Covering behavior of two co-occurring Jamaican sea urchins: Differences in the amount of covering and selection of covering material

In Discovery Bay, Jamaica, Tripneustes ventricosus (Lamarck, 1816) and Lytechinus variegatus (Lamarck, 1816) co-occur on shallow substrata with varying proportions of turtle grass (Thalassia testudinum Banks and Soland. ex Koenig) and coral rubble. Both cover their aboral surface with grass fragments and small pieces of rubble or shell. We tested the hypothesis that these species differ in covering behavior and actively select different covering material. Field observations in three habitats with different proportions of grass cover (< 30% grass, 50% grass, and > 70% grass) showed that L. variegatus covered significantly more of its surface than T. ventricosus in all habitats. The proportion of covering material due to grass increased significantly with the availability of grass in the habitat, and was significantly greater for T. ventricosus. In the mostly rubble habitat (< 30% grass), T. ventricosus still selected 78% grass while L. variegatus selected 92% rubble. A laboratory experiment was conducted in which single sea urchins, collected from each of the three habitats, were presented with equal amounts of grass and pieces of rubble or shell. Consistent with the field observations, L. variegatus covered significantly more of its surface than T. ventricosus, and was significantly more likely than T. ventricosus to select rubble or shell over grass. Clearly, the amount and type of covering material used by these sea urchin species differs and reflects both the availability of material and active choice.