Short Notes©Malacological Society of Japan
41

Short Notes

Growth of the Submarine Cave-dwelling Micro-bivalve Carditella iejimensis
Akihisa Kitamura1*, Nagisa Yamamoto1,2 and Konatsu Kobayashi1
1

Shizuoka University, 836 Oya, Suruga-ku, Shizuoka 422-8017, Japan; *seakita@ipc.shizuoka.ac.jp
2Marine Works Japan, 3173-25 Showa-machi, Kanazawa-ku, Yokohama,Kanagawa 236-0001, Japan

More than 60 unique cave-dwelling bivalve
species have been reported from submarine caves
throughout the Japanese Islands, Philippine Islands,Saipan, Palau, and Guam (Kase & Hayami, 1992;
Hayami & Kase, 1993, 1996). Kase & Hayami(1992) stated that many of these species share thefollowing characteristics: 1) very small adult size(usually less than 5 mm in length), 2) unusuallylarge prodissoconch I that commonly exceeds 250
μm in length, and 3) the absence of prodissoconch IIin many species. The small adult size and large
prodissoconch I strongly suggest that the minutebivalve species produce a relatively small number oflarvae (Kase & Hayami, 1992). We deduced thatsuch a reproductive strategy appears to be suitable
for the oligotrophic conditions experienced insubmarine caves, where suspension feeders depend
for their survival on the limited phytoplankton
brought into the cave from external environments byweak currents. Indeed, Kase & Hayami (1992)
predicted that the growth of cave-dwelling microbivalves would be slow because nutrition isgenerally deficient in submarine caves.

However, to the best of our knowledge, noprevious study has investigated the life-history
characteristics of cave-dwelling micro-bivalve
species, although millennia-scale variations in cavedwelling bivalve assemblages and the larval shellmorphology of micro-bivalves have been examined(Kitamura et al., 2007; Ubukata et al., 2009;
Yamamoto et al., 2009). To address thisshortcoming, we examined the shell growth ofCarditella iejimensis Hayami & Kase, 1993
(Ca rditidae ), via a mark–re le ase survey inShodokutsu submarine cave on the fore-reef slopeof Ie Island, Okinawa, Japan. According to Hayami& Kase (1993), the most common species of living
bivalves within surface sediment in Shodokutsu
cave are C. iejimensis, Cosa waikikia, Cosa kinjoi,
Dacrydium zebra, and Parvamussium crypticum.
Among these, C. iejimensis is the most suitablespecies for a mark–release survey, because thespecies is a mobile infaunal suspension feeder, while
the others attach to the terminal part of polychaetetubes or small rock fragments above the sedimentsurface by a byssus (Hayami & Kase, 1993).

Fig. 1. Location maps of Daidokutsu and Shodokutsu submarine caves at Ie Island, off Okinawa, Japan.

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VENUS 70 (1–4), 2012

Study Area and Material
The entrance to Shodokutsu cave isapproximately 19 m below sea level. The cave is 35
m long, and the interior is straight and horizontal(Fig. 1). Mean monthly water temperature within
the cave varies between 20°C (February) and 29°C
(September) (Yamamoto et al., 2010). Salinity
within the cave ranges from 33.5‰ to 34.5‰(Yamamoto et al., 2010).
C. iejimensis has only been found in two caves(Daidokutsu and Shodokutsu) upon Ie Island, Japan(Hayami & Kase, 1993, 1996) (Fig. 1). The species
is less than 3.5 mm in height and length even atmaturity (Fig. 2). The maximum diameter ofprodissoconch I ranges between 130 and 157 μm,
while prodissoconch II is not distinguishable(Hayami & Kase, 1993). We inferred from this that
C. iejimensis has lecithotrophic development. An
ana ly sis of sediment cores recovered from

Daidokutsu cave reveals that C. iejimensis waspredominant within the cave during at least the past
7,000 years (Yamamoto et al., 2009). Althoughseveral species of Carditella (e.g., C. hanzawai(Nomura, 1933), C. torresi Smith, 1885 and C.
rugosa (Sowerby, 1833)) were described from open
seas in the low-middle latitudinal regions of theIndo-Pacific, no previous study has investigatedtheir life-history characteristics.

Methods
Five mark–release-recapture surveys werecarried out, with varying durations (Table 1). For
each survey, different individuals were collected,released, and recaptured. Living individuals werepicked from the >1.0 mm fraction of surface
sediment collected from a plot of 0.25 × 0.25 m2 insize in the middle part of Shodokutsu cave (Fig. 1).
In the laboratory, the very edge of the shell margin
was marked using a felt pen with water-insoluble
ink. Marked individuals were then released into thesampling plot within 10 hours of their collection.
Subsequently, we recaptured the shells from surface
sediment of the sampling plot of 0.25 × 0.25 m2 andmeasured shell heights of living individuals (Fig. 3).
For measurements, a picture of the shell was takenfrom above with a digital camera connected to a
microscope. To estimate the growth rate, themeasured increase in shell height was divided by thenumber of days between release and recapture.

Results and Discussion

Fig. 2. SEM of Carditella iejimensis; left valve,
Shodokutsu submarine cave, Ie Island, Okinawa.

We found significant differences among the fivesurveys in terms of the recapture rates of alive anddead individuals (Table 1). This result may berelated to spatial heterogeneity of the environment

Table 1. Results of five sets of mark–release and recapture surveys.

Releaseday

Recaptureday

29 Jul 200920 Sep 2008
3 Jun 2009 29 Jul 200929 Jul 2009 21 Apr 201018 Apr 2010 12 Jun 201029 Jul 2008

Shell height
No. of(mm) ofDays
releasedreleased inds
individuals
min.–max.
365
5356266
5640

0.81–2.213952
29

1.39–2.10
0.81–1.90
1.40–2.74

Shell heightTotal
No. of(mm) of
number ofrecaptured
recaptured
recaptured
individuals
living inds
individuals(alive)min.–max.
(alive + dead)35
24
1.13–2.76
33
1.67–2.01
77
1.47–2.14
178
1.40–2.52
98
1.40–2.74

Short Notes

43Fig. 3. Growth rate of mark– recapture specimens. The photograph in Fig. 3a shows anindividual recaptured 365 days after release.
within the cave, movement of individuals or
disturbance of substrata by tourist-diving. We did
not perform a detailed examination of changes ingrowth rate with ontogeny, because we did notrecapture living individuals with shell heights
greater than 2.8 mm on the day of release (Fig. 3).

The growth rates of the marked specimens were lessthan 4.1 μm day –1 (Fig. 3). Since there is no
statistically significant correlation between shellheight and growth rate among the five sets ofsampled shells (Fig. 3), we estimated the mean
growth rates of the five sets in Fig. 4. These values

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are averaged over released periods. so it is notpossible to evaluate changes in the seasonal growthof the shells from these data. However, sinceevidence of seasonal growth cessation is notdiscernible from these data, it is possible that thespecies undergoes year-round continuous growth. If
this interpretation is correct, the shell probably
requires more than 1 year to reach 1 mm in height,
and 4 years to reach 3 mm; consequently, thelifespan for C. iejimensis is estimated to be at least
4 years. The growth rate of C. iejimensis is much
lower than that of bivalves which live in the opensea (Fig. 5). Our data support the previous
hypothesis that the growth of cave-dwelling micro-bivalves would be slow (Kase & Hayami, 1992).
Acknowledgements: We thank Dr. T. Ubukataand Dr. L. Harper for helpful comments that helped
to greatly improve the article. We also thank
Koushin Yasumura for collecting samples and A.
St a l l a r d f or i m pr o vi n g t h e E n gl i s h in t h emanuscript. This study was funded by Grants-in-Aid(19540492 and 22540475) awarded by the Japan
Society for Promotion of Science, The Japan
Science Society, a Sasakawa Scientific Research
Grant, and a Grant-in-Aid from the Fukada
Geological Institute.

Fig. 4. Mean growth rate of the five surveys. Note that the unit of mean growth rate is mm per year. The
vertical lines indicate the interval of ±1 standard deviation.

Fig. 5. Shell-growth rate for many bivalve species which live in the open sea. 1, Mya
arenaria oonogai from Goshima (1982); 2, Scapharca kagoshimensis from Yoshida (1964);
3, Phacosoma japonicum from Tanabe & Oba (1988); 4, Pinna nobilis from Richardson et al.
(1999); 5, Modiolus barbatus from Peharda et al. (2006); 6, Donax dentifer from Riascos(2006); 7, Arctica islandica from Zettler et al. (2001). SH, shell height; SL, shell length.

Short Notes

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(Accepted July 13, 2011)

海底洞窟生微小二枚貝
Ca
rditellaieji
mensisの成長
北村晃寿・山本なぎさ・小林小夏

要約9沖縄県伊江島沖の海底洞窟“小洞窟”（深度 1m）に生息する微小二枚貝 Ca
rditellaieji
mensis（殻.
5mm以下）について，標識個体
長・殻高ともに 3の殻成長追跡調査を行った。その結果，同種は 1年を通じて殻成長を行なう可能性が高いことが分かった。また，殻の成長速度から，殻長 3mm に達するまでに，少なくとも 4年間を要するものと推定される。