Trade-offs between nutritional quality and abundance determine diet selection in juvenile benthic green turtles

Abstract

Herbivores consume foods that are often low in many essential nutrients and high in structural constituents difficult to digest. In this situation, nutritional quality is of paramount relevance for food selection. Neritic juvenile green turtles (Chelonia mydas Linnaeus 1758) in the western South Atlantic are plant-based omnivores relying primarily on red macroalgae, although brown macroalgae are highly available. We investigated the digestibility coefficient and calculated the intake passage time of the red macroalgae Pterocladiella capillacea, the brown macroalgae Sargassum cf. vulgare and the fish Cynoscion leiarchus for juvenile green turtles ranging 42.7–62.0 cm in curved carapace length to test the hypothesis that macroalgae selection is based on nutritional quality. Results indicated that the apparent digestibility coefficient of Pterocladiella capilacea was similar to that of fish fillets (93.9% and 98.9% respectively) and significantly higher than that of the Sargassum cf. vulgare (75.8%), with a significant effect of body weight on the digestibility of certain macronutrients. Those differences arose partially because of the higher apparent digestibility of insoluble fiber of Pterocladiella capillacea compared to that of Sargassum cf. vulgare (95.2% and 84.0% respectively). The intake passage time, at 24.5 °C, was 20.6 ± 3.8 days for the three diets. The overall evidence indicates that foraging on Pterocladiella capillacea is more profitable than foraging on Sargassum cf. vulgare, which may explain the prevalence of red algae in the diet of green turtles off Brazil. Furthermore, evidence indicates that red algae diets are similar to animal diets as far as digestibility is considered, although a higher daily intake is necessary to acquire the same energy intake, due to a lower energy density.