Bull. Natn. Sci. Mus., Tokyo, Ser. B, 30(2), pp. 35–54, June 22, 2004

Taxonomy, Ultrastructure, and Biogeography of the
Aulacoseira subarctica Species Complex
Akihiro Tuji1 and Akiko Houki2
1

Department of Botany, National Science Museum, Tokyo, 4–1–1, Amakubo, Tsukuba, 305–0005, Japan
E-mail: tuji@kahaku.go.jp
2
Faculty of Science, Nara Women’s University, Nara, 630–8506, Japan

Abstract We examined the A. subarctica species complex using type materials. The type specimen of A. subarctica lacks areolae on the valve face, and we recognized individuals with areolaeon the valve face as another taxon, Aulacoseira pusilla (F.Meister) Tuji et Houki comb. nov. We examined two endemic Japanese Aulacoseira taxa, which are very similar to A. subarctica, and transferred both to varieties of A. subarctica because of their similarity. We also designated several lectotypes for the A. subarctica species complex and A. islandica species complex.
Key words : Aulacoseira pusilla (F.Meister) Tuji et Houki comb. nov., Melosira pusilla F.Meister, Aulacoseira subarctica var. logispina (Hust.) Tuji et Houki stat. nov. et comb. nov., Melosira
logispina Hust., Aulacoseira subarctica var. tenuis (Hust.) Tuji et Houki comb. nov., Melosira
longispina var. tenuis Hust., lectotype.

Introduction
Aulacoseira subarctica (O.Müll.) E.Y.Haw.
is a very common plankton species (Lund 1971,
Amblard and Bourdier 1990) and its taxonomy,
biogeography, and ecology have been well studied (Gibson et al. 2003). However, a type examination of this species has not been done and therelationships between this species and similar
taxa from Japanese lakes have not been examined. Therefore, we examined type specimens of
the A. subarctica complex using light (LM) andscanning electron microscopy (SEM), and discuss the taxonomy of the A. subarctica complex.
Materials and Methods
We examined the following material:
1. A slide labeled “plankton. Heidi, Sud-Island. 28. VIII. 1901” (S)2. A slide labeled “Thingvallavatn. 23. I.
1903” and “o.m” (S)3. A slide labeled “Thingvallavatn. 31. III.
1903” and “o.m” (S)4. A slide labeled “Thingvallavatn. 30. 6.

1903” (S)The information for these three slides concurs
with the sampling date and place in the description in Müller (1906), and these slides should be
the syntypes for the six new taxa described in
Müller (1906). We could not find any other slideor raw material for Muller (1906).
5. Raw material numbered 54 for Foged(1974), sampled from moss and stone atThingvallavatn, South-west Island, Iceland,
dated “1954. 16/7” in the Foged collection(C)
We used this raw material as the material fromtype locality for the new taxa described byMüller (1906) for SEM observations.
6. A slide numbered “A2/21” in the Hustedt
collection (BRM)
7. Raw material numbered “As163” in the
Hustedt collection (BRM)
This slide and a raw material should be used
for the new description of Melosira longispina
Hust. (Simonsen 1987).
8. A slide numbered “A2/22” in the Hustedt
collection (BRM)
9. Raw material numbered “As162” in the

36

Akihiro Tuji and Akiko Houki

Hustedt collection (BRM)
This slide and a raw material should be used
for the new description of Melosira longispina
var. tenuis Hust. (Simonsen 1987).
10. A slide numbered “A3/61” with Meister’slabel in the Hustedt collection (BRM)
11. A slide numbered “A3/30” with Hustedt’slabel in the Hustedt collection (BRM)
These slides are labeled “M. pusilla” and aremarked as one individual. These slides should be
used for the new description of Melosira pusilla
F.Meister by Meister.
12. Raw material sampled from Katata Lagoon,
Shiga, Japan by A. Tuji in 1998/11/10(TNS)
13. Raw material sampled from Katata Lagoon,
Shiga, Japan by A. Tuji in 2000/3/18 (TNS)
14. Raw material sampled from Lake Biwa,
Shiga, Japan by A. Tuji in 1999/6/4 (TNS)The specimens examined are kept in the
Botany Department of the Swedish Museum of
Natural History (S), the Botanical Museum andLibrary of the University of Copenhagen (C), theFriedrich Hustedt Study Centre for Diatoms, theAlfred Wegener Institut fur Polar- und Meeresforschung (BRM), and the Department of Botany,
National Science Museum, Tokyo (TNS).
Results & Discussion
Müller (1906) described three taxa of Aulacoseira islandica (Melosira islandica f. recta, M.
islandica f. curvata, and M. islandica f.
helvetica) and three A. subarctica taxa (M. italicassp. subarctica f. recta, M. italica ssp. subarctica
f. curvata, and M. italica ssp. subarctica f.
tenuis). A. islandica is easily distinguished fromother Aulacoseira species using the character of
the areolae, the coarse density of the rows, andthe pattern of straight rows parallel to the pervalvar axis (Figs 15–22, 36–39, 40).
Taxonomy of A. islandica
Müller (1906) described three Aulacoseira islandica group taxa: Melosira islandica f. recta,
M. islandica f. curvata, and M. islandica f. hel-vetica. These three taxa were characterized usingcolony form and cell diameter (Müller 1906).
These three types of individuals were seen insyntype slides and we designated the lectotypes
for these taxon.
Melosira islandica f. recta O.Müll., Jahrb. f.
wiss. Bot. 43: 56. pl. I, f. 3. 1906.
Lectotype (designated here): A slide labeled“Thingvallavatn. 30. 6. 1903” and “o.m” (S).
(Figs 38, 39)
Melosira islandica f. curvata O.Müll., Jahrb. f.
wiss. Bot. 43: 56. pl. I, f. 1–2. 1906.
Lectotype (designated here): A slide labeled“Thingvallavatn. 31. III. 1903” and “o.m” (S).
(Fig. 22)
Melosira islandica subsp. helvetica O.Müll.,Jahrb. f. wiss. Bot. 43: 67. pl. I, f. 8–9 (7, 10,11?). 1906.
Lectotype (designated here): A slide labeled“Thingvallavatn. 30. 6. 1903” and “o.m” (S).
(Figs 36, 37)
The morphological variation within one taxon
is thought to be very large in Aulacoseiraspecies, and the morphological variation within
A. islandica described in Müller (1906) should
represent the variation within one taxon.
Aulacoseira islandica (O.Müll.) Simonsen, Bacill.
2: 60. 1979.
(Figs 15–22, 36–39, 40, 45–51)
Basionym: Melosira islandica f. recta O.
Müll., Jahrb. f. wiss. Bot. 43: 56. pl. I, f. 3–6.
1906.
Synonym: Melosira islandica f. curvata O.
Müll., Jahrb. f. wiss. Bot. 43: 56. pl. I, f. 1. 1906.
Synonym: Melosira islandica subsp. helvetica O. Müll., Jahrb. f. wiss. Bot. 43: 67. pl. I, f.
8–9. 1906.
Type locality: Thingvallavatn, Iceland.
Valves 7–27 m m in diameter, mantle height
4–21 m m. Valves usually longer than broad. The
circular form areolae on the mantle always run instraight rows parallel to the pervalvar axis with
11–12 pervalvar striae/10 m m and 12–13 areolae/10 m m. Linking spine sector form. Sulcus exists. Several rimoportulae exist off sulcus andAulacoseira subarctica species complex

sometimes near valve face. This species should
not belong to the A. subarctica complex.
Taxonomy of A. subarctica
Müller (1906) described three A. subarctica
taxa: M. italica ssp. subarctica f. recta, M. italicassp. subarctica f. curvata, and M. italica ssp.
subarctica f. tenuis. These three taxa were characterized using colony form and cell diameter.
These three types of individuals were seen insyntype slides and we designated the lectotype
for these taxa. We also examined the ultrastructure of these taxa using the material from thetype locality (Thingvallavatn, Iceland).
Melosira italica subsp. subarctica f. recta O.
Müll., Jahrb. f. wiss. Bot. 43: 79. pl. II, f. 7.
1906.
Lectotype (designated here): A slide labeled“Thingvallavatn. 31. III. 1903” and “o.m” (S).
(Figs 1–2)
Melosira italica subsp. subarctica f. curvata
O.Müll., Jahrb. f. wiss. Bot. 43: 79. pl. II, f. 8.
1906.
Lectotype (designated here): A slide labeled“Thingvallavatn. 31. III. 1903” and “o.m” (S).
(Fig. 14)
Melosira italica subsp. subarctica f. tenuis O.
Müll., Jahrb. f. wiss. Bot. 43. pl. II, f. 9. 1906.
Lectotype (designated here): A slide labeled“Thingvallavatn. 31. III. 1903” and “o.m” (S).
(Fig. 9)The morphological variation within one taxon
is thought to be very large for Aulacoseiraspecies and this morphological variation within
A. subarctica is likely the variation seen within
one taxon.
Aulacoseira subarctica (O.Müll.) E.Y.Haw., Dia.
Res. 5: 195. 1988.
(Figs 1–14, 23–35, 41–44, 52–54)
Basionym: Melosira italica subsp. subarctica f. recta O.Müll., Jahrb. f. wiss. Bot. 43: 79. pl.
II, f. 7. 1906.
Synonym: Melosira italica subsp. subarctica
f. curvata O.Müll., Jahrb. f. wiss. Bot. 43: 79. pl.
II, f. 8. 1906.
Synonym: Melosira italica subsp. subarctica

37

f. tenuis O.Müll., Jahrb. f. wiss. Bot. 43. pl. II, f.
9. 1906.
Type locality: Thingvallavatn, Iceland.
Valves 3–10 m m in diameter, mantle height 5–
14 m m. Valves usually longer than broad (Figs 1–
14, 23–35, 41–44, 53–54). The circular areolaeon the mantle run in a spiral with 16–20 pervalvar striae/10 m m and 22–24 areolae/10 m m (Figs
53–54). Valve face areolae do not exist (Fig. 53).
Two rows per simple spine (Figs 53–54). Linking
spine does not exist. Sulcus exists. Several rimoportulae exist off the sulcus (Figs 5–8, 14, 23–24,
28, 41, 43–44) and sometimes near the valve face(Figs 9, 42).
The morphological variation within thisspecies is thought to be very large. The length/
width ratio and density of striae could changewith environmental conditions (Kilham et al.
1986). Several studies agree with this hypothesis(Edgar & Theriot 2003).
Haworth (1988, 1990) transferred this taxon to
the genus Aulacoseira using samples from Cambrian waters. She included two morphologicaltypes in A. subarctica: one lacks areolae on thevalve face and usually is not wider than long(Type A) and the other has areolae on the valve
face and is markedly wider than long (Type B).
Only Type A individuals are found in the syntype
from Iceland in this study (Figs 1–14, 23–35,
41–44). Type B is found widely in Japan (Kawashima & Kobayasi 1993, Tuji and Houki 2001;
figs 69–105), and Type A is not observed in thesame waters. Type A is observed from Lake
Shikotsu, Hokkaido, Japan (Tuji personal observation), while we could not find Type B in thislake. Therefore, there are ecological differences
between the two types. Furthermore, several
morphological differences in the rimoportulae
have been observed in the two types. In Type B,
rimoportulae are observed only on the sulcus
using SEM (Figs 103, 104) and three to five rimoportulae exist per a valve. Unfortunately, we
have not succeeded in observing the rimoportulae of Type A directly using SEM and the material from type locality. However, shadows of the rimoportulae are observed using LM. Some exist

38

Akihiro Tuji and Akiko Houki

near, but not on, the sulcus (Figs 5–8, 14, 23–24,
28, 41, 43–44), and some are distant from thesulcus (Figs 9, 42). Likhoshway & Crawford(2001) observed the rimoportulae of Type A directly using SEM and reported that two rimoportulae occur near, but on, the ringleist per a valve.
Although the outline of frustules has large variation, the position of the rimoportulae relative tothe sulcus is very stable in the same locality; theposition of the rimoportulae should be very useful for the taxonomy of Aulacoseira species.
Likhoshway & Crawford (2001) also show theimportance of rimoportula for the taxonomy of
Aulacoseira species. Therefore, Type B is another taxon of Type A, which is A. subarctica.
Meister (1913) described Melosira pusillafrom Lake Suwa, Japan. The iconotype of thisspecies is similar to Type B of the A. subarctica
complex (Tuji & Houki 2001). Two marked specimens labeled “Melosira pusilla” are found in the
BRM, and should be the slides upon which thattype illustrations are based. These specimens
were mounted with air, and are difficult to observe in detail (Figs 55–68). Unfortunately, we
could not find this taxon from the type locality.
However, this species is found in many lakes andponds in Japan, and we examined materials from
Lake Biwa and its lagoon (Katata Lagoon) for
this taxon (Figs 69–105). From our examination,
we made a new combination for this taxon.
Aulacoseira pusilla (F.Meister) Tuji et Houki
comb. nov.
(Figs 53–105)
Basionym: Melosira pusilla F.Meister, Arch.
Hydrobiol. 8: 306. pl. IV, f. 2. 1913.
Lectotype (designated here): A slide numbered“A3/619” with Meister’s label (BRM).
Type locality: Lake Suwa, Nagano Prefecture, Japan.
Valves 6–10 m m in diameter, mantle height
2–10 m m. The circular areolae on the mantle run
in a spiral with 20–26 pervalvar striae/10 m m and
25–30 areolae/10 m m (Figs 69–99). Valve faceareolae exist (Figs 100–102, 105). Two striae persimple spine (Figs 100–101, 105). Linking spine
does not exist. Sulcus exists (Figs 103–104). Several rimoportulae exist on the sulcus and notaway from the sulcus (Figs 103–104).
We also examined two taxa that are very similar to A. subarctica and Japanese endemic
species: A. longispina (Hust.) Simonsen var.
longispina and A. longispina var. tenuis (Hust.)
Simonsen. Hustedt described both taxa from
Lake Cyuzenji and Lake Yuno in HuberPestalozzi (1942). The water of L. Yuno flows
into L. Cyuzenji via the Yukawa River, and bothlakes can be thought of as the same body ofwater. Simonsen (1987) examined the slides
upon which that type illustrations are based, andpresented photographs of these specimens withthe description of “holotype”. However, Hustedt
described only the information for type locality(“Japan, Chuzenij-See” and “Japan, Juno-See”)
and did not cite any specimen. In this case, these
specimens can not be holotype (International
Code of Botanical Nomenclature (Saint LouisCode): ICBN, Art 37.3 Note 1), and they can beconsidered lectotype (ICBN Art 9.8) designatedby Simonsen (1987). Although, both lectoypeslides include several taxa. these are very purespecimens, and do not cause a problem for thetaxonomy of either taxa. Although, both species
are very similar under LM, the rimoportulae
form differs markedly in the two taxa. The rimoportula of A. longispina var. longispina is very
simple and exists on the sulcus (Figs 128-129),
while the rimoportula of A. longispina var. tenuis
has a very complex structure. The rimoportulae
are away from the sulcus and exist on a hollow
nodule structure (Figs 159–161). These structures can be seen in LM photographs (Figs 131–
133, 138–156) and observed from photographsof lectotype specimens (Simonsen 1987). Thishollow nodule is not known from other Aulacoseira species. Although both taxa are very similar
to A. subarctica, the long spines and coarse density of the striae differ. These differences may begeological variation and should be at the variety
level.
Aulacoseira subarctica var. longispina (Hust.)
Tuji et Houki stat. nov. et comb. nov.
(Figs 100–130)
Basionym: Melosira longispina Hust. inAulacoseira subarctica species complex

Huber-Pestalozzi, Phytopl. Susw. 2: 388. pl. 115,
f. 469a. 1942.
Lectotype (designated in Simonsen 1987):
A/2/21 in the Hustedt collection (BRM), figs.
460/1–3 in Simonsen (1987).
Type locality: Lake Cyuzenji, Tochigi Prefecture, Japan.
Valves 5–14 m m in diameter, mantle height
12–15 m m (Figs 100–125). The circular areolaeon the mantle run in a spiral with 10–12 pervalvar striae/10 m m and 11–14 areolae/10 m m (Fig.
130). Valve face areolae exist (Fig. 127). Two
striae per simple spine (Fig. 130). Linking spine
does not exist. Sulcus exists. Several rimoportulae exist on the sulcus and not away from the sulcus (Figs 128–129).
Aulacoseira subarctica var. tenuis (Hust.) Tuji et
Houki comb. nov.
(Figs 131–161)
Basionym: Melosira longispina var. tenuis
Hust. in Huber-Pestalozzi, Phytopl. Susw. 2: 339.
pl. 115, f. 469b. 1942.
Lectotype (designated in Simonsen 1987):
A/2/22 in the Hustedt collection (BRM), Figs.
460/4, 5 in Simonsen (1987).
Type locality: Lake Yuno, Tochigi Prefecture, Japan.
Valves 4–6 m m in diameter, mantle height
15–18 m m (Figs 131–156). The circular areolaeon the mantle run in a spiral with 12–16 pervalvar striae/10 m m and 15–20 areolae/10 m m (Figs
157–160). Valve face areolae exist. Two striae
per simple spine (Figs 157–158). Linking spine
does not exist. Sulcus exists (Figs 159–160). Several rimoportulae exist away from the sulcus(Figs 160–161).
Conclusions
For Aulacoseira species, the morphologicalvariation within one taxon is very large, andthere are few taxonomic characters, making it
difficult to distinguish Aulacoseira species. The
morphological characters of the rimoportulae,such as its position relative to the sulcus, number,and form, are very useful for the taxonomy of
Aulacoseira species (Likhoshway and Crawford

392001). For direct observation of therimoportulae, it is very important to use SEM
and broken frustules. However, it is sometimes
difficult to find type material and to break thefrustules for observation.
In this study, we could not observe the rimoportulae of A. subarctica directly and did not observe North American and European specimens,
except from Iceland. We plan to study the taxonomy of the A. subarctica complex further.
Acknowledgments
We extend our thanks to Dr. Anders Tehler and
Dr. Lars Hedenäs of the Swedish Museum of
Natural History (S), Dr. Ruth Nielsen of the
Botanical Museum and Library, University of
Copenhagen (C), and Dr. Richard Crawford and
Dr. Friedel Hinz of the Friedrich Hustedt Study
Centre for Diatoms, Alfred Wegener Institut fürPolar- und Meeresforschung (BRM) for help intheir herbarium and the loan of specimens. We
are also grateful to the Dr. Richard Crawford forthe kind advice on this paper.
References
Amblard, C. & G. Bourdier, 1990. The spring bloom of
the diatom Melosira italica ssp. subarctica in Lake
Pavin (Massif Central, France): Biochemical, energetic
and metabolic aspects during sedimentation. J. Plankt.
Res. 12(3): 645-660.
Edgar, S. M. & E. C. Theriot, 2003. Heritability of areolar
characters in Aulacoseira subarctica (Bacillariophyta).
J. Phycol. 39: 1057–1066.
Foged, N., 1974. Freshwater diatoms in Iceland. Bibl.
Phycol. 15: 1–118, 1–36 pls.
Gibson, C. E., J. N. Anderson and E. Y. Haworth. Aulacoseira subarctica: taxonomy, physiology, ecology andpalaeocology. Europ. J. Phycol. 38: 83–101.
Haworth, E. Y., 1990. Diatom name validation. Dia. Res.
5: 195–196.
Haworth, E. Y., 1988. Distribution of Diatom taxa of the
Old Genus Melosira (now mainly Aulacoseira) in Cumbrian waters in Algae and the aquatic environment.
Eds. F. E. Round: 138–167.
Huber-Pestalozzi, G., 1942. Das Phytoplankton desSuswassers. Die Binnengewasser. E. Schweizerbart’sche
Verlagsbuchandlung, Stuttgart. 16(2): 367–549, pls.
108–178.

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Akihiro Tuji and Akiko Houki

Kawashima, A. & H. Kobayasi. 1993. Diatoms from
Akan-ko (Lake Akan) in Hokkaido, Japan. l. Centric
Diatoms. Nat. Envir. Sci. Res. 6: 41–58.
Kilham, P., S. S. Kilham, et al. 1986. Hypothesized resourc relationships among African planktonic diatoms.
Limnol. Oceanogr. 31: 1169–1181.
Likhoshway, Y. V. & R. M. Crawford, 2001. The rimportula-a neglected feature in the systematics of Aulacoseira. 16th Intenational Diatom Symposium. 33–47.
Lund, J. W. G., 1971. An artificial alteration of the seasonal cycle of the plankton diatom Melosira italica subsp.

subarctica in an English lake. J. Ecol. 59: 521–533.
Meister, F., 1913. Beitrage zur Bacillariaceenflora Japan.
Arch. f. Hydrobiol. Plankt. 8: 305–312.
Müller, O., 1906. Pleomorphismus Auxosporen undDauersporen bei Melosira-Arten. Jahrb. f. wissensch.
Botanik. 43: 49–88, 2 pls.
Simonsen, R., 1987. Atlas and Catalogue of the Diatom
Types of Friedrich Hustedt. J. Cramer, Berlin &Stuttgart, 1: 525 pp., 2: 597 pp, 3: 619 pp.
Tuji, A. & A. Houki, 2001. Centric Diatoms in Lake
Biwa. Lake Biwa Study Monographs, 7, 1–90.

Aulacoseira subarctica species complex

1–14. Aulacoseira subarctica (O.Müll.) E.Y.Haw.
1–14. A slide labeled “Thingvallavatn. 31. III. 1903” and “o.m” (S).
1–14. Bar10 m m. 1, 2. LM (normal light microscopy image). 3–14. DIC (differential interference contrast
image). Arrowrimoportula.
1, 2. Lectotype. Melosira italica subsp. subarctica f. recta O.Müll.
9. Lectotype. Melosira italica subsp. subarctica f. tenuis O.Müll.
14. Lectotype. Melosira italica subsp. subarctica f. curvata O.Müll.

41

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Akihiro Tuji and Akiko Houki

15–22. Aulacoseira islandica (O.Müll.) Simonsen. A slide labeled “Thingvallavatn. 31. III. 1903” and “o.m” (S).
15–22. DIC, bar10 m m. Arrowrimoportula.
22. Lectotype. Melosira islandica f. curvata O.Müll.

Aulacoseira subarctica species complex

23–35.
23–39.
21–39.
36–37.
38–39.

Aulacoseira subarctica (O.Müll.) E.Y.Haw. 36–39. Aulacoseira islandica (O.Müll.) Simonsen.
A slide labeled “Thingvallavatn. 30. 6. 1903” (S).
LM, bar10 m m. Arrowrimoportula.
Lectotype. Melosira islandica subsp. helvetica O.Müll.
Lectotype. Melosira islandica f. recta O.Müll.

43

44

Akihiro Tuji and Akiko Houki

40. Aulacoseira islandica (O.Müll.) Simonsen, 41–44. Aulacoseira subarctica (O.Müll.) E.Y.Haw.
40–44. A slide labeled “Thingvallavatn. 30. 6. 1903” (S).
40–44. DIC, bar10 m m. Arrowrimoportula.

Aulacoseira subarctica species complex

45–48. Aulacoseira islandica (O.Müll.) Simonsen.
45–48. “54” for Foged (1974), Thingvallavatn, South-west Island, dated “1954. 16/7” in Foged collection. (C).
45–48. SEM. Arrowrimoportula. 45. Whole frustule, bar10 m m. 46. Rimoportulae exist off the sulcus andsometimes near the valve face, bar1 m m. 47. Linking spine, bar1 m m. 48. Rimoportulae exist off the sulcus, bar1 m m.

45

46

Akihiro Tuji and Akiko Houki

49–51. Aulacoseira islandica (O.Müll.) Simonsen.
49–51. “54” for Foged (1974), Thingvallavatn, South-west Island, dated “1954. 16/7” in Foged collection. (C)49–51. SEM. Showing a rimportulae off the sulcus. Arrowrimoportula. 49–50. A rimoportula exist off the sulcus, bar1 m m. 51. A rimoportulae, bar100 nm.

Aulacoseira subarctica species complex

52–54. Aulacoseira subarctica (O.Müll.) E.Y.Haw.
52–54. “54” for Foged (1974), Thingvallavatn, South-west Island, dated “1954. 16/7” in Foged collection. (C)
52–54. SEM, bar1 m m. 52. Sulcus. 53. valve face areolae is not exist. 54. whole frustule.

47

48

Akihiro Tuji and Akiko Houki

55–68. Aulacoseira pusilla (F.Meister) Tuji et Houki comb. nov.
55–68. Bar10 m m. 55–56. LM. 57–58, 61–68. DIC. 59–60. DC (Dark contrast image).
55–60. Lectotype. A slide numbered “A3/61” with Meister’s label in Hustedt collection (BRM).
61–68. A slide numbered “A3/30” with Hustedt’s label in Hustedt collection (BRM).

Aulacoseira subarctica species complex

69–99. Aulacoseira pusilla (F.Meister) Tuji et Houki comb. nov.
69–99. TNS-AL-55478s (TNS). Katata lagoon dated 2000/3/18.
69–99. DIC, bar10 m m. Arrowrimoportula.

49

50

Akihiro Tuji and Akiko Houki

100

102

101

103
104

105
100–105. Aulacoseira pusilla (F.Meister) Tuji et Houki comb. nov.
100–101. Katata lagoon dated 98/11/10. 102,105. Akanoi, Lake Biwa dated 98/06, 103–104. Katata lagoon
dated 99/12/10.
100–105. SEM, bar1 m m. 100. Frustule showing areorae on the valve face, 101. frustule of long type. 102,105. whole frustule and bands. 103–104. rimoportulae on the sulcus.

Aulacoseira subarctica species complex

106–125. Aulacoseira subarctica var. logispina (Hust.) Tuji et Houki stat. nov. et comb. nov.
106–125. TNS-AL-55476s (TNS) from a material As163 (Type material) in Hustedt collection (BRM).
106–125. DIC, bar10 m m. Arrowrimoportula.

51

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Akihiro Tuji and Akiko Houki

126–130. Aulacoseira subarctica var. logispina (Hust.) Tuji et Houki stat. nov. et comb. nov.
126–130. As163 (type material) in Hustedt collection (BRM).
126–130. SEM. 126–128. Arrowrimoportula. Bar1 m m. 126. Sulcus. 127. No areolae on the valve face. 128.
Rimoportula on the sulcus. 129. Rimoportulae on the sulcus and not off the sulcus, bar10 m m. 130. Whole
frustule, bar1 m m.

Aulacoseira subarctica species complex

131–156. Aulacoseira subarctica var. tenuis (Hust.) Tuji et Houki comb. nov.
131–156. TNS-AL-55477s (TNS) from a type material As162 in Hustedt collection (BRM)
131–156. DIC, bar10 m m. Arrowrimoportula.

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Akihiro Tuji and Akiko Houki

157–161. Aulacoseira subarctica var. tenuis (Hust.) Tuji et Houki comb. nov.
157–161. As162 (type material) in Hustedt collection (BRM).
157–161. SEM. Arrowrimoportula. 157, 158. Whole frustule. 157. Bar10 m m. 158–160. Bar1 m m. 159.
Rimoportulae exist off the sulcus. 160, 161. Rimoportula exist on the hollow nodule. 160. bar100 nm.