DISTRIBUTION OF LIVING LARGER FORAMINIFERA NW OF SESOKO-JIMA, OKINAWA, JAPAN

Living, symbiont-bearing, larger foraminiferids restricted to the phot ic zone incorporate mechanisms blocking solar radiation and water curr ent transportation. They can be divided in two groups according to the ir wall structure: 1. Families with opaque test walls, reducing light penetration: Peneroplids inhabit shallow-water environments from the i ntertidal (Peneroplis in large numbers) down to 40 m (Dendritina, pref erring sandy substrates). Soritids are restricted to subtidal environm ents with less water turbulence and range from the reef moat down to 6 0 m. Amphisorus and Marginopora are common down to 30 m, whereas Paras orites is restricted to deeper parts of the reef slope. Alveolinella c an be found in the upper 40 m of the reef slope. 2. Families with hyal ine test walls, adjusted for light penetration: amphisteginids inhabit the whole photic zone, demonstrating a correlation of test flattening with increasing depth. Calcarinids cling with their spines to firm su bstrates such as algal thalli or macroids. Baculogypsina is restricted to high energetic regions on the reef hat. Calcarina inhabits similar environments, extending its distribution range within fore reefs down to 80 m. Baculogypsinoides is a dominant calcarinid genus from 30 to 70 m depth. Within nummulitids, Heterostegina prefers hard substrates on the reef slope, while Assilina (= Operculina) is frequent in deeper parts and is independent of substrates. Sandy bottoms from 30 to 60 m are inhabited by the last living representative of the genus Nummulit es, N. venosus. The giant foraminifer Cycloclypeus, with test sizes up to 7 cm, is restricted to fore reef areas below 50 m down to the base of the photic zone.