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

Abstract

Larger foraminifera living in the upper 50 m in front of the fringing coral reef northwest off Sesoko Island, Japan show strong habitat differences. This study closely examines the distributions of larger foraminifers and relates these to a number of key environmental factors using rigorous statistical methods. Since all larger foraminifera house symbiotic algae, light attenuation by the water column is the most important limiting factor that must be dealt with wall structures. Water energy is also countered by test structure. The local topography is responsible for different intensities of hydrodynamic forces, which are expressed in various substrates, mostly coral rubble and coarse-grained sand. The genus Peneroplis, very common on the reef flat, clearly prefers hardgrounds of the shallowest slope parts down to 30 m, while Dendritina is restricted to sandy bottoms and avoids the uppermost meters of the slope. It can be found down to 50 m at least. Alveolinella shows a similar depth distribution to Dendritina, but is common on hard bottom. The distribution of Parasorites, which is restricted to sandy substrates, starts at 20 m and extends to 80 m. Sorites, on the other hand, was found 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 hardgrounds, while Amphistegina radiata is also common on sand. The calcarinids, capable of withstanding high water energy, are abundant on firm substrates close to the reef edge. Only Baculogypsinoides inhabits deeper parts of the slope on sandy bottom and avoids the shallowest parts. Sections with hard substrates are settled by Heterostegina, even down to 80 m, although this genus was occasionally found on sandy bottoms. Nummulites, in contrast, is restricted to sands between 20 and 70 m. Operculina, starting at 20 m, also prefers sandy substrates, but rare individuals were detected on coral rubble and macroids.