Larval settlement of an algivorous snail species (Trochidae) induced by symbiotic sea urchins and conspecific snails

Abstract

The rock-boring sea urchin, Echinostrephus molaris, excavates soft rocks, and the pits are later used by non-boring sea urchins such as Echinometra mathaei and Anthocidaris crassispina. The pits used by these non-boring sea urchins are also characteristically inhabited by a limpet-like algivorous trochid snail, Broderipia iridescens (Gas-tropoda, Trochidae). To determine how this unique symbiotic association is maintained, we observed reproduction and larval development of the snail, and performed assays to detect what induced settlement in the snail larvae. As candidates of settlement inducers, a habitat biofilm plate, the host and the non-host sea urchins, and conspecific snails were kept in separate glass bowls, and newly hatched B. iridescens veliger larvae were introduced to each bowl. Sixty to 80% of larvae settled in bowls that contained the non-boring host sea urchins and conspecific snails. On the other hand, only approximately 8% of the larvae settled in bowls containing the boring non-host sea urchins. This is the first report demonstrating that larval settlement of an algivore is induced not by the habitat biofilms, but by the symbiotic hosts.