Evolution of submarine cave molluscs

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The present study has been continued since 1992 in order to clarify the characteristics of diversity and life history of cryptic molluscs in submarine caves around coral reefs as well as their evolutionary signifcance. With assistance of skillful divers we surveyed the biota of many caves in Okinawa, Miyako, Yonaguni and Bonin Islands. Materials from the Philippines, Palau and various localities of Japan are also compared.As the result the following common features are found in cavernicolous bivalves : 1) Most bivalves are very small in size, 2) Many species are defenseless against predators (ecologically primitive) ; some species can be regarded as "living fossils", 3) Cave bivalves are mostly undescribed and include many species referable to deep-water genera, 4) Many species show paedomorphosis (progenesis), 5) Non-planktotrophic species is unusually common like deep-water and arctic bivalves, 6) Many species are characterized by huge prodissoconch I,suggesting large egg size and K-strategy, 7) Some species may be endemic to particular caves, but widely distributed species in the western Pacific are common in spite of their non-planktotrophic larvae.These features are interpreted as due to the adaptation to low predation pressure and poor nutrition in such caves. We already published the systematic description of bivalves from Ryukyu and some discussion about their evolutionary significance.