Genus Bathyarca Kobelt, 1891

Bathyarca sp. indet.

Figures 65-69

The present species is represented only by a few dead specimens (RM19392) from the bottom sediments of "Fool's Palace" of Shimoji Islet, Miyako Islands. It clearly differs from the above described species of Bentharca and may belong to Bathyarca, which has been known as another deep-water arcid genus.

The shell is smaller than 2 mm in length, not carinate, relatively short, and characterized by much reduced anterior part, distinct byssal indentation, undeveloped marginal crenulations, only two or three short anterior and posterior teeth, numerous vertical striations on the middle part of hinge plate, and radially sculptured surface without developed commarginal lamellae. Pd I is D-shaped and about 135µm in maximum diameter; prodissoconch II is not clearly marked.

The outline and hinge teeth of the present species somewhat resemble those of Bathyarca pectunculoides (Scacchi, 1834), the type species of this genus, from wide areas of the Atlantic (Oliver and Allen, 1980), but the shell is much smaller, the anterior part more reduced, and the byssal notch more deeply indented. Bathyarca kyurokusimana Nomura and Hatai, 1940, from the lower sublittoral and bathyal bottom of central-south Japan (Habe, 1958), differs from the present species in its more semicircular outline and shorter dorsal margin.

The present species is probably closely related to Bathyarca perversidens Hedley, 1902, from the lower sublittoral bottom of New South Wales. Though the present specimens show smaller shell size, less elongated anterior and posterior teeth and less developed marginal crenulations than Hedley's illustrated specimens, further comparison with immature stages of the Australian species seems to be necessary.

Superfamily Limopsoidea Dall, 1895

Family Philobryidae Bernard, 1897

Philobryids consist of small-sized paedomorphic prionodonts which have been known from waters of various depth mainly in the southern hemisphere (Cotton and Godfrey, 1938; Powell, 1958; Nicol, 1966; Cox et al., 1969; Tevesz, 1977; Dell, 1990; etc.). In the northwest Pacific (except Hawaii) there is no formally described species of this family, but Kaneko (1984, 1991) preliminarily illustrated a few unnamed species from beach and dredged sands at several localities in south Japan, which he regarded as belonging to Cosa and Cratis.

Philobryids are often dominant in the cave bivalve fauna, exhibiting great species diversity. Thanks to Kaneko's personal communications (August 12 and 30, 1991) replying to our inquiry, the relation between his species and the present cave species became clearer. Dell (personal communication, March 25, 1993) also gave us very useful comments about the relation between these cave species and southern Pacific and Hawaiian philobryids.

This family in general is an interesting group from the viewpoint of evolutionary biology. Many species are of diminutive size and retain numerous denticles of provinculum until the latest growth stage, indicating significant paedomorphosis. In a comprehensive systematic study of the Philobryidae, Tevesz (1977) regarded philobryids as derived from the limopsids and as constituting a neotenous group. Morton (1978) also regarded the ligament of Philobrya as having evolved neotenously from some ancestors of the Limopsidae. Considering the significant reduction of adult size, however, the evolutionary change, if real, may be better called paedomorphosis by progenesis (instead of neoteny) in accordance with Gould's (1977) and McKinney and McNamara's (1991) definitions.

Prezant (1990) studied the ontogeny of an Antarctic brooding philobryid, Lissarca notorcadensis Melvill and Standen, 1907, in detail. He described that species as having very small Pd I and large Pd II, which were regarded as atypical features for a brooding bivalve. However, the Pd I is generally very large in this family (commonly exceeding 200 µm in diameter), indicating large size and relatively small numbers of eggs. Moreover, many species of this family are characterized by hat-shaped Pd I, which, we presume, indicates parental incubation of juveniles. In an individual of Cosa waikikia from the cave "Devil's Palace", several hat-shaped juveniles have been found in situ within the valves (Figures 79, 80). Although many philobryids have been known in exposed environments, this family may have been able to adapt easily to such an oligotrophic cavernicolous environment owing to its intrinsically stunted and progenetic properties.

Genus Cosa Finlay, 1927

Four philobryids from submarine caves are regarded as belonging to Cosa in view of the persistent denticles of provinculum, undeveloped prionodont teeth and small triangular (not much elongated) ligament pit interrupting the denticles of provinculum. The outline, surface sculpture, position of umbo and shape of Pd I, however, are different between the species. In shell outline and surface features some species resemble Philobrya species, but the ligament pit is acline or only a little prosocline, and never obliquely elongated.

Several species of Cosa were described from New Zealand and Australia, as listed by Powell (1955, 1958) and Laseron (1953). Because of the diminutive shell size, diagnostic characters of some early proposed species are often difficult to recognize from their original descriptions and accompanying line-drawings. Though more ex-haustive study on this genus is needed, we here regard these cavenicolous species except for Cosa waikikia as distinct from those Oceanic species.