FEEDING AND PHARMACOLOGICAL PROPERTIES OF ANISODORIS NOBILIS INDIVIDUALS (MOLLUSCA; NUDIBRANCHIA) SELECTING DIFFERENT NOXIOUS SPONGES PLUS DETRITUS

¹ Hopkins Marine Station of Stanford University, Pacific Grove, California 93950

Individual differences in Anisodoris nobilis food selection were sought in the natural environment, to determine why their digestive glands differed pharmacologically. Of 73 stationary A. nobilis specimens turned over in the initial survey, 84% of those on sponges and 65% of those on detritus had their mouths everted; no others were feeding. Microacoustic observations showed that an individual stops on a sponge, everts its proboscis for a long period, then rasps the sponge. Field studies of individual diets were made without disturbing the animals, by tabulating the foods on which each nudibranch was found to be stationary.

Twelve individual nudibranchs observed subtidally about three times daily for 8 days differed significantly in the sponge species they selected. Each individual was found stopped on detritus during roughly 50% of the observations, remaining there for 1 day at a time. Each nudibranch stopped for <2 days on a sponge, sometimes returning to that spot during subsequent days. Among individuals transplanted to different feeding sites, each everted its mouth only when on the sponge species it had been eating at the previous site. Two months of subsequent, scattered observations showed occasional longer-term feeding differences.

Noxious histamine (present in each sponge) and doridosine (found only in the nudibranch) accounted for different pharmacological properties in bioassays of Anisodoris digestive gland extracts. Histamine disappeared when the nudibranchs ate detritus, which suggests that this mixed diet purges sponge compounds from the nudibranch. Extracts from edible sponges administered to nudibranchs caused fleeing, while extracts of avoided sponges caused violent contractions, increased mucus production, and even death.