Although several authors paid attention to the evolutionary significance of pantropical cryptic organisms (especially sclerosponge-brachiopod fauna) ill and around coral reefs, little had been known about the features of molluscs in such sheltered environments. The newly discovered cavernicolous fauna from the Ryukyu Islands contains numerous characteristic diminutive bivalves as well as gastropods and other taxonomic groups. Further taxonomic, ecologic and environmental studies by specialists are, of course, needed to understand the unique community and ecosystem, but the composition and characteristics of the bivalves seem to suggest the possibility that submarine cave biota offers substantial data to test various theories and ideas of evolutionary ecology, historical biology and marine biogeography.

As described in this paper, 13 sublittoral limestone caves of Ie, Shimoji and Irabu Islets o.fthe Ryukyu Islands yield 48 characteristic bivalves, which are mostly new and at least in part indigenous to such cryptic environments. The common and predominant features of these cavernicolous bivalves are:
1) small adult size and frequent stunting,
2) dominance of epibyssate and semi-infaunal suspension feeders,
3) absence or rarity of cemented species and deep burrowers,
4) presence of many deep-water type genera (e.g. Huxleyia, Pronucidci, Bentharca, Gratis, Dacrydium, Parvamussium, Chlamydella, Divarilima, Carditella, Kelliella and Halonympha),
5) dominance of paedomorphosis by progenesis, though little is as yet known about the actual growth rate,
6) dominance of non-planktotrophic (lecithotrophic and direct) development which is comparable with that of deep-water and high-latitudinal bivalves,
7) small clutch size and low fecundity suggested by the large-sized Pd I in many species and parental incubation of embryos in several species.

The marked species diversity of cavernicolous bivalves, in spite of narrow space, is certainly due to their diminutive body size. The acknowledged relation between area and number of species (MacArthur and Wilson, 1967) would be invalidated, if mean body size were significantly different between regions. The abundant occurrence of many deep-water type genera from. upper sublittoral caves in this subtropical region indicates that neither high hydrostatic pressure nor low water temperature is a decisive factor for the restricted distribution of these genera.

These remarkable features are probably related to one another and mainly caused by the steadily oligotrophic condition for suspension feeders, because no primary production of nutrition (phytoplanktons) by photosynthesis occurs in dark caves. Low predation pressure may be also a factor analogous to the deep-sea bottom. K-selection, if anything, seems to prevail in the cavernicolous bivalves like deep-water species, although the tendency of stunting and progenesis is not compatible with current theories of evolutionary ecology, probably because the deficiency of nutrition is a decisive factor in the survival strategy.

Prodissoconchs are generally well preserved in these cave bivalves. Remarkable plasticity in prodissoconch size and shape within a genus is strongly indicated by the cave species of the Arcidae, Philobryidae, Propeamussiidae and Limidae. Submarine caves seem to offer refuges to relatively defenseless organisms of archaic life mode. Rapid evolutionary changes of adult size and developmental strategy may have occurred, as the ancestral populations adapted to such a sheltered environment.

The origin and speciation of these cavernicolous bivalves are still an unsolved problem because fossil and extant bivalves of such diminutive size have been insufficiently described in this region. It is still not known why some brooding bivalves (e.g. Cosa waikikia and Chlamydella incubata) actually show such a wide geographic distribution. We expect, however, that further comparative studies of cryptic biota from different regions would solve these problems, because there are numerous similar submarine caves around coral reefs of tropical-subtropical regions. Discovery of various "living fossil" organisms is anticipated as well in future research.