ABSTRACT
Aggregations of the sea urchin S. granulans have been studied on the Island of Giglio (Tuscany/Italy) in situ using SCUBA.
Two types of aggregations were found:
Grazing aggregations on coralligenous gravel incrusted with calcareous red algae and sometimes on rocks covered with algae. Both habitats provide good food supply. The animals stayed together as a group up to several weeks. The attempt to induce spawning artificially (KCl-injections) was little successful. Thus the gonad maturity was estimated to be low.
Spawning aggregations were found on coarse sand where little or no food was available. They lasted only for a short time (1–2 days), then the animals spread to other habitats. Spawning was easy to induce. The amount of gametes shed after KCl-injections was high.
In-situ experiments proved that aggregations in S. granulans is true social behaviour and not only a result of environmental conditions.
