Digestibility of the sponge Chondrilla nucula in the green turtle, Chelonia mydas

Bjorndal, Karen

The green turtle, Chelonia mydas, is an herbivore that feeds primarily on sea grasses and algae; animal tissue is not commonly ingested after the turtle shifts from pelagic to benthic habitats (Mortimer, 1982; Bjomdal, 1985). Sponges are often a major portion ofthe animal matter that is consumed. Stomachs from 243 Nicaraguan green turtles contained only 1.4% animal matter, two-thirds of which was sponge (Mortimer, 1981). Sponges were found in 4 of 94 green turtles from Brazil and were the third most common animal group (Ferreira, 1968). The only animal tissue found in digestive tracts from 14 green turtles from Antigua-Barbuda was a sponge in the order Haplosclerida in 1 of the turtles (Bjomdal and Bolten, unpubl. data). Balazs (1980) reported that sponges are occasionally included in the diet of Hawaiian green turtles. Sponge was the only animal tissue found in the stomachs of four green turtles in Honduras (Carr, 1952). Feeding on sponges is not common in vertebrates. Spongivory is limited to a small number of fish species, sea turtles and four species of freshwater turtles (Meylan, 1988). In marine turtles other than the green turtle, sponges comprise the major part of the diet of the hawks bill, Eretmoche/ys imbricata (Meylan, 1988) and a small portion of the diet of the loggerhead, Caretta caretta (Dodd, 1988). The extent to which sponge tissue is digested has apparently not been measured . in a vertebrate. Sponges have a variety of chemical and structural components that proteCt them from predation and may lessen their digestibility. These characteristics include toxic compounds, silica spicules and spongin (Meglitsch, 1967; Green, 1977). The possibility that the small amount of animal matter ingested by green turtles may make a major contribution to their nutrition has been discussed (Hirth et al., 1973; Bjomdal, 1985). To approach this question, and to investigate the nutritional yield of sponges to vertebrate spongivores, the digestibility of the chicken liver sponge, Chondrilla nucu/a, was measured in three size classes of green turtles in the southern Bahamas.

Digestibility of the sponge Chondrilla nucula in the green turtle, Chelonia mydas

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