CHARACTERISTICS OF SUBMARINE CAVE BIVALVES IN THE NORTHWESTERN PACIFIC

This paper discusses the diversity, common features, and geographic di stribution of submarine cave bivalves collected with SCUBA from a numb er of islands around the Philippine Sea (Okinawa, Miyako, Yonaguni, Da ito, Bonin, Bohol and Cebu of the Philippines, Palau, and Guam). Commo n significant characteristics of cave bivalves are: (1) unique taxonom ic assemblage, (2) reduced adult size, (3) many deep-water genera (4) occurrence of several ''cavity-dwelling'' shallow-water genera on the exposed wall and sediment surface, (5) frequent paedomorphosis by prog enesis, (6) relative abundance of non-planktotrophic species, (7) low fecundity and dominance of brooding, and (8) archaic life mode reminis cent of a fauna before the ''Mesozoic marine revolution'' (rarity of s edentary species and deep burrowers). These features must be related t o one another and are generally regarded as due to a common adaptive s trategy toward the oligotrophic condition and low predation pressure o f cave habitats. It is still mysterious how cave bivalves, even broodi ng species, have become so extensively distributed in the western Paci fic region. Although there is no positive evidence, rafting is a possi ble mechanism of transoceanic dispersal for minute epibyssate bivalves .