Settlement of the Larvae ofSpirorbis SpirorbisL.

Among the marine invertebrates whose settlement behaviour is well studied, the polychaete tubeworm,Spirorbis spirorbisL. ( =borealisDaudin), the barnacle,Balanus balanoides, and the oyster,Ostrea edulis, are different in form whether in the free-swimming or adult stage, yet the settlement behaviour patterns are basically similar (Crisp, 1973). The larvae have a sequence of movements during exploration of the substratum, with each activity initiated by a hierarchy of stimuli. Given a positive set of responses to a suitable habitat, the distances moved on the substratum are limited progressively so that finally the larva investigates a very restrictred area. When the exact site for settlement is determined the larva ceases to move, attaches permanently and immediately commences metamorphosis into the adult form. However, if certain stimuli are lacking or obnoxious to the searching larva, it recommences the initial free-swimming activity, in order to reach alternative substrata.