Feeding rates and absorption efficiencies of four species of sea urchins (genus Echinometra) fed a prepared diet

Four closely related species of sea urchins belonging to the genus Echinometra, Echinometra sp. A (Ea), E. mathaei (Em), E. sp. C (Ec), and E. oblonga (Eo), occur sympatrically but in different microhabitats on Okinawan coral reefs. Feeding rates and absorption efficiencies of the four species were investigated in the laboratory by feeding sea urchins ad libitum a diet prepared from turf algae and agar over a 7-day period. Feeding rates differed significantly among the four species of Echinometra (Ea > Em ≈ Ec > Eo). Absorption efficiencies of protein and lipid did not differ significantly among the four species. Carbohydrate, a major nutrient component in the diet, was absorbed by the four species at significantly different efficiencies (Eo > Ec ≈ Em > Ea), which resulted in similar interspecific differences in absorption efficiencies of dry matter, total organic matter, and energy. The amount of nutrients absorbed from the diet was directly related to the feeding rate, indicating that the increase in absorption efficiency was not sufficient to completely compensate for low feeding rate. The interspecific difference in physiological performance in relation to feeding and absorption is consistent with taxonomic differentiation among the four species.