Functional shell geometry of symbiont-bearing benthic Foraminifera

Abstract

An important function of symbiont-bearing benthic foraminifera is to provide their endosymbiotic microalgae with light. At the same time, these foraminifera have to resist hydrodynamic forces. Foraminifera match these demands by constructing shells (tests) functioning as glasshouses. In shallowest regions of oligotrophic tropical seas, at the one end of the scale, foraminifera with spherical and thick lenticular tests develop special fixing mechanisms to resist extreme water motion. In the deepest euphotic zone, at the other end of the scale with extremely weak light and quiet water, foraminifera with flat discoid and blade-shaped tests possessing a high surface/volume-ratio position their symbionts just beneath the transparent test walls, intensifying the weak light through elevated test surface structures. Between these two extremes, foraminifera react to decreasing light intensity and water motion by the transition in shell form from spherical to extremely flat tests. A second way in test form from high energetic shallow water to less, but still energetic water is the transition from spherical to fusiform tests, again raising the surface/volume-ratio but not in the same degree as performed by test flattening.