Habitats of larger foraminifera on the upper reef slope of Sesoko Island, Okinawa, Japan

Larger foraminifera living in the upper 50 m in front of the fringing coral reef northwest off Sesoko Island, Japan show strong habitat differences. T his study closely examines the distributions of larger foraminifers and rel ates these to a number of key environmental factors using rigorous statisti cal methods. Since all larger foraminifera house symbiotic algae, light att enuation by the water column is the most important limiting factor that mus t be dealt with wall structures. Water energy is also countered by test str ucture. The local topography is responsible for different intensities of hy drodynamic forces, which are expressed in various substrates, mostly coral rubble and coarse-grained sand. The genus Peneroplis, very common on the re ef flat, clearly prefers hardgrounds of the shallowest slope parts down to 30 m, while Dendritina is restricted to sandy bottoms and avoids the upperm ost meters of the slope. It can be found down to 50 m at least. Alveolinell a shows a similar depth distribution to Dendritina, but is common on hard b ottom. The distribution of Parasorites, which is restricted to sandy substr ates, starts at 20 m and extends to 80 m. Sorites, on the other hand, was f ound only on firm substrates between the reef edge and 50 m. The same depth distribution was recorded for Amphisorus, but this genus is not correlated with specific substrates. Most of the Amphistegina species prefer hardgrou nds, while Amphistegina radiata is also common on sand, The calcarinids, ca pable of withstanding high water energy, are abundant on firm substrates cl ose to the reef edge. Only Baculogypsinoides inhabits deeper parts of the s lope on sandy bottom acid avoids the shallowest parts. Sections with hard s ubstrates are settled by Heterostegina, even down to 80 m, although this ge nus was occasionally found on sandy bottoms. Nummulites, in contrast, is re stricted to sands between 20 and 70 m. Operculina, starting at 20 m, also p refers sandy substrates, but rare individuals were detected on coral rubble and macroids. (C) 1999 Elsevier Science B.V. All rights reserved.