Suborder Ostreina Férussac, 1822
Superfamily Ostreoldea Rafinesque, 1815
? Family Gryphaeidae Vyalov, 1936
Subfamily Pycnodonteinae Stenzel, 1959
Genus Pycnodonte Fischer de Waldheim, 1835
Subgenus Pycnodonte Fischer de Waldheim, 1835

Pycnodonte (Pycnodonte) taniguchii Hayami and Kase

Frontispiece (figures 1-2)

1992. Pycnodonte (Pycnodonte) taniguchii Hayami and Kase, Trans. Proc. Palace-in. Soc. fapan, n. s., no,165, p.1076, figs,2-7.

1993. Pycnodonte (Pycnodonte) taniguchii, Hayami and Kase, Univ. Mus. Univ. Tokyo, News, no.27, p.1, 2 figs.

As fully described by us, this species is a significantly bradytelic "living fossil" oyster. Until now, the present oyster has been found alive on the gloomy walls and ceilings of "W-arch" (type locality), "L-arch" and "Cross Hole" of Irabu Islet, "Devil's Palace", "Toriike", "Black Hole" and "Coral Hole" of Shimoji Islet, "Daidokutsu" of Ie Islet, and below an overhang at Seragaki Beach of Okinawa. The largest colony (more than 100 living individuals) has been found on the ceiling near the entrance of "Coral Hole" (RM19604). Thus, this oyster is regarded as an important constituent of the sclerosponge fauna, at least in this region.

Color dimorphism exists in the present species, as shown in the frontispiece. One phenotype is characterized by a purplish brown shell, and the other has an ivory white shell. No intermediate individual has been found, though a few ivory individuals have dark streaks. Brownish individuals are more abundantly found than ivory individuals, according to divers who observed the colonies at "Coral Hole" and "Toriike". As preliminarily assumed, they should be regarded as due to genetic dimorphism, because the two phenotypes occur strictly sympatrically at every locality.

Order Limoida Waller, 1978
Superfamily Limoidea Rafinesque, 1815

Family Limidae Rafinesque, 1815

All the limids from these submarine caves are extremely small in size, commonly exhibiting large prodissoconch I and paedomorphic features.