Analysis of Grain-Size Selection by Deposit-Feeding Holothurians and Echinoids (Echinodermata) from a Shallow Reef Lagoon, Discovery Bay, Jamaica

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Abstract
The size-frequency distribution of sediments from the foreguts of the aspidochirote holothurians Isostichopus badionotus, Holothuria mexicana and H. arenicola, and the spatangoid echinoids Meoma ventricosa and Plagiobrissus grandis, collected in the field, closely resembled those of sediments sampled adjacent to each animal. The differences in graphic mean grain size fell within the range of variability of the available sediment. Laboratory experiments using 2 grades of poorly sorted sand confirmed that I. badionotus, H. mexicana, H. arenicola, H. grisea, Actinopyga agassizi and the apodous holothurian Euapta lappa did not discriminate between different size grains. Small and large specimens of the first 3 spp. showed no difference in the spectrum of grain sizes ingested in the laboratory. Critical evaluation of previous reports reveals no unequivocal evidence of selective feeding by deposit-feeding holothurians. In general, deposit-feeding holothurians and spatangoids appear to be incapable of exercising any preference for particular grain sizes. The significance of the differences in surface texture of the buccal podia between holothurian species is unclear, but texture does not appear to be related to size of food particles in any simple way. Feeding might not be a purely mechanical process, as previously thought; adhesive mucus on the buccal podia might also play a role. In contrast to other deposit-feeding taxa, grain size does not appear to be a significant mechanism for resource partitioning between coexisting species of tropical holothurians; other studies suggest microhabitat differences may be more important.