Growth of the Submarine Cave-dwelling Micro-bivalve Carditella iejimensis

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 the following characteristics: 1) very small adult size (usually less than 5 mm in length), 2) unusually large prodissoconch I that commonly exceeds 250 μm in length, and 3) the absence of prodissoconch II in many species. The small adult size and large prodissoconch I strongly suggest that the minute bivalve species produce a relatively small number of larvae (Kase & Hayami, 1992). We deduced that such a reproductive strategy appears to be suitable for the oligotrophic conditions experienced in submarine caves, where suspension feeders depend for their survival on the limited phytoplankton brought into the cave from external environments by weak currents. Indeed, Kase & Hayami (1992) predicted that the growth of cave-dwelling microbivalves would be slow because nutrition is generally deficient in submarine caves. However, to the best of our knowledge, no previous study has investigated the life-history characteristics of cave-dwelling micro-bivalve species, although millennia-scale variations in cavedwelling bivalve assemblages and the larval shell morphology of micro-bivalves have been examined (Kitamura et al., 2007; Ubukata et al., 2009; Yamamoto e t a l . , 20 09) . To address t h i s shortcoming, we examined the shell growth of Carditella iejimensis Hayami & Kase, 1993 (Cardit idae), via a mark–release survey in Shodokutsu submarine cave on the fore-reef slope of 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 suitable species for a mark–release survey, because the species is a mobile infaunal suspension feeder, while the others attach to the terminal part of polychaete tubes or small rock fragments above the sediment surface by a byssus (Hayami & Kase, 1993).