EFFECTS OF MACROPHYTE SECONDARY CHEMICALS ON FOOD CHOICE AND DIGESTIVE EFFICIENCY OF CEBIDICHTHYS-VIOLACEUS (GIRARD), AN HERBIVOROUS FISH OF TEMPERATE MARINE WATERS

Laboratory experiments using prepared seaweed food strips coated with the extracts of two dietary and four nondietary macrophytes provided evidence that the feeding of a cold-water herbivorous fish is deterred chemically by at least one brown algal species. Neither polar nor nonpolar extracts of the dietary algae Iridaea splendens (Turn.) Bory and Ulva lobata (Kutz) S. et G. or the nondietary macrophytes Laurencia spectabilis Post. & Rupr., Macrocystis integrifolia Bory and Phyllospadix scouleri Hook applied to the food strips reduced feeding by the stichaeid fish Cebidichthys violaceus (Girard). Only food strips coated with a polar extract of the nondietary fucoid brown alga Fucus gardneri Silv. significantly reduced feeding by C. violaceus. Moreover, the polar extract of F. gardneri but not that of the laminarian brown alga M. integrifolia coated on the dietary alga Ulva lobata reduced the digestibility of this alga when force-fed to C. violaceus. Specifically, the assimilation of N by C. violaceus was significantly reduced. These results partially explain food choice in this herbivorous fish and help explain why fucoid algae are not eaten by north temperate herbivorous fishes.