Plankton Benthos Res 15(2): 146–155, 2020

Plankton & Benthos
Research© The Plankton Society of Japan

Seasonal changes in the community structure ofchaetognaths and the life cycle of the dominantchaetognath Eukrohnia hamata in the Oyashio region,
western subarctic Paciﬁc
Yoshimi Tone Matsumoto1,# & Atsushi Yamaguchi1,2,*
1

Graduate School of Fisheries Sciences, Hokkaido University, 3–1–1 Minatocho, Hakodate, Hokkaido 041–0821, Japan
Arctic Research Centre, Hokkaido University, Kita–21 Nishi–11 Kita-ku, Sapporo, Hokkaido 001–0021, Japan#
Graduated Hokkaido University in 2008, living in Sendai City
2

Received 17 August 2019; Accepted 6 March 2020 Responsible Editor: Hiroomi Miyamoto
doi: 10.3800/pbr.15.146
Abstract: The community structure and life cycle of dominant chaetognath species were studied in a 0–1000–m
water column in the Oyashio region, western subarctic Paciﬁc, six times during the year from March 2003 to February
2004. The abundance of chaetognaths ranged from 3235 to 6691 inds. m−2. Throughout the study period, ten chaetognath species belonging to nine genera were identiﬁed. Among the chaetognaths, Eukrohnia hamata accounted for
39–62% of the abundance, followed by E. hamata/Eukrohnia bathypelagica juveniles, Parasagitta elegans, and E.
bathypelagica. The species diversity (H') of chaetognaths varied between 1.12 and 1.50. For E. hamata, individuals
had a body length ranging from 2.3–23.8 mm. The abundance of juveniles ﬂuctuated from 22.0–48.3% and was higher
in June 2003. Based on cohort analysis, recruitment of E. hamata juveniles occurred from spring to summer, and theyreached a body length of 8 mm in one year. The overwintered population showed rapid growth from June to October,when they reached 15 mm in body length. Eukrohnia hamata showed little growth in winter and reproduced the subsequent spring. Thus, a two–year life cycle of E. hamata is proposed for the population in the Oyashio region. For
comparable information about the life cycle of E. hamata, an eight to ten–month generation length was reported for the
population in the eastern subarctic Paciﬁc. These regional differences in the generation length of E. hamata are attributed to differences in habitat temperature, with much higher temperatures in the eastern (3.8–6.0°C) than the western(2.3–2.9°C) subarctic Paciﬁc.
Key words: chaetognaths, Eukrohnia hamata, life cycle, mesopelagic, Oyashio region

Introduction
Chaetognaths occur only in marine environments, having adapted to pelagic, benthic, coastal, and oceanic environments. For pelagic chaetognaths, 115 species have beenreported globally (Bieri 1991), with more than 40 species
occurring in the Paciﬁc (Pierrot–Bults & Nair 1991) and32 species present in the neighbouring waters of Japan(Terazaki 1996). Since each species prefers certain thermaland haline ranges, pelagic chaetognaths are used as water–mass indicator species (Bieri 1959). For most oceans,* Corresponding author: Atsushi Yamaguchi; E-mail, a-yama@ﬁsh.
hokudai.ac.jp

the chaetognath biomass is large, following copepods, andit accounts for 10–30% of the copepod wet weight biomass(Ryther 1969, Reeve 1970). Planktonic chaetognaths arecarnivores, feeding mainly on copepods, with protozooplankton, euphausiids, amphipods, crustacean larvae, andﬁsh eggs and larvae having been reported as other food organisms (Feigenbaum 1991). Chaetognaths feed upon higher trophic levels such as ﬁsh (David 1955, Reeve 1966),
which suggests that they have an important role in thematerial transfer that connects secondary producers andhigher trophic levels.
Many studies of chaetognaths have reported the life
cycles (Hirota 1959, Murakami 1959, 1966, Nagasawa &Marumo 1972, 1977, 1978, Nagasawa 1991, Kotori 1999)

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Life cycle of the chaetognath Eukrohnia hamata

and food organisms (Nagasawa & Marumo 1972, King
1979) of neritic and epipelagic species of the family Sagittidae in the North Paciﬁc, however, ecological information
on mesopelagic species is scarce. The life cycle of mesopelagic chaetognaths has been reported in the Gulf of Alaska(Terazaki & Miller 1986) and the Oyashio region (Nishiuchi 1999). Regarding food items for mesopelagic chaetognaths, several studies have been published (Sullivan 1980,
Terazaki & Marumo 1982). Despite their numerical dominance, information on the life cycles of mesopelagic chaetognaths is scarce.
In the present study, we analysed seasonal changes in
the community structure of epi– and mesopelagic chaetognaths in the Oyashio region. The life cycle of the dominant mesopelagic chaetognath Eukrohnia hamata (Möbius,
1875), was evaluated by cohort analysis of body length andgonad maturation. These life cycle data were comparedwith those in the Gulf of Alaska, and specialized characteristics in the Oyashio region were evaluated.

chaetognaths were performed under a stereomicroscope.
For species identiﬁcation, we referred to Yamaji (1984) andTerazaki (1996).
Gonad maturation and body lengths were measured forthe most abundant species, Eukrohnia hamata. Eukrohnia
hamata is reported to be difﬁcult to distinguish from Eukrohnia bathypelagica according to phylogenetic (mtDNA)
analyses (Miyamoto et al. 2012). Notwithstanding, theabundance of E. bathypelagica was low enough in thisstudy (cf. Table 1) that we discounted. Thus, if any contamination with E. bathypelagica occurred, the effect on
the results was considered to be small, and we have treatedthe E. hamata/bathypelagica species group as E. hamata
in this study. Gonad maturation was separated into ﬁve
stages following Thomson (1947) and Terazaki & Miller(1986):
Juvenile: the ovary is not visible
Stage I: the ovary is developed
Stage II: small eggs are visible in the ovary
Stage III: some large eggs are visible in the ovary
Stage IV: all the eggs in the ovary are large
For juveniles, species identiﬁcation between E. hamata

Materials and Methods
Field sampling
Sampling was conducted at Site H (41°30′N, 145°50′E)in the Oyashio region during cruises of the T/S Oshoro–
Maru and Ushio–Maru on six occasions with two–three–month intervals from 13 March 2003 to 8 February 2004
(Fig. 1, Table 1). Zooplankton samples were collected byoblique hauls of a ﬂowmeter–incorporating Bongo net (70cm mouth diameter, 335-µm mesh size) from 0–1000-m
depths and preserved in borax–buffered 5% (v/v) formalinseawater. On each occasion, the temperature and salinity
were measured by CTD (Sea–Bird Electronics, SBE911Plus).
Sample analyses
After measuring the weights of whole samples immersed in formalin–seawater, the samples were stirred
well with the aid of a glass bar. Then, 500 ml of seawatercontaining zooplankton was injected into a jar to generate the subsample. The volumetric split ratio of such subsamples were 14.8–23.8% (Table 1). Based on the subsamples, sorting, species identiﬁcation, and counting ofFig. 1. Location of the Oyashio region in the western subarctic
Paciﬁc (A) and sampling site (Site H) in the Oyashio region (B).
Depth contours (2000, 4000, 6000 and 8000 m) are superimposedin (B).

Table 1. Sampling information for chaetognaths collected by Bongo nets (335-µm mesh) in the Oyashio region. Abbreviations of the shipʼs
names are Os: T/S Oshoro Maru, Us: T/S Ushio Maru.
Sampling date

Sampling Day/Night

Ship

Volume of water ﬁltered (m3)Subsample examined (%)

13 Mar. 2003
4 June 2003
23 Aug. 2003
5 Oct. 2003
17 Dec. 2003
8 Feb. 2004

Night
Day
Night
Day
Day
Day

Os
Os
Os
Us
Os
Os

1736
1726
1691
1323
2525
2149

23.6
12.9
16.9
21.4
22.2
14.8

148

Y. T. Matsumoto & A. Yamaguchi

Fig. 2.

Seasonal changes in vertical proﬁles of temperature (A) and salinity (B) in the Oyashio region, western subarctic Paciﬁc.

and E. bathypelagica was difﬁcult, and they were treatedas E. hamata/E. bathypelagica juveniles in this study. For
small specimens (<10 mm), body length was measured
with an eye–piece micrometre with a precision of 0.01 mm.
For large specimens (≥10 mm), body length was measured
using a slide calliper with a precision of 0.1 mm.

where t is the age (year), lt is the estimated body length
at time t (mm), K is the growth coefﬁcient, t0 is the age
when the body length is zero, and lmax is the asymptotic
size. For analysis of the von Bertalanffy equation, we used
the Microsoft Excel Solver function (Gorie 2001).
Results

Community structure
The number of chaetognath individuals (n) was converted to abundance for the 0–1000 m water–column standing
stock (N: inds. m−2) using the following equation:N=(n × D)(s × F )where D is the depth (m) to which the net was lowered, sis the split ratio of each sample, and F is the volume of ﬁltered water (m3). We applied D=1000 for all samples usedin this study.
Based on the abundance data for each sampling period,
the species diversity (H') was calculated using the following equation (Shannon & Weaver 1949):
H ′ = –  pi ln pi

where pi indicates the proportion of the ith species in totalabundance.
Life cycle of Eukrohnia hamata
For E. hamata and E. hamazta/E. bathypelagica juveniles, cohort analyses of body length histogram data were
performed using Microsoft Excel Solver (Aizawa & Takiguchi 1999). To evaluate the growth of E. hamata andE. hamata/E. bathypelagica juveniles, the body length
growth curve was expressed using the von Bertalanffy
equation:lt = lmax (1– e – K [t – t0 ] )Hydrography
Throughout the study period, the sea surface temperature ranged between 0.7°C (March 2003) and 14.5°C (August 2003), and the temperature below 250 m ranged from
2.5–3.5°C (Fig. 2A). Salinity at the sea surface ranged
from 31.4 (June 2003) to 33.5 (February 2004). Salinity increased with increasing depth and reached 34.4 at 1000 m(Fig. 2B). T–S diagrams at each sampling date are shownin Fig. 3. Based on the water mass classiﬁcation by Ohtani(1971), cold (≤3°C) and fresh (33.0–33.3) Oyashio water
was observed at depths above 200 m, except in February
2004. The effect of a warm core ring (temperature ≥4°C
and salinity ≥33.5) was detected for the surface layer in
December 2003 (0–50 m) and February 2004 (0–200 m)(Fig. 3).
Chaetognath community structure
The chaetognath standing stock in the 0–1000 m watercolumn ranged from 3235 inds.m−2 (December 2003) to6591 inds.m−2 (June 2003) (mean: 4722 inds.m−2, Table 2).
Throughout the study period, ten species belonging to nine
genera were observed: Caecosagitta macrocephala (Fowler, 1904); Eukrohnia bathypelagica Alvariño, 1962; Eukrohnia hamata (Möbius, 1875); Krohnitta subtilis (Grassi,
1881); Mesosagitta minima (Grassi, 1881); Parasagitta elegans (Verrill, 1873); Pseudosagitta lyra (Krohn, 1853); Sagitta bipunctata Quoy & Gaimard, 1827; Solidosagitta zetesios (Fowler, 1905); Zonosagitta nagae (Alvariño, 1967).

Life cycle of the chaetognath Eukrohnia hamata

149

Fig. 3. Seasonal changes in the T–S diagram in the Oyashio region. T–S ranges of Oyashio (OY) and the Warm Core Ring (WCR) arepresented as shaded rectangles (cf. Ohtani 1971).

Eukrohnia hamata dominated throughout the year (39–
62% of the abundance), followed by E. hamata/E. bathypelagica juveniles, P. elegans, and E. bathypelagica. The
above three species composed 83–99% of the chaetognathabundance (Table 2). For each sampling date, ﬁve to seven chaetognath species were observed. For June 2003, K.
subtilis and S. bipunctata were detected. From December
2003 to February 2004, M. minima, S. zetesios, and Z.
nagae were identiﬁed. The species diversity (H') ranged
from 1.12 (October 2003) to 1.50 (December 2003) (Table
2).
Population structure of Eukrohnia hamata
For the dominant chaetognaths, the population structureof Eukrohnia hamata and E. hamata/Eukrohnia bathypelagica juveniles was analysed. The abundance of E. hamata and E. hamata/E. bathypelagica juveniles ranged from

2314 inds.m−2 (December 2003) to 5006 inds.m−2 (June2003). Body length ranged from 2.3 to 23.8 mm (Fig. 4).
Within the population, E. hamata/E. bathypelagica juveniles and stage I dominated for more than 60% of the year
(Fig. 5). The composition of juveniles was high (45–48%)
in March and June 2003 and low (22–25%) in October andDecember 2003. Reproductive individuals (stage IV) wereabundant (3.3%) in February 2004 and did not occur inAugust 2003.
On each sampling date, two to three cohorts were observed for E. hamata (Fig. 6). The mean body length ofeach cohort varied from 4.7 mm to 15.6 mm. The cohorts
with both the smallest and largest mean body length wereobserved in June 2003. The means and standard deviationsof each cohort were determined using a time course beginning with the smallest cohort in June (Fig. 7). Based on
this analysis, one cohort was found to reach a body length

150

Y. T. Matsumoto & A. Yamaguchi

Table 2. Abundance (inds. m−2) and species composition (%) of chaetognaths in the Oyashio region (0–1000 m), western subarctic Paciﬁc.
The species diversity index (H′) is also calculated.
2003
Species

13 Mar.

Eukrohniidae
Eukrohnia bathypelagica
Eukrohnia hamata
E. hamata/E. bathypelagica
juveniles
Eukrohnia spp.
Sagittidae
Caecosagitta macrocephala
Krohnitta subtilis
Mesosagitta minima
Parasagitta elegans
Pseudosagitta lyra
Sagitta bipunctata
Solidosagitta zetesios
Zonosagitta nagae
Sagittidae spp.
Unidentiﬁed
Total

4 June2004

23 Aug.

5 Oct.

17 Dec.

8 Feb.

inds.m−2%

inds.m−2%

inds.m−2%

inds.m−2%

inds.m−2%

inds.m−2

%

365
2063
1682

7.8
44.3
36.1

383
2586
2414

5.8
39.2
36.6

299
1941
1300

7.2
46.9
31.4

220
2595
731

5.2
61.9
17.4

144
1738
575

4.5
53.7
17.8

282
3090
1616

5.1
56.1
29.3

10

0.2

̶

̶

̶

̶

̶

̶

4

0.1

6

0.1

5
̶
̶
197
5
̶
̶
̶
2
334

0.1
̶
̶
4.2
0.1
̶
̶
̶
0.1
7.2

̶

10
̶
̶
577
3
̶
̶
̶
7
3

0.3
̶
̶
13.9
0.1
̶
̶
̶
0.2
0.1

̶
̶
̶
576
7
̶
̶
22
11
32

̶
̶
̶
13.7
0.2
̶
̶
0.5
0.3
0.8

4
̶
274
209
70
̶
̶
83
5
130

0.1
̶
8.5
6.5
2.2
̶
̶
2.6
0.2
4.1

9

̶
1154
36
4
̶
̶
̶
9

̶
0.1
̶
17.5
0.5
0.1
̶
̶
̶
0.1

3
127
115
̶
3
̶
̶
257

0.2
̶
0.1
2.3
2.1
̶
0.1
̶
̶
4.7

4662

100.0

6591

100.0

4142

100.0

4193

100.0

3235

100.0

5509

100.0

Number of species
Diversity index (H')

5
1.28

4

6
1.25

of 15 mm in two years. These ﬁndings indicated that E.
hamata had a two–year generation length in the Oyashio
region. Growth in body length through time can be expressed by the von Bertalanffy equation:

lt = 34.8 (1– e – 0.25[t + 0.37 ] )where t is time (year) and lt is the body length (mm) at timet. The mean body length in the cohort was larger than the
von Bertalanffy equation during summer to autumn, while
it was smaller than the von Bertalanffy equation during
winter to spring (Fig. 7).
Discussion
Chaetognath community structure
The number of species was lower in our study (10 species) than in a previous study in the Oyashio region (15species; Nishiuchi 1999). A deep–sea species, Eukrohnia
fowleri (Ritter–Záhony, 1909), and four warm–water species, Aidanosagitta regularis (Aida, 1897), Decipisagitta
decipiens (Fowler, 1905), Flaccisagitta enﬂata (Grassi,
1881) and Serratosagitta pseudoserratodentata (Tokioka,
1939), were reported by Nishiuchi (1999) but did not occur
in our study. While the sampling frequency employed by
Nishiuchi (1999) was high (ﬁfteen times per year), these

5
1.22

5
1.12

7
1.50

̶

7
1.17

warm–water species were collected on only one or two
occasions. They were considered to be transported by awarm core ring (Terazaki 1992, Johnson & Terazaki 2003).
Compared with Nishiuchi (1999), the low frequency ofsampling (six vs ﬁfteen times per year) in this study might
have prevented the collection of these rare warm–water
species.
Ozawa et al. (2007) reported that the species diversity
varied greatly between the subarctic and the subtropicalregions; the species diversity was lower (H′=1.22–1.38)
in the subarctic region and higher (H′=2.17–2.35) in thesubtropical region. The regional pattern corresponded wellwith that known on larger geographical scales (e.g. Indo–
Paciﬁc region) (Miyamoto et al. 2014). The species diversity in this study (H′=1.12–1.50; species number 5–7) corresponded well to the values in the subarctic region reported
by Ozawa et al. (2007).
With regards to seasonal changes in the chaetognathcommunity structure in the Oyashio region, interestingly,
the timing of warm–water species occurrence varied withthe species. Thus, Krohnitta subtilis and Sagitta bipunctata were observed in June 2003, while Mesosagitta minima, Solidosagitta zetesios, and Zonosagitta nagae wereabundant from December 2003 to February 2004 (Table 2).
These periods (June, December 2003, and February 2004)
corresponded with the timing of the occurrence of theLife cycle of the chaetognath Eukrohnia hamata

Fig. 4. Body length distribution and maturity stage compositionof Eukrohnia hamata (Stage I–IV) and E. hamata/E. bathypelagica
juveniles in the Oyashio region (0–1000 m), western subarctic
Paciﬁc.

151

Fig. 6. Seasonal changes in the cohort composition of Eukrohnia hamata in the Oyashio region, western subarctic Paciﬁc. G0,
G1, and G2 represent the cohort aged +0, +1 and +2 years old.
Triangles indicate the peak length of each cohort.

warm–core ring observed in the T–S diagram (cf. Fig. 3).
Life cycle of Eukrohnia hamata

Fig. 5. Seasonal changes in maturity stage composition of Eukrohnia hamata, including E. hamata/E. bathypelagica juveniles, in
the Oyashio region, western subarctic Paciﬁc.

Eukrohnia hamata, the dominant chaetognath in the
Oyashio region, is distributed worldwide in oceanic regions and is abundant and distributed in the near–surface
layer in high–latitude oceans (Froneman & Pakhomov
1998, Kruse et al. 2010, Kosobokova & Isachenko 2017)
but scarce in the meso– and bathypelagic layers of low–latitude oceans (Alvariño 1965). The life cycle of E. hamata
varied among the regions: a two–year generation length

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Y. T. Matsumoto & A. Yamaguchi

Fig. 7. Estimated growth curve of Eukrohnia hamata in the
Oyashio region, western subarctic Paciﬁc. Dots and bars indicate
means and standard deviations of body length, respectively. Symbols of G0, G1, and G2 (+0, +1 and +2 years old) correspond withthose in Fig. 6. The solid line denotes the ﬁtted von Bertalanffyʼs
growth curve.

in a Nordic fjord (Korsfjorden), Barents Sea, Bafﬁn Bay,and Beaufort Sea (Bogorov 1940, Sands 1980, Sameoto
1987, Grigor et al. 2017), eight to ten months with three
discrete generations in a year in the Gulf of Alaska (Terazaki & Miller 1986), and a four–year generation length in
the Oyashio region (Nishiuchi 1999). Our study showed a
two–year generation length with reproduction occurring
during October to March (Fig. 4).
For the studies in the Oyashio region, the ﬁltered volumes in our study were higher (1,323–2,525 m3) comparedto the other study (4–432 m3; Nishiuchi 1999). This difference was due to differences in the sampling methods,
where Nishiuchi (1999) sampled much deeper (0–2000 m),
owing to employing vertically stratiﬁed hauls of a smallermouth ring (60-cm) and a smaller mesh size (100 µm). In
this study, the sampling depth was shallow (0–1000 m),
and samples were collected by oblique hauls of large
mouth rings (70-cm) with a large mesh size (335 µm).
Thus, because of the large ﬁltered volumes, the numberof chaetognath individuals collected was much greater inthis study. Because the number of collected individuals
was low, Nishiuchi (1999) combined data into two periods;
December–May and June–October, and based analyses onthese two periods, thereby assessing the life cycle of E.
hamata. Conversely, since a sufﬁcient number of individuals was collected at each sampling date during the present
study, a cohort analysis could be performed at each sampling date with two–month intervals throughout the year.
As mentioned in Materials and methods, distinguishing
the two species (E. hamata and E. bathypelagica) is difﬁcult for juveniles without a visible ovary. In a molecularanalysis, both E. hamata and E. bathypelagica are classiﬁed into a single–species group (Miyamoto et al. 2012).
From the perspective of phylogenetic distance, regional
differences in E. hamata have been detected, with differences between E. hamata and E. bathypelagica within aregion therefore being inevitable (Miyamoto et al. 2012,
Kulagin et al. 2014). These ﬁndings suggest that combining E. hamata/E. bathypelagica juveniles may be allowable for life cycle analysis of the single–species group of

Fig. 8. Growth curves of Eukrohnia hamata in the Gulf of
Alaska (Terazaki & Miller 1986) (A) and in the Oyashio region(B). Note that three discrete cohorts (shown as a, b–b′ and c–c′) are
observed over the year in the Gulf of Alaska while only one cohort
occurred in the Oyashio region.

E. hamata/E. bathypelagica.
Both the ﬁndings of Nishiuchi (1999) and the present
study deﬁne E. hamata/E. bathypelagica juveniles. The
vertical distribution of E. bathypelagica is known to bemuch deeper than that of E. hamata (distribution centre
ranging from 583–1500 m for the former and 169–308 m
for the latter, Ozawa et al. 2007). Since the sampling depthof Nishiuchi (1999) is much deeper than in this study(0–2000 m vs 0–1000 m), the contamination effect of E.
bathypelagica may have been greater in his study. The
smaller mesh size of Nishiuchi (1999) than in the present
study (100 µm vs 335 µm) may have induced a much larger
proportion of smaller E. hamata/E. bathypelagica juveniles. These features (deeper sampling depth with smaller
mesh sizes) suggest a much more severe contamination effect due to the presence of E. bathypelagica juveniles, withregards to the life cycle analysis of E. hamata conductedby Nishiuchi (1999).
The generation lengths reported in the Gulf of Alaska(eight to ten months, Terazaki & Miller 1986) were much
shorter than those in the Oyashio region (two years; 24months) (Fig. 8). Comparative studies on zooplankton life
cycles in the Gulf of Alaska and the Oyashio region have

153

Life cycle of the chaetognath Eukrohnia hamata

been conducted mainly only for the dominant calanoid copepods. The generation length is longer in the Oyashio
region for Metridia paciﬁca Brodsky, 1950 (Padmavati etal. 2004), which has been attributed to the habitat temperature. The habitat temperature for E. hamata ranges from
3.8–6.0°C at 100–500 m in the Gulf of Alaska (Miller etal. 1984), while it was 2.3–2.9°C at 250 m in the Oyashio
region (Fig. 2). Thus, a colder habitat temperature is observed for E. hamata in the Oyashio region.
A negative relationship between generation length andhabitat temperature is well known for the epipelagic chaetognath Parasagitta elegans. According to Sameoto (1971),
the generation length (GL: day) is expressed by the habitat
temperature (T: °C):

GL = 611.51/ T 0.811.
As an example, inserting the habitat temperatures of E.
hamata in the Gulf of Alaska and Oyashio region (5°C
and 2.6°C) into the above equation, the generation lengthis predicted to be 166 days (Gulf of Alaska) and 282 days
(Oyashio region), respectively. The difference in generation lengths of 1.7 times (=282/166) may be usable as a
guide to extrapolate differences in the generation lengthof E. hamata in the two different locations, although theequation was derived for P. elegans.
The life cycle of E. hamata has been studied in several locations. A two–year generation length has been reported for E. hamata in a Nordic fjord (Korsfjorden), theBarents Sea, Bafﬁn Bay, and the Beaufort Sea (Bogorov
1940, Sands 1980, Sameoto 1987, Grigor et al. 2017). The
correspondence in the generation length of E. hamata in
the Oyashio region with these locations may be due to thecorrespondence in habitat temperature (2–3°C) with those
high–latitude oceans (Sameoto 1987).
Interestingly, E. hamata growth exhibits a clear seasonalpattern, i.e., their growth is much greater (above the vonBertalanffy curve) during summer to autumn, while it ispoor (below the von Bertalanffy curve) during winter tospring (Fig. 7). The seasonal phenology of E. hamata issummarized as follows. The high proportion of juveniles
that formed a new generation (G0) occurred from March
to August. The matured population was found in G1 from
October to March, suggesting that reproduction occurred
mainly in this season. This gap between the occurrenceof G0 and the reproductive season was caused by the sampling efﬁciency of the 0.335 mm mesh net. Nagasawa &Marumo (1977) showed that small individuals (less than
5 mm in body length) were underestimated in numericalabundance by using 0.335 mm mesh net sampling. In one
year, the G0 reached a body length of ca. 8 mm (Fig. 6).
During winter to spring, the growth of G0 was slow andpoor, and negative growth might have occurred between
January and May (Fig. 7). From June to October, and during the next summer, E. hamata showed rapid growth,reaching a body length of ca. 15 mm, followed by slow
and poor growth in the subsequent winter (Fig. 7). Thus,

the two-year population (G1) may enter reproduction from
October to March (Fig. 4). Some remains occurred as G2 inMarch and June (Fig. 6).
Temperature has been reported as an environmental factor affecting the growth and generation length of chaetognaths (Sameoto 1971). However, the habitat temperatureof E. hamata (2.3–2.9°C) is mostly stable throughout theyear. These ﬁndings suggest that other environmental factors such as food may have important roles in governing
the seasonal growth rate. In regards to the prey of E. hamata, the small calanoid copepod Pseudocalanus minutus(Krøyer, 1845) may be important. Pseudocalanus minutus
was the only species detected in the gut contents of E. hamata in this study (Matsumoto, unpublished data). Since P.
minutus undergo diapause at deeper depths around 250 m,
and their descent occurs during summer to autumn (Yamaguchi & Shiga 1997), the greater growth of E. hamata thanthe von Bertalanffy curve during summer to autumn might
be a reﬂection of the abundant availability of food (prey)
for this species during that season.
Acknowledgements
We thank the captains, ofﬁcers, crews, and researchers onboard the T/S Oshoro–Maru and T/S Ushio–Maru,
Hokkaido University for their great efforts during the ﬁeld
sampling. Part of this study was supported by a Grant–in–
Aid for Scientiﬁc Research 19H03037 (B) and 17H01483(A) from the Japan Society for the Promotion of Science(JSPS). This work was partially conducted for the Arctic
Challenge for Sustainability (ArCS) project.
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