A number of nearly or totally dark sublittoral limestone caves open to the fore-reef slopes of Ie, Shimoji and Irabu Islets of the Ryukyu Islands are inhabited by various interesting cryptic organisms. 48 cavernicolous bivalves, 31 of which were found alive, are systematically described here mainly with SEM photomicrography. Dominant species are mostly new and belong to Huxleyia, Bentharca, Casa, Cratis, Dacrydium, Urumella (gen, nov.), Parvamussium, Cyclopecten, Chlamydella, Divarilima, Ctenoides, Limatula, Cardita, Carditella, Salaputium and Halonympha. The generic assemblage is considerably unique and extraneous for Indo-West Pacific sublittoral fauna, though several diminutive species of shallow-water genera (e.g. Septifer, Streptopinna, Lima, Epicodakia, Rochefortina, Coralliophaga, Irus) are often found alive in association. Except for a bradytelic species of Pycnodonte (s. s.), which is a member of the sclerosponge-brachiopod community, and for Glossocardia obesa, most cavernicolous bivalves are very small in adult size (generally less than 5 mm in length), often exhibiting remarkable stunting and paedomorphosis by progenesis.

The cavernicolous bivalves resemble deep-water ones not only in generic composition but also in developmental strategy: the predominance of non-planktotrophic (lecithotrophic and direct) development is strongly indicated by the large-sized prodissoconch I and in several species by parental incubation of embryos. These common characteristics may be related and are mainly caused by oligotrophic conditions in the caves, where suspension feeders have to depend on the limited phytoplanktons brought from exposed environments by weak currents. The archaic life mode of cavernicolous bivalves as well as the rarity of durophagous predators indicates that such submarine caves offer refuges for defenseless animals. Emphasis is laid upon the importance of the study of such cryptic faunas in evolutionary biology.