ)

~

8.0
6.0

4.0

2.0

Carapace margin length (mm)Fig. 24. Allomorphosis of leg in Thysanoessa euphausiids. Relation between length ofcarapace margin length and total length of thoracic legs.

0.8
0.7
0.6

Female

0.5
E

0.4

E

~

0.3

u
0

1:3

0.2
0

0.1

•
1.0

•

2.0

3.0

l.O

2.0

3.0

length of merus in first leg Imm I

Fig. 25. Allomorphosis in Thysanoessa euphausiids.
merus in first leg and length of dactylus.

Relation between the length of

132

T. NEMOTO

In second legs, the allometric tribes are given for the length of merus. Although T. longipes stands a little apart from the main line of the shorter group of thedactylus in the first leg, they fairly well coincide with the allomorphosis in the first
dactyl us.
In third and fourth legs, the wide band includes all species of Thysanoessa as
illustrated in Fig. 24. In these figures, T. inermis and T. raschii form the obliqueline which cross the main line. This is the same tendency in the first leg, and T.
inermis and T. raschii form the distinct allometric tribes in fifth and sixth legs bythemselves. But T. spinifera shift the tribe in fifth and sixth legs as illustrated inFig. 24. It makes allometric line with T. inermis and T. raschii in sixth and fifthlegs. These are confirmed by the skimming structure with well developed setae inthose legs. On the other hand, T. macrura T. longipes, T. gregaria and T. vicina haveshorter and poorly developed setae in those legs.
0.5

-=·
0.4
c
E

:

0.3

CJ)

-0

c

N

0.2

c

"'::>

::-u
0
-0

O.l

0
-;:::
Ol

c
~

l .O

2.0

4.0

6.0

8.010.0

Length of merus in 2nd leg Imm l

Fig. 26. Allomorphosis in legs of Tkysanoessa euphausiids.
length of merus and dactylus in 2nd leg.

Relation between

The allomorphosis in thoracic leg joints in Thysanoessa euphausiids are also
very characteristic. The length of dactylus of the first legs are plotted against thelength of merus of the first legs in logarithmic chart. They form only one allometric
tribe except T. macrura. The structure of dactyl us of those species bearing somewhat
peculiar characters as discussed. The position of T. vicina may shift from the maingroup if more data were obtained as it is basing rather small number of data.
The length of dactylus of other legs are also plotted against for the length ofmerus. They are shown in Fig. 26, and 27. In second leg, three allometric tribes
are observed. T. inermis and T. raschii form the same tribe as the first leg, but
T. spinifera differs considerably from the group. The oceanic group, T. macrura,T. inspinata, T. vicina, T. gregaria and T. longicaudata form one tribe and position of

THYSANOESSA EUPHAUSIID

133

T. longipes is somewhat vague as shown in Fig. 26. T. longipes also show some shift
from those species in the relation between length of merus in second leg and carapace
margin length. The length of dactylus is very variable in second legs of Thysanoessa euphausiids, and its variation in length is closely connected with the lengthof other joints of legs. T. inermis and T. raschii has shorter merus and longer dac0.5

x'

3rd

/o
1;1~

0.4

+}
0/

>
0

0.2

0

I
a:•

e/ I

;.

--;:
Ol

c
~

O.l

I

./

/

00

®
®

.)''

I

1·

o/x

x

!°

I

L

~ 0 3

'""()

t>

!'!.

E
E

u

4th

I
•

l .O

3.0

2.0

l.O

2.0

3.0

length of merus Imm I

Fig. 27. Allomorphosis of leg in Thysanoessa euphausiids.
merus and length of dactylus in 3rd and 4th legs.
8.0

Relation between length ofMale'E 1.0.S 6.0gi 5.0i! 4.0N·'= 3.0
~
~

Q.

2 2.0
0

1.0

2.0

3.0

5.0

7.0

2.0

3.0

4.0 5.06.0

Length of merus in 2nd leg Imm J

Fig. 28. Allomorphosis in legs of Thysanoessa euphausiids.
merus and carpus in 2nd leg.

Relations between length oftylus. T. spinifera has somewhat longer merus and shorter dactylus than the former
two species. It should be noted that T. spinifera differs in this point because it belongs
to the T. inermis group in many other characters.
In the third legs of Thysanoessa euphausiids, the dactylus length against thelength of the merus of the same legs form one allometric tribe except T. macrura

134

T. NEMOTO

which has rather shorter dactylus still in the third leg.
From fourth legs to sixth legs, all Thysanoessa euphausiids run the same line inallometric tribe in the relation between length ofmerus and length ofdactylus. Andsetae in merus and carpus are developed in T. inermis, T. spinifera and T. raschii.
The setae of T. longipes, T. gregaria and other pelagic species have rather rigid spinesand number of setae are small.
The characteristic position of T. macrura in the allomorphosis in the dactylus
of first and third legs may suggest the relation to the related species Tessarabrachion
oculatus which has elongated second and third legs and shorter dactylus in thoselegs. The allomorphosis in the relation between length of carapace margin lengthand total second legs, Tessarabrachion oculatus stands the same line with T. longipes,
T. gregaria, and T. macrura (Nemoto, 1962). In the allomorphosis in the relation
between dactylus and merus of third legs, it shifts from the group of T. longipes with
T. macrura.

1;/

2nd

I

20.0

3rd

4th

5th

6th

0

110.0

/

~::~I:"
0

i

4.0

•

/

/

/

2.0
2.0

4.0

6.0 8.0 2.0

4.0

6.0 8.0

3.0

5.0 7.0

LeniJ.lh of first leg Imm)

3.0

5.0 7.0

3.0

5.0 7.0

Fig. 29. Allomorphosis in legs of Thysanoessa euphausiids in the world ocean. Relation
between first leg and other legs.

The relation between carpus and merus in second legs in Thysanoessa euphausiids is as follows. Both in males and females, there are two allometric tribes inThysanoessa. T. inermis, T. raschii and T. spinifera form one line and other six species
form another one. It clearly seems that T. spinifera belongs to the same tribe with
T. inermis and T. raschii, although it belongs to the different tribe alone in dactylus.
The total leg length of each species are plotted against the length of the firstleg. In the second legs, T. spinifera, T. raschii and T. inermis form one allometric
tribe and other Thysanoessa species having longer second legs form another group.
The third and fourth legs show no difference between above two groups in the total
leg length, but T. macrura shifts slightly in fourth leg. In fifth and sixth legs, thegeneral degeneration is observed in T. macrura group which has elongated second
legs. The degeneration is heavy in sixth legs as illustrated in Fig. 29.

Dactylus of the first legs
The shapes of the dactylus of the first legs are very various in each species.
Thysanoessa inermis and T. raschii have rather long dactylus and shorter top spines.
The spines along the inner margin of the dactylus are ranging 5-8 in inermis and135

THYSANOESSA EUPHAUSIID

8-12 in raschii. T. spinifera has also long dactylus and the spines of inner margins
are ranging 8-14 and has shorter top spines.
T. longipes has somewhat shorter dactylus and longer top spines, but it has many spines
in the inner margin of the dactylus. It should be noted T. longipes often has a half
size spine along the inner margin at the most basic position as illustrated in Fig. 30.
The fringes in the spines are different in T. spinifera and T. gregaria groups. T.
spinifera and T. inermis have fringes of the same length even in the top of the spines
TABLE 7.

NUMBER OF SHORT SPINES IN INNER MARGIN OF DACTYLUS
OF THE FIRST LEG IN THYSANOESSA EUPHAUSIIDS

Species
T. spinifera
T. raschii
T. inermis
T. longipes
T. inspinata
T. longicaudata
T. gregaria
T. macrura
T. vicina

Male
8-14
8-12
5 or 6
5- 7
4- 6
3- 5

Female
8-14
8-12
6- 8
7- 9
5 or 63- 5
1 or 22 or 31

2 or 3
1Type of fringe
lnermisGregaria(Inermis)
Vicina

/

I

I

~g

I
I

7

c

A

B

c

F

G

Fig. 30. Dactylus of first leg of Thysanoessa euphausiids. A-T. spinifera: B-T. raschii: C-T.
longipes: D-T. inspinata: E-T. longicaudata: E-T. macrura: G-T. vicina.
a- spine of dactylus in T. spinifera: c- spine of T. longipes: g- spine of T. vicina.

although they curve to the acute point. In T. gregaria and related speciees, the fringe
are uneven in length and the top fringes are comparatively short as illustrated.
The fringe of T. macrura has small number of spines along the inner side,
but type of the fringes is seemed to be inermis type. The closely related species,
T. vicina is gregaria type dactylus which has few spines in the inner margin of thedactylus. But it has short and pecurial fringes differing from those of T. inermis
and T. gregaria.

136

T. NEMOTO

Shape of the dactylus in the second legs
T. inermis and T. raschii have slender dactylus which bears the long spines along
the margin of the dactyl us. The number of the side spines in T. raschii are ranging
4 to 6 in one row, and T. inermis has 2 to 4 spines. These two species are
considered as the same group also in the allometric tribe. T. spinifera, however, isconsidered to belong to another group. The dactylus is not so slender, rather
short and the side row of spines is not observed. Those spines are growing in the
distal edge of the top as illustrated. Other Thysanoessa have also shorter dactylus,
but the arrangement of spines are the same with T. spinifera. There are two rowsof small setae in the spines.

gI,/,,~j!IIC

I II "/ ' ' 1' 1!

I

I

"

/11!;/''//I I
[I

E

//,f'

i f h'/ .l1

i !14)

J

E

I

•!
11

i

1i

Ii/!
JI!

I 1.,; III /1 1.1/1·
lJ .1'//!//
ii'
f ! 1A
11

j

J

Ii

Ir{ 11I\

1~ 'r''JJ

I~ if' ;fFL'i/11''

11.1 ff!1! ,' :/1 11

J

i1 1q1I !/1~,!
r1:/111·.1\•,·•\n•·I~
11«1
I ,\IH~1·'\l

G L--1

Fig. 31. Dactylus of second, and third legs in Thysanoessa euphausiids. A-T. raschii; B-T.
inermis; C-T. spinifera; D-T. longicaudata; E-T. macrura; F-T. inspinata; G-T. macrura, third
leg. Upper figures show the shorter spines.

These shapes of the dactylus are closely connected with the relative growth,
allomorphosis of dactylus against the merus and carpus in the elongated secondlegs.
The shape of dactylus in second legs are discussed on Euphausia superba by
Barkley (1940) and pecurial spines (Putzborsten according to Zimmer, 1927) arefound in the inner side of the dactylus. These spines are yellowish or red in theliving condition, and easily observed in their activity. But the sweeping or cleaning
for their legs or setae is not observed by Barkley (1940).
None of this peculiar spines is found in Thysanoessa euphausiids. In T. gregariagroup, the dactylus is shorter and the corresponding series of spines or setae become
strong spines in the margin or inner edge of the dactylus.
It is very interesting that not only hervivorous Euphausia species but it is also
found in the carnivorous euphausiids such as Nematobrachion species, although thesecond leg of Nematobrachion species is not elongated.

THYSANOESSA EUPHAUSIID137

MOUTH APPARATUS

Here T. gregaria and T. inermis are mainly examined which are belonging to different type respectively comparing with other Euphausia and Nematobrachion species
according to structures of thoracic legs.

Upper lip
The general shape and function of the upper lip in T. gregaria is almost thesame with Euphausia superba and T. inermis which have also nearly the same shape of
upper lip.
Lower lip
The lower lip of T. gregaria and T. inermis are also nearly the same with thatof E. superba but having edge spines in the outer margins of the false jaws. These
spines are characteristic denticles in Thysanoessa euphausiids but they may not havespecial mechanism in feeding.
Mandible
The general shape of mandible of Thysanoessa seems not to be so different fromother euphausiids, however, the number of furrows in the mill-like process are notso much as Euphausia superba (Barkley, 1940).
First maxilla
The first maxilla has very strong processes along the edge of basis. T. gregaria
has about seven tough processes. Euphausia vallentini and E. superba (Barkley, 1940)
have much more processes along the basis of the first maxilla. These processes areaccompaied with other small spines. The coxa has also processes and many spines.
But these processes and spines reduced in carnivorous euphausiids, Nematobrachion
boopis. Other euphausiids, Euphausia superba, E. vallentini have considerably manyspines and processes. It is considered Thysanoessa euphausiids stand the intermediate
position between carnivorous and hervivorous euphausiids from the structure ofthose maxilla.
Second maxilla
The second maxilla is long and covers the first maxilla. The general shape of thesecond maxilla is different between T. inermis and T. gregaria. T. inermis has longer
segment in the last endopodite which corresponds to Euphausia species. (E. superba,
1940 Barkley: E. vallentini, Nemoto). But the shape of the last segment of T.
gregaria is rather short and rounded. The carnivorous Nematobrachion boopis has
also rather short and rounded segment, and setae are long but fragile and sparsely
scattered. The mechanism of those second maxilla is considered to collect andmove the odds which congregated in front of the mouth from the outer side to the
opening of lower and upper lip and then to the mandible. The spines and setaeof the coxa and the first ishium of the endopodite are the most important for the

138

T. NEMOTO

action, but the long spines and setae in the last segment of the endopodite are also
usefull for the collection and not to miss the food. The carnivorous animals use
the long sparse spines bearing special structure which is very effective to seize thosefood such as copepods and other active animals.

H

c

6
D

M

Fig. 32.

First (Left) and second (Right) maxilla of Tftysanoessa gregaria and spines of maxilla.
INNER STRUCTURE OF STOMACH

The inner wall of euphausiids are constructed to crash the foods of them such as
diatoms, foraminfera, other zoo-planktons and phyto-planktons. The preliminary
figures are given in the previous book on foods of whales and another paper (Nemoto,1965). The food organisms are described by Ponomareva (1955). According to
her list, Thysanoessa generally feeds on zooplanktons especially in T. longipes as well
as phytoplanktons.
The stomach structure of T. inermis is given in Fig. 33. The general patternof structure is the same with other euphauiids. It has, however, peculier bunch ofspines in the middle of the lower margin of the side plate of the stomach wall. They
may be called ' cluster spine ' of the euphausiids. This type of the cluster spine ischaracteristic in Thysanoessa euphausiids and related species Tessarabrachion. Every
species of Thysanoessa euphausiids have row of cluster spines ranging from 10 to 40
in the middle of the side plates. The number of the cluster spines is small in T.
vicina and T. macrura, and other Thysanoessa have 20 or more number of spines. The
general shape of spines are not different among each species. T. raschii and T.
inermis have about 20-25 cluster spines, T. longipes and T. inspinata have about 30
spines, and T. spinifera has about 40 cluster spines. Thysanoessa euphausiids have

TH'YSANOESSA EUPHAUSIID

139

other spines to crash in the anterior part of the side plates as illustrated in Fig. 33.
Especially T. inermis, T. longipes and T. inspinata and T. vicina have strong spines in
the anterior edge parts of the side plates. These spines have some taxonomical
characters as given in T. longipes and T. spinifera. In Euphausia genus, these spines
congregate and form the circle bunch of spines as illustrated in a former report(Nemoto, 1965, Fig. 8). This type of cluster spines are characteristic in Euphausia,
but they lack the row of cluster spines in the low or middle part of the side plate of
the stomachs. The cluster spines in circles and isolated spines in Thysanoessa euphausiids are made to crash foods with the spines in upper side plates in the stomach
walls. The circle cluster spines are fit for the porpose to crash hard wall cell of diatoms
and related organisms. So Thysanoessa euphausiids lacking this circle cluster spines

;1

D

E

c
B

F

H

G

Fig. 33. Stomach of Thysanoessa inermis. A-Lateral view: B-Cross section at middle: C-Basepart of the stomach: D-spine cluster in side plate of stomach: E,F,G,H,-Chewing spines inthe stomach.

show good adaptation to catch zooplankton or other animals without hard shells.
In the central part of the side plates, there are many isolated spines as illustrated inFig. 32. These spines are also considered to crash their foods, and are most dominant
in the central part of the side plates in the Thysanoessa and Euphausia species. In
some species in the deep sea living Thysanopoda such as Thysanopoda monacantha has
this type of spine, but other Thysanopoda, T. tricuspidata has one row of spines in the
middle of the side plates. And Nematobrachion boopis completely lacks those spines
in the side plates. All Thysanoessa euphausiids have central row of spines in the bottom plates. This row of spines is clear in Thysanoessa longicaudata, and T. inermis,
but it is not so strong in other Thysanoessa.
This central row of spines is also typical in Thysanopoda monachantha, Nematobrachion

140

T. NEMOTO

boopis, (Nemoto, 1965 Fig. 9.) and Euphausia species. But Nematoscelis has two
branch rows and Stylocheiron longicorne and S. abbreviatum have only slight or no row
of these spines.
The stomach of Thysanoessa is considered as a polyphagus type from the arrangement of cluster spines and isolated spines in the side plates. The side view of thestomachs of Thysanoessa shows no swell of the posterior portion of the stomach justabove the pyloric region of the intestine. N. boopis has very swollen stomach in theposterior part (Nemoto, 1965 Fig. 9) which fit to take vast quantity of undigested
foods such as copepods and other living zooplanktons at one occasion.
COPULATORY ORGANS

The male copulatory organs of euphausiids are considered one of the best specificcharacteristics and the female copulatory organs, namely thelycum, are also pointed
out as one of the best character for taxonomy by Einarsson (1942). In some closely
related euphausiids, Thysanopoda aequalis and T. subaequalis (Boden & Brinton, 1956),

I
A
'---'

10µ

B

50µ

c

•11
Iii

D

E

F

Fig. 34. Spine clusters of stomachs of Thysanoessa euphausiids. A-Thysanoessa vicina:
macrura: C-T. gregaria: D- T. longipes: E-T. inspinata: F-T. raschii: G-T. spini,fera.

G

B-T.

Euphausia nana and Euphausia paci.fica (Brinton, 1962a), the copulatory organs do notexhibit so well their specific characters. In a former report, (Nemoto, 1963) I also
discussed the possible difference in the relative growth of the length of the process
and spines in the male copulatory organs in closely related T. longipes and formarlyspineless form, T. inspinata. On Euphausia nana and E. paci.fica, the microstructure of
the male copulatory organs are considered as the taxonomic characters, however,the body length, horizontal distribution are also main discriminating points about
two species (Brinton, 1962a).
The copulatory organs in T. gregaria group in males has terminal process distally expanded, but in the younger stages it is not so expanded as discussed in theprevious report on T. macrura (Nemoto, 1958). The young Thysanoessa macrura has
also broader base of terminal process but the tip is pointed.
The copulatory organs of Thysanoessa euphausiids are divided into four or fivegroups. T. gregaria, T. macrura, T. vicina and T. parva have rather related male copulatory organs as illustrated in Fig. 35.

THYSANOESSA EUPHAUSIID141

The edge of the tip in terminal process of T. gregaria and T. vicina are serrated,and no clear serration is observed in T. macrura and T. parva.
T. raschii and T. inermis have similar male copulatory organs. Both species
have two long menbranous wings or expansions from the tip almost to the bases.
(Boden, Johnson & Brinton, 1955). The proximal and lateral processes are welldeveloped but not expanded in the ends. T. spinifera has not the broad terminal
spine. It has, however, the terminal spine of broad base and both proximal andlateral spines have acute ends. The broad base of a terminal spine is identicalwith T. inermis and T. raschii and it is considered as the original type of the copulatoryorgans of Thysanoessa euphausiids of inermis group.

Fig. 35. Male copulatory organs and possible relationships in dendrogram in Thysanoessa
euphausiids. A-T. inspinata, B-T. longipes, C-T. longicaudata, D-T. gregaria, E-T. rnacrura, F-T.
vicina, G-T. parva, H-T. inermis, I-T. raschii, J-T. spinifera.

T. longipes and T. inspinata have three main processes which are not expandedin the tapered ends. But those are slender and the base is not broad as T. spinifera.
T. inspinata has short and robust processes and main characters are the samewith those of T. longipes. The base of terminal process is not so broad as T. spinifera.
T. longicaudata has pointed terminal process, it has thin keel on the outer side.
The proximal process flattens out considerably above the middle and forms somewhat spoon-shaped plate with an uneven upper edge. The terminal end of the proximal process is not serrated. This character is bearing a resemblance to T. parva
or T. gregaria, but its base of terminal process is rather narrow and pointed end isquite different from those of T. gregaria group.
DISTRIBUTION
Three main distributions in the World Ocean contain following species listed in142

T. NEMOTO

Table 8 and 9. Two Arctic and boreal species T. inermis and T. raschii, are common
in the Atlantic and the Pacific.
T. longicaudata is only found in the Atlantic and T. longipes and T. inspinata, T.
spinifera are only distributing in the boreal region of the Pacific. Other two species
T. macrura and T. vicina are Antarctic species, and two species T. gregaria and T.
parva are distributing both in southern and northern temperate water regions.
T. spinifera is distributing only along the American side, the eastern part of the
Pacific and T. inspinata and T. longipes are abundant throughout the boreal regionsof the Pacific. The latter two species are rather rare in the shallow Arctic Sea.
(Johnson, 1956) and the occurrence of T. inspinata in the sea of Japan is somewhat
uncertain still in recent studies (Komaki & Matsue, 1958), although Ponomareva
considered it also distributed in the Sea of Japan.
TABLE B.

OCCURRENCES OF THYSANOESSA EUPHAUSIIDS IN THE WORLD OCEAN
SpeciesNorth
Atlantic

North
Pacific

+
+

+
+
+
+

T~ysanoessa raschii

inermislongipes
inspinatalongicaudata
spinifera
gregaria
parva
macruravicinaTABLE 9.

ArcticSea

Thysanoessa raschii
inermislongipes
inspinataspinifera
gregaria
parva

+

±
±

+

Southern
Hemisphere

+
+
+

+
+

+
+
+
+

OCCURRENCES OF THYSANOESSA EUPHAUSIIDS
IN THE NORTH PACIFIC
BeringSea
West

BeringEast

Okhotsk
Sea

Japan
Sea

+
+
+
+

+
+
+

+
+
+

+
+
+

+

-?

-?

Northwest
Pacific++++

Alaskan American
Gulf
Coast

+
+
+
+
+

±

+
+
+
+

T. spinifera shows pecurial distribution in the eastern part of the North Pacific.
(N emoto, 1962 : Brinton, 1962b). It has not been recorded from the Arctic Ocean,
north Bering Sea, Okhotsk Sea and the Sea of Japan. It is only distributing in theshallow neritic waters of the east coast of the North America. It is found dominantlyin the stomachs of baleen whales in the coastal waters of the east Aleutian Is. (Banner,
1954: Nemoto, 1959). The most northern record is given in a former report(Nemoto, 1962) and the most western record is given by Hansen, (1915). The
concentration of this species is also observed along the continental shelf of the Gulf
of Alaska from the observation of the stomach contents of baleen whales. The

THYSANOESSA EUPHAUSIID

143

southern range of the distribution is given by Banner (1949) and Brinton (1962b)
along the California coast of North America.
The range of water temperature and salinity is considered slightly different
from other neritic T. raschii and T. inermis. T. spinifera occurres in a little wamer
temperature and lower salinity and T-S diagram chart for T. spinifera is different
from those euphausiids. (Nemoto, 1962).
Th)'sanoessa inermis is distributing in the boreal Arctic water in the Atlantic
and the Pacific. In the Atlantic, general distribution is observed between the
February ice limit and the surface isotherm for l0°C (Einarsson, 1945). And thePacific distribution is also limited within the February surface isotherm 10°C
(Brinton, 1962b: Nemoto, 1962) and the main distribution is found within 15°C
in summer isotherm (Nemoto, 1962). T. inermis is considered to spawn in the shallow waters along the continental shelf, and Glover (1952) described T. inermis as
the shallow water species. In one former report, I reported that T. inermis occurresin the stomachs of baleen whales as their foods very often in the coastal waters ofKamchatka peninsula and along the continental shelf of the Alaskan peninsula in
the Bering Sea (Nemoto, 1962). T. inermis also found in the Okhotsk Sea and Japan
Sea. But latter occurrence is restricted in the northern half waters (Komaki &Matsue, 1958: Ponomareva, 1957). The concentration of T. inermis is also observed in the southern edge of the shelf along the Alaskan peninsula and the Gulf
of Alaska, and it goes down to the waters off Vancouver (Brinton, 1962b).
The possible relation between T. inermis in the Atlantic and the occurrence of
it in the western part of the Pacific is expressed in the two and one spine forms of
T. inermis. In recent investigations, Johnson (1956) reported it from the Chukchi
Sea but only few specimens. The main range of the T. inermis distribution is limited
the line of continental shelf of the Bering Sea, and the ice limit in summer in theshallower Atlantic north waters. Thus the interchanging of Atlantic and Pacificstocks in recent age may not occur to effect the main distribution and of two andone spine forms as shown in percentage occurrence of them.
T. raschii is uaully distributing along the shallow continental shelf or the neritic
shore waters in the Arctic boreal regions. It plays the role of foods of balleen whalesin the off waters of Anadyle Gulf, Bristol Bay and main occurrences are also observed
along the Kamtchatka peninsula, Anadyl Gulf, and the shelf of the Gulf of Alaska.
Other investigations prove that it also occurres in the Japanese coast of Okhotsk Sea(Nemoto, 1959), north shallow waters of Okhotsk Sea and northern part of the Sea
of Japan (Ponomareva, 1955).
The main stocks of T. raschii in the Pacific and the Atlantic are living within ice
covered waters in winter seasons. It is also more common in the North pole sea
than other euphausiids (Johnson, 1956). The interrelation between the Pacificand Atlantic stocks may be closer than other euphausiids, and possible continuityof distribution through the north pole waters is considered.
The southern range of T. raschii is restricted to the cold current and summer
concentration of this species is found even in the low salinity (Einarsson, 1945:
Nemoto, 1962). This position may be compared with the Antarctic Euphausia

144

T. NEMOTO

cristallorophias along the Antarctic continental shelf.
T. longipes is differeing from other neritic Thysanoessa spinifera, T. inermis andT. raschii, considered as pelagic form. Its main distribution is given by Brinton(1962b) and Nemoto (1962) in the northern part of the North Pacific. T. longipes
is transported to the north American secter of the Arctic sea (Johnson, 1956), whichcoincides with the flow of the Arctic water current from the southern waters throughBering strait (Johnson, 1956). Considering from the occurrences in the stomach
contents of baleen whales (Nemoto, 1959: 1962) and plankton net investigations(Banner, 1949: Brinton, 1962b), T. longipes concentrations are observed in the eastern
Bering Sea (Nemoto, 1962 : Brinton : l 962b) in the off waters of Alaskan continental
shelf and mid waters along the Bowers Bank. The former occurrences are found incompany with T. inermis and dominant occurrences are found in a little more offwaters in the north waters of Unimak Is. The heavy concentrations are also
observed in the southern waters of Aleutian Is. along the latitudinal belt of 50°N
(Brinton, l 962b) by plankton investigations.
The closely related species T. inspinata (Nemoto, 1963) or the spineless form of
T. longipes (Banner, 1949) is found in more southern waters. It consitutes thesmaller part of the food of baleen whales in the southern Bergin Sea (Nemoto, 1959)
which is possibly the northern limit of this species. The general distribution of
T. inspinata is given by Brinton (1962b) as the unspined form of T. longipes. The
southern limit of this species is considered in the western side of the Pacific coincidewith the range of cold current Oyashio. Brinton gave the most southern record
34-l 7N, 156-1 lE, where the submergence at the southern margin of the range is observed (Brinton, 1962b). The present data collected by Tansei-maru in KT-64-17
cruise, T. inspinata occurred from 33°-SON and 138-30E as the more western waters
in the southern waters of Japan.
T. macrura and T. vicina are only distributing in the southern hemisphere.
Those two Antarctic species have been found in the circumpoler belts from thesubantarctic to the Antarctic. From the external characteristics, they are considered to come from gregaria-parva relatives not longipes or inermis, and its distribution
suggests also phylogenetic speciations like the relation between T. longipes and T.
inspinata.
The distribution of T. gregaria is well discussed by Brinton ( l 962b) with somephylogenetic consideration in view of biantitropical panoceanic distribution of thisspecies in southern and northern hemispheres.
The vertical and latitudinal distribution of Thysanoessa euphausiids is given in
Fig. 36. In both Arctic and Antarctic seas, Thysanoessa euphausiids extend to thedeeper waters following to the equator. Thysanoessa raschii, the most northern species
is living in shallow water usually upper than 200 metre, and T. inermis upper than
300 metre in the North Pacific. The southern waters species, T. macrura and T.
vicina are also living in the layers upper than 300 metre (Hardy & Gunther, 1935).
T. longipes is generally considered to live in the layers upper than 500 metre, anddeeper waters in the southern waters near 40N latitudes. The closely related species
T. inspinata which is living more southern waters (Brinton, 1962) is considered toTH'YSANOESSA EUPHAUSIID

145

distribute into deeper waters (Nemoto, 1962). The deep water species T. parua is
usually found from 750 to 2000 metre depth in the waters. The vertical zones
of euphausiids distribution are illustrated by Brinton ( l 962b) as the mid Pacificprofile in the North Pacific. The Arctic and subarctic species of euphausiids
found in the surface layers often found in the middle layers of temperate watersof lower latitudes.
The deep sea living T. parua has far reduced spine and setae in the thoracic
legs, although T. gregaria and other species have also reduced setae in thoracic
legs especially in second legs.
This tendency is also observed in carnivorous euphausiids Nematobrachion
species. But the remarkable point in T. parua is the lack of strong fringes in setae
as well as the reduction of setae. As a taxonomic character in T. parua, these fine
fringes in setae in legs are described. These fine fringes may be also adoptation to thedeep sea living where the small food organisms such as diatoms are comparatively
scarce as compared with surface layers.
0 s

60'
I

I

200

E
-C

500

0.

~ I .OOO
2.000

I
I

T.macrura!~-

A.C

40'

20'

0

20'

40'

60'

i ,!

I
I

, 1

i !I

I

I

T. vicina T.gregaria

T.gregorio)---- ---1
T.porvo'. ! :• iT.inermis:
i 1'-----+-t_J
1
i IT.longipes

-I

I
\

N

' I: '1:T.rasc--h ii

II______ _]I,---- ______ ,,
i

'

'

''

I T.parvo [L---:::r~-r.inspinata

1___________ .)Fig. 36. Profile of the vertical and latitudinal distribution of Thysanoessa euphausiids along
the section approximately 160E.

Among Thysanoessa euphausiids, T. inspinata is found by KT-64-17 cruise fromthe south waters ofJapan by deep oblique haul, where T. gregaria possibly distributes
from the surface to about 1000 m deep. The Pacific boreal species Tessarabrachion
oculatus is also collected by this cruise in the same position. This submargence of
boreal species in temperate waters is observed in other groups of plankton such as
chaetognata Sagitta elegans in the south waters of Honshu (Marumo, 1965).
Brinton considered the biantitoropical panoceanic distribution of Thysanoessa
gregaria may be explained by Pleistocene cold waters distribution. The 7°C and
11 °C isotherms at 200 m are associated with the limitation of the recent range ofT. gregaria. and T. gregaria crossed the equater by this cooling of 2 1/2° to 3°C at
200 m in Pleistocene age.
At this circumstance, the North Pacific subarctic species, T. longipes may
still occupy the coastal fauna in the region 20-30N where the colder California
current runs and it never has the chance of transgressing the equator.
From above explanations for the distribution of Thysanoessa euphausiids two
hypotheses may be derived.
One is the trespass of T. uicina and T. macrura or their ancestor in the Antarctic
waters is earlier than the transgression of the equator by T. gregaria, and speciation

146

T. NEMOTO

of T. inspinata from T. longipes or its relatives is possibly later than that age.
The second hypothesis is that T. parva is also specialized into southern andnorthern hemisphere at about the same age of T. gregaria or a little later age, and it
also transgressed the equator after the specialization as T. parva, not the age of theoriginal type of T. parva in the older ages.
DISCUSSION
The allometric study and comparative morphology explain fairly well the taxonomicand ecological positions of each species in Thysanoessa euphausiids.
TABLE 10.

T. inermis
T. raschii
T. spinifera
T. gregaria
T. macrura
T. vicina
T. longicaudata
T. longipes
T. inspinata
T. parva

CHARACTERISTICS IN COMPARATIVE MORPHOLOGY ANDECOLOGY IN THYSANOESSA EUPHAUSIIDS

++
+±
++
+±
+±
+±
+

<>+
<><>+

·-··-·?

<> -£::,. +x -+£::,. +(!) +(!) +<)?-<)?+<)?++-

-+
++

±-

++
+-

<> <><><> <><><> x<)• •<>•?•C!J
•?•<>
0 •<>
0 •<>
0 •<>
?

±
±

±

+
+
+

?

+
+
±±
+
+
+
±±
+
+
+

+ ±
+ ±
+ ±
+(±)
+(±)
+(±)
++
+(±)
+(±)
?

+--+--+±-++±++-++-+++++±++±--++The similarity in morphological characters of Thysanoessa euphausiids is listed
in Table 10. In general consideration T. inermis and T. raschii resemble each other.
T. inermis differs in the sexual dimorphism in the first antenna, presence of spines in
the posterior abdominal segments, shape of the rostrum and the absence of the carapace denticle in the lower margins. But other main characteristics are the samewith T. raschii. The arrangement of thoracic legs resembles to Euphausia species.
T. spinifera shows the first characteristic point in the sexual dimorphism in thefirst antenna. The rostrum of the carapace has also specific difference among thesethree species. The most peculiar point is that the eye hole spine along the carapace
margin is present in T. spinifera. None of other Thysanoessa has this spine. The
shape of dactylus in thoracic legs approximately the same among those T. inermis,
T. raschii and T. spinifera. Only the dactylus of T. spinifera in the second leg differsfrom other two species. It bears the intermediate characteristics between T.

THTSANOESSA EUPHAUSIID

147

raschii group and T. gregaria and other species which have elongated second legs.
Third dactylus does not differ among every species except T. macrura as it is discussed in the former part. And no special difference is given in other dactylus inposterior legs.
Preanal spines in those Inermis species show slight sexual dimorphism, and othertwo groups also divide themselves from those species as fairly clear dicisions. The
group of T. parva is still unknown, but it may possibly belong to T. gregaria group.
The Atlantic T. longicaudata has non-sexual different preanal spines in denticulation
and belongs to T. longipes group. Longipes group species have the same character inthe dactylus of the first legs and shape of the rostrums, but T. longicaudata lacks thelateral denticle of the carapace. The second leg of T. spinifera stands peculiar intermediate position between T. inermis group and T. gregaria group. And this tendency
is also observed in the relation between merus and dactylus in the second leg. These
characters show that the second leg of T. spinifera becomes elongating and the dactylus
shortening which is more relating to T. gregaria group. T. gregaria group is considered
off shore living species and they are now developing to more carnivorous feeder inselection of the food. The short and bearing tough spines in the dactylus of theelongated legs are also often observed in other carnivorous euphausiids. And alittle elongated second leg in some younger forms of T. inermis (Hansen, 1911, Einarsson, 1945) is noted already, especially in T. neglecta form in the Atlantic. According
to the Einarsson's consideration, T. neglecta is only a transitory form in the developmental history of T. inermis. It has longer second legs and shorter dactylus fromthe figures by Einarsson, (1945 Fig. 19) and constricted eye. The illustrated
specimens stand clearly in the gregaria group from the length of each joint. The
larval forms of T. inermis usually have eyes longer vertically, which is almost the sameas T. longipes eyes in those stages. Thus T. inermis sometimes shows in its developmental stages the different pattern of allomorphosis in second leg, and it is considered as the one explanation for the different group of allomorphosis in Thysanoessa
euphausiids is originated from the above different developmental stages.
Another neritic species T. raschii has non-elongated second leg, and non-constricted eye in adults, and it has no clear form of elongated second leg in furcilia
stages like neglecta form of T. inermis. But it has lateral denticles in the anterior
part of the margin of the carapace, although the denticle is situated far posterior
part of the lateral margin of the carapace in the furcilia stages of T. raschii. This
would suggests full development in this character if the speciation is attributable toexcessive growth. There are three main types are observed in Thysanoessa euphausiids in male copulatory organs. One is inermis type which includes T. raschii and T.
inermis. T. spinifera has this character, but it bears also longipes type as discussed
in the former part. T. gregaria is considered to belong the same group with T.
macrura, T. vicina and T. parva. T. longicaudata is standing in the intermediate position between T. longipes and T. gregaria. This is also one of the expression of thephylogenetic position of T. longicaudata that it does not belong to the gregaria group.
The position of T. longicaudata is also confirmed by the presence of central spines
in the base plate of the stomach structure. This central spine is clear also in some148

T. NEMOTO

other euphausiids. Thysanopoda, Nyctiphanes, and Nematobrachion species have welldeveloped central spines in the base plate. Only T. longicaudata among Thysanoessa
has well developed central spines and T. inermis group has vague spines. T. longipes
and T. gregaria have trace of central spines but sometimes they have spines like T.
inermis. The type of cluster spines in the anterior part of the side plate of the stomachs
are nearly the same in Thysanoessa species, but some difference in related species
such as T. longipes and T. inspinata is observed.
The bilobate eye of seven Thysanoessa euphausiids in adult may directly beconnected to the broader living range in the depth of the sea at present or theirancestors (Brinton, I 962b).
TABLE 11.

,j

LIST OF SIMILARITY IN ALLOMORPHOSIS IN
THYSANOESSA EUPHAUSIIDS

...l

0

~

::a....: ::a~"Ou<".::lii. a:l....

~<..!l 8 ..!l ~ ~~ t; ]'E-5'5'5
0

bO

"bll

T. inermis
T. raschii
T. spinifera
T. gregaria
T. macrura
T. vicina
T. longicaudata
T. longipes
T. inspinata-~-...:i"O'"t:l..s::<a,_,_£'5
E-<"'c<')'<I< ""<O
0 ......

0

E-<

<"
C'lc<')'<!<l!'l<O~Cl<.. < .. ..........
0

...:i
E-<
rr.i

<>?<><><><><> <><><> <><>/':-,.
<>?<><><><><> <><><> <><>
x x<><><><> <><><> <><>•<><>?•••0 •• •<><>•••<><>•• •<><>?••
0 • •<><>•• •<><> ••• •<><>•• •<><> ••0• •<><>•• •<><> ••+ • •<><>•?
•<><> ••

:l

:l

'"t:l

oj

0.

oj

....

u

o]"E.s.s.s

rr.i'"t:l;::i a~"'~ ..

::;s" i.;

::;s
0 -a

~:i-o

~"
rr.i

<~

Cl "' 0

O a.n
oj

~a.s

8
. ~a

~...c:.!!?

~<<D< ....

<,;:;~><: ••••

0rr.i<><><><><> <> <><> <><>

~C'lc<')'<!< l!'l<O

~

~

•

'Eu-

::;s

E-<
a z
"8bO

...l

..........

::;s<><><><><> <>x<><><><> <>•<><><><>•0<><><>•<><><><>•<><><><>•<><><><> ••<><><><> •Of':..

<><>?
••
•0
•0
+0?
./':-,.

•••• ./':-,.

f':..0

xx
••
00
00
+0
+o
+o

The similarity in allometric tribes are illustrated symbolically in the Table 11.
As the general aspects of the list, those tribes are also divided into two main groups.
In the relative growth of the lateral denticle and carapace length, however, eachspecies demonstrates respective tribe except T. vicina and T. macrura.
It is suggested the same allometric tribe of T. macrura and T. vicina in theposition of lateral denticle means they closely related each other and position of
T. vicina is larval stage of T. macrura in this relation. Because the position of lateral
denticle of the carapace is situated in the posterior part and goes up to forward withthe body development especially in the furcilia stages. The body size of T. vicina
is smaller than T. macrura and lateral denticle is situated a little posterior position.
These characters are the larval characteristics. The general distribution of thosespecies show that the adult form, namely the lateral denticle of the carapace situatingin the anterior position, of each group is found in the colder waters. The body sizeof T. inspinata is smaller than T. longipes, and T. vicina is smaller than T. macrura.
Thus those specific differences are connected with the body development in thoseTHYSANOESSA EUPHAUSIID

149

species. The wamer water type may be the larval type, which has the posterior
carapace denticles, and having smaller body length. The phenomena like thishave been found on American fresh water fish as a consideration of the evolutionof those fish (Hubbs, 1940).
The legs of euphausiids are generally considered as the feeding and swimming
apparatus and legs vary very much among euphausiid species.
The general allomorphosis in legs of Thysanoessa euphausiids shows also two
main groups. The total leg length against carapace margin length and total firstleg are both the same in third and fourth legs, but it.differs in first, second, fifth andsixth legs. As for the relation of total length of the first legs for carapace length,
T. inermis, T. raschii and T. spinifera form one allometry group and other six andpossibly T. parva form another group.
T. spinifera stands itself in the second leg aparting from T. inermis but it forms
the same allometric tribes in fifth and sixth legs with T. inermis and T. raschii.
The allometric group of second legs are also divided into two groups. These
groups coincide with also groups of fifth and sixth legs. From the observations on
structure of legs, the reduction in the posterior thoracic legs is considered as thesteps to the carnivorous feeding type. Thysaneossa euphausiids have even in thegregaria group, rather long and skimming setae in sixth and fifth legs. But othercarnivorous Nematobrachion species have none functional setae and reduced fifthand sixth legs. The surface feeding and hervivorous euphausiids, such as Euphausia,
have functional and long sixth and fifth legs in general. Thus it may be consideredsome of Thysanoessa euphausiids bear both types of feeding and omnivorous feeding
type. Among them, T. inermis, T. raschii are surface feeder type. The generalfeatures of setae in thoracic legs as the feeding apparatus are discussed by Barkley(1940). T. inermis, T. raschii and T. spinifera resemble to E. superba in the arrangement of those setae in ishium and merus and in the allomorphosis relationship ofevery leg length.
In the allomorphosis in the relation between merus and dactylus of each legs
shows also two main allometric tribes as in the relation between merus and carpus
in second legs. But T. macrura shows peculiar position in first and third legs which
occupys the independent allometric tribe. T. spinifera occupys also the tribe itself
in second leg.
Although the different allometric tribes are observed in second legs, no difference is found in the allomorphosis from fourth to sixth legs in the relation between
merus and dactylus. Thus ·the speed in the development in each joints from small
and oceanic species to large neritic species is the same for each species. Only
second legs, and first and third legs partly, show the significant allomorphosis difference, which also have special means for ecological and morphological structures.
The allomorphosis in the relation between carapace margin length and abdominal
segments varies considerably in each species. It is very interesting that T. raschii
forms the same tribe with T. longipes and T. inspinata. T. macrura, T. vicina and T.
longicaudata form one tribe and T. gregaria forms itself one tribe. In the relation
between the length of sixth and fifth abdominal segment, T. gregaria also forms tribe

150

T. NEMOTO

only itself. T. gregaria is only one species living in the tempeate surface waters, andposterior parts of the abdominal segments are considered as the swimming apparatus
of euphausiids. If it is true, T. raschii takes somewhat curious position to form thesame allometry line with other two oceanic species. T. vicina and T. macrura form
the same tribes not only in the denticle length but also the legs and abdominal
segments relationship in general. The former relation in lateral denticles is considered as the close interspecific relation and the latter are partly the similarity
in ecological characteristics such as feeding and swimming in the sea.
The preliminary phylogenetic development in Thysanoessa euphausiids is in considered as follows. The original form of Thysanoessa is possibly belonging to thegroup of Nematoscelis and Tessarabrachion relatives.
From the ancestor of Thysanoessa, two branches are considered which comparatively earlier times developed to the different directions. Those are T. longipes
and T. gregaria group, and T. inermis and T. spinifera group. The former stem isdeveloping along the Thysanoessa line, that is oceanic and wider vertical range ofdistribution. The later stem is developing to the different neritic inhabitance suchas T. inermis, T. raschii and T. spinifera.
In the Antarctic, Euphausia crystallorophias is living as the real neritic species.
Its colonization of Antarctic waters is considered by John (1936) as the series of
E. lucens-E. vallentini-E. frigida and this series is divided into two branchs, thoseare E. superba and E. crystallorophias. And the transgression of the ancestor of E.
frigida series from the south as the E. pacifica series is possibly at the same time thatthe genus Thysanoessa become antitropical (Brinton, l 962b). At these time, thelater stem became developing as neritic and surface living species.
T. longipes or its ancestor is considered coming from the stem different T.
inermis-ancestors and T. gregaria-ancestors as it bears the male copulatory organs andpreanal spines of different forms. Further the distribution of T. longipes andT. inspinata is fixed in only North Pacific. This shows the transgression of equatorof these T. longipes group did not occur at the age of transgression of T. gregariagroup. They possibly occupy the northern colder waters already at that times,
and they shift the postulated distributions of transgression of equator at the coolingage discussed by Brinton ( l 962b).
Thysanoessa inermis and T. raschii are common in the north Atlantic and Pacific,
and they had possibly contact through North pole sea still in later ages. This may
be endorced by the distribution of one and two spine forms of T. inermis and neritic
species T. raschii. Before the differenciation between T. inermis and T. raschii, T.
spinifera or its ancestor is fixed as the neritic and shallow water species from the consideration on the structure of male copulatory organs and the allomorphosis of bodyparts. The restricted distribution of T. spinifera along the west coast of North
America may contradict this assumption. But it is generally accepted that it isclosed in Aleutian and California current, and the cold North Bering Sea and generalwater transport from the west to the east in the North Pacific drift may be the barriers for the extension of T. spinifera to the west Pacifiic (Nemoto, 1962). The
position of Atlantic T. longicaudata is very characteristic. It is considered it comesTHYSANOESSA EUPHAUSIID

151

from the way between T. longipes stem to the ancestors of T. gregarza or directlyfrom ancestor of T. longipes. T. longicaudata completely lacks the carapace denticle
but it is sometimes absent in many species which usually have carapace denticles andsome species have two forms which have denticles and none (Nematoscelis difficilis,
Mclaughlin, 1965). It is apparent that the external characters such as male copulatory organs and carapace denticles and preanal spines, the T. gregaria ancestor has
four other present Thysanoessa species, T. macrura T. vicina, T. parva and T. gregaria
at least.
ACKNOWLEDGEMENT

I would thank here Emeritus Professor Y oshiyuki Matsue of the Ocean ResearchInstitute, University of Tokyo for his kind guidance and helpful criticism of thispaper. Dr. Ryuzo Mammo, Professor of the Ocean Rseearch Institute, Universityof Tokyo, kindly gave me guidance as well as fruitful criticism and kind cooperations through the collection of some deep sea materials on board of R. S. Tanseimaru in the cooperative research cruise KT-17 in 1964. Drs. Yutaka, Kaharada,
Masataka Kitou and Kenzo Furuhashi of the Meteological Agency of Japankindly gave me samples of euphausiids collected in Japan Deep Sea Investigation(JEDS cruise) and the sea of Japan, which are used in one part of this study, andI thank also their helpful discussions.
The Atlantic specimens of Thysanoessa longicaudata have been sent by courtesyof Dr. L. T. Jones of the Oceanographic Laboratory at Edinburgh. I would
thank Dr. Sigeru Motoda, Professor of the Hokkaido University and Mr. R. S.
Glover, Director of the Oceanographic Laboratory of the Edinburgh for their kindarrangement for the sending of these T. longicaudata specimens.
SUMMARY

Thysanoessa euphausiids are investigated under the comparative morphology andallomorphosis studies, to get general considerations on morphological and taxonomical characters as well as ecological peculiarities. The main results are summarized
as follows.
I. Modified and simple key for the taxonomy of Thysanoessa genus is given.
2. Three main groups, namely longer, intermediate and shorter forms are foundon the first antenna. T. inermis, T. spinifera and T. raschii form the longer group,and they run also the same allometric tribe in allomorphosis in the relation between
carapace length and length of the flagellum. T. vicina, T. macrura, T. inspinata,
T. longipes and T. gregaria form the shorter group and also the same allometric
tribe. T. longicaudata forms different tribe apart from those two groups.
3. The shape of the rostrums, lateral denticles are examined. The positionof the lateral denticle have very important taxonomical characters especially inclosely related species under the evolution of the neoteny-like speciation.
4. Shape of eyes of Thysanoessa are very various. Neritic and shallow water152

T. NEMOTO

species have round and none constricted small eyes which show no sexual dimorphism. The allometric tribes in eyes are very various among each species and sex,
which is possibly due to the reflection of the complex ecological characteristics.
5. Arrangement of dorsal keels and abdominal spines are examined. One
and two spine forms of Thysanoessa inermis show the geographical dine in the North
Pacific in occurrences of each form. One spine form occurrence is high in western
Pacific especially in the sea of Okhotsk, in which it is ranging about 80 to 90 percent
like the Atlantic specimens. But it decreases following to the east and only about
25 percent in the waters of the Gulf of Alaska. Three spine form of T. inermis isagain described from the North Pacific.
6. T. gregaria stands in peculiar position among Thysanoessa species in the
allomorphosis in telson, fifth, and sixth abdominal segments. It is possibly due
to the wamer water habitat in the transition zone apart from other Thysanoessa.
T. macrura and T. vicina have close relations also in those points, which shows theecological and phylogenetic similarities are very close also in these body parts.
7. In T. spinifera and T. longicaudata, the allomorphosis in each sex is different
in the relation of abdominal segments and other body parts. The dalay in thedevelopment of the carapace in males may bring this variation, and these sexual
differences among allometric tribes are considered as one of speciation of the direction from the female type to the male type.
8. Preanal spines of Thysanoessa euphausiids are divided into three groups,
according to the sexual dimorphism. It is present in T. macrura, T. gregaria, T.
vicina and possibly in T. parva. Only slight dimorphism is observed in T. inermis
and T. raschii, and nearly absent in T. longipes, T. inspinata and T. longicaudata.
These sexual dimorphisms are already found still in younger adlescents.
9. Allomorphosis and structures of thoracic legs especially dactylus of them
are examined. In the relation between carapace length and second thoracic legs,
three allometric tribes are observed. T. inermis and T. raschii run the same line,
and T. longipes, T. inspinata, T. gregaria, T. macrura, T. vicina and T. longicaudata form
another tribe. T. spinifera forms one tribe itself. These tribes are nearly the samewith the first legs, and T. longipes group is corresponding to the shorter dactylus
group. There is no difference in the allomorphosis in third and fourth legs, but
T. inermis, T. raschii and T. spinifera form another tribe apart from T. gregaria groupwhich has shorter and poorly developed setae in those legs.
The characteristic positions of the dactylus in the first and third legs of T.
macrura are observed.
10. The structures of the dactylus of first and second legs are investigated.
The number of the spines along the inner margin of the dactylus of the first leg varies
in each species, and corresponding to the shorter and longer groups of the dactylus.
T. spinifera and T. raschii, the longer group, have 8 to 14 spines and T. inermis has
5 to 8 spines. The shorter group has less than 9 and T. gregaria, T. macrura andT. vicina have only 1 to 3 inner spines.
The dactylus of second legs is long in T. inermis and T. raschii, and T. spinifera
has rather shorter dactylus. Other Thysanoessa euphausiids have shorter dactylus

THYSANOESSA EUPHAUSIID

153

and elongated merus and carpus, which may be the simpton of carnivorous habit.
11. The first and second maxilla of Thysanoessa species show the carnivorousand hervivorous characters, which stand the intermediate position between thecarnivorous euphausiid, Nematobrachion and the mainly hervivorous euphausiids
Euphausia and other species.
12. The inner structure of the stomachs of Thysanoessa euphausiids has special
row of spines which is named as ' Cluster spines '. This row of cluster spines is foundalong the base part of the side plate of inner wall of the stomach. The isolated
tough spines are also found along the anterior part of the side plate, which also very
effective for the crashing their foods such as diatomes and other hard shell organisms.
But Thysanoessa euphausiids have no circle ' cluster spines ' in the anterior part ofthe side plate.
13. Male copulatory organs of Thysanoessa prove also T. macrura T. vicina
and possibly T. parva are belonging to the relative of T. gregaria. T. longicaudata
stands peculiar position, and T. inspinata shows close similarity to T. longipes.
14. The similarity and difference in allomorphosis and morphological study
are compared to get characteristics and phylogenetic considerations.
From the morphological observations, three main types are found. T. longipes,
T. inspinata and T. longicaudata form one group, T. raschii, T. inermis and T. spinifera
second group, and T. gregaria, T. vicina and T. macrura (possibly T. parva is includedin this group) third group. The most typical difference among those three groups
is the sexual dimorphism in preanal spines. The characteristics expressed in preanalspine and male copulatory organs of T. longicaudata suggest that it comes from theT. longipes ancestors rather than T. gregaria group.
15. General distribution of each species shows the possible correlation between
the evolution and present distributions, suggesting that T. vicina and T. macrura are
fixed in the Antarctic before the transgression of the equator by T. gregaria, and thespeciation of T. inspinata from T. longipes is possibly later than that age in the North
Pacific.
16. The preliminary consideration of the phylogeny of Thysanoessa is stated.
T. longipes and T. gregaria groups are developing along the Thysanoessa line, whichis oceanic and wider vertical range of distribution, in which T. longipes and T. inspinata or their ancestor is coming not through the T. gregaria or gregaria ancestor,
although some morphological characters are related to it. T. longicaudata alsocomes from the way between T. longipes and the ancestors of T. gregaria or directly
T. longipes group but possibly not from T. gregaria group. The close relations between two groups, T. longipes and T. inspinata, T. macrura and T. vicina may be explained as the neoteny type evolution.
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