Optimal foraging by deposit-feeding invertebrates: Roles of particle size and organic coating.

Feeding experiments were conducted on marine, deposit-feeding benthic invertebrates to test the predictions of an optimal foraging model. Food item selection based on sediment particle size and presence or absence of an organic coating on particles was investigated. Animals displaying a wide range of feeding mechanisms were studied in particle size-selection experiments using artificial sediment of closely controlled size composition. Nine of 10 species from 4 phyla ingested smaller particles in greater proportions than the particles were present in the sediment. In experiments where animals fed on a mixture of two particle types, one with and one without a surface protein coating, 6 of 7 species from 3 phyla ingested preferentially the protein-coated beads. While these trends of selection of smaller particles and protein-coated particles follow qualitatively the predictions of the optimal foraging model, the animals did not ingest exclusively the preferred particle types. Mechanics of particle handling rather than behavioral responses to particle characteristics appear to offer the better explanation for the observed selection patterns. In particular, the results support strongly the recently proposed role of mucous adhesion in particle selection by deposit feeders.These and other results from studies of deposit feeders suggest that factors in addition to food item selection must be considered when testing the assumptions and predictions of optimal foraging theory. Specifically, feeding energetics are also affected by postfood-selection processes such as variation of ingestion rate. Furthermore, the effects of abiotic environmental factors on foraging behavior cannot be overlooked in evaluating the optimality of foraging behavior; variable water velocity affected differently the particle size selectivity of 3 sympatric polychaete species in these studies.