Evolution of submarine cave molluscs

为了阐明珊瑚礁周围海底洞穴中隐栖软体动物的多样性特征、生活史及其演化意义，自1992年以来，本研究一直持续进行。在熟练潜水员的协助下，我们调查了冲绳、宫古、与那国岛和小笠原岛的许多洞穴的生物群。本文还比较了菲律宾、帕劳和日本各地的资料，发现洞穴栖双壳类具有以下共同特征：（1）大多数双壳类的体型都很小;（2）许多种类对捕食者没有防御能力（生态学上的原始）; 3）洞穴双壳类大多未被描述，包括许多可归入深水属的种类，4）许多物种表现出幼态（原生代）; 5）非亲营养型物种异常常见，如深水和北极双壳类; 6）许多物种的特征是巨大的原盘壳I，表明卵的大小和K-策略; 7）一些物种可能是特定洞穴特有的，但在西太平洋广泛分布的物种是常见的，尽管他们的非-这些特征被解释为由于适应低捕食压力和营养不良，在这样的洞穴。我们已经发表了琉球双壳类的系统描述和一些关于它们进化意义的讨论。

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.