Dietary assimilation and the digestive strategy of the omnivorous anomuran land crab Birgus latro (Coenobitidae)

Joanne E. Wilde, Stuart M. Linton, Peter Greenaway

On Christmas Island, Indian Ocean, the diet of robber crabs, Birgus latro (Linnaeus) was generally high in fat, storage polysaccharides or protein and largely comprised fruits, seeds, nuts and animal material. The plant items also contained significant amounts of hemicellulose and cellulose. In laboratory feeding trials, crabs had similar intakes of dry matter when fed artificial diets high in either fat or storage polysaccharide, but intake was lower on a high protein diet. Assimilation coefficients of dry matter (69-74%), carbon (72-81%), nitrogen (76-100%), lipid (71-96%) and storage polysaccharide (89-99%) were high on all three diets. B. latro also assimilated significant amounts of the chitin ingested in the high protein diet ( 93%) and hemicellulose (49.6-65%) and cellulose (16-53%) from the high carbohydrate and high fat diets. This is consistent with the presence of chitinase, hemicellulase and cellulase enzymes in the digestive tract of B. latro. The mean retention time (27.2 h) for a dietary particle marker ((57)Co-labelled microspheres) was longer than measured in leaf-eating land crabs. The feeding strategy of B. latro involves the selection of highly digestible and nutrient-rich plant and animal material and retention of the digesta for a period long enough to allow extensive exploitation of storage carbohydrates, lipids, protein and significant amounts of structural carbohydrates (hemicellulose, cellulose and chitin).