DOES CACO3 IN FOOD DETER FEEDING BY SEA-URCHINS

Calcified algae typically rank low in preference to generalist herbivo res; however, this correlation is confounded by the fact that many cal cified algae are also chemically defended. Few studies have experiment ally examined the role of CaCO3 in deterring herbivory. CaCO3 could de ter herbivory by increasing either the toughness or the mineral conten t of the algal thallus. By incorporating powdered CaCO3 (calcite and a ragonite) into an artificial food, we decouple these 2 possibilities a nd focus on the second. Diadema setosum and Echinometra sp. fed prefer entially on food containing powdered calcite; Mespilia globulus fed si milarly on experimental and control foods. Powdered aragonite deterred feeding by D. setosum, may have deterred feeding by M. globulus (depe nding upon whether data were analyzed as mass eaten or volume eaten), and had no effect on feeding by Echinometra sp. The absence of large n egative effects of powdered CaCO3 on feeding by sea urchins is consist ent with the fact that most urchins have a neutral gut pH and regularl y ingest some carbonate material. The most likely explanation for the more negative effect of aragonite than calcite is that the aragonite p articles were larger than particles of the commercially available calc ite. Further work needs to evaluate the relative importance of mineral content and toughness to see which is more important in rendering cal cified algae unpalatable to a variety of herbivores.