Asynchrony between dietary and nutritional shifts during the ontogeny of green turtles ( Chelonia mydas) in the Mediterranean

Abstract

Young green turtles ( Chelonia mydas) spend their early lives as oceanic omnivores, which consume primarily animal prey. Once they settle into neritic habitats (recruitment), they appear to shift rapidly into an herbivorous diet in tropical regions. However, the ontogeny of the dietary shift and the relevance of animal prey in the diet of neritic green turtles are poorly understood in subtropical and warm temperate regions. Stable isotopes of carbon and nitrogen in the carapace scutes of 22 green turtles from the eastern Mediterranean, ranging from 28 to 83 cm in curved carapace length (CCLmin), were analysed to test the hypothesis of a rapid nutritional shift following recruitment. Seagrasses prevailed in the stomach contents of all the green turtles that were larger than 30 cm CCLmin, but the concentration of stable isotopes in the carapace scutes revealed that turtles shorter than 40 cm CCLmin derived a negligible amount of assimilated nutrients from seagrasses. The concentration of stable isotopes in the carapace scutes also suggested that the contribution of seagrasses to the nutrients assimilated by green turtles increased steadily with turtle size and that some, but not all, of the turtles larger than 62 cm CCLmin were fully herbivorous. The overall evidence (gut contents analysis and stable isotope analysis) indicates that green turtles in the Mediterranean shift to a seagrass-based diet immediately after recruitment but turtle growth continues to rely on animal-derived nutrients for several years after recruitment. This asynchrony between the dietary and nutritional shifts is thought to be caused by the temperature sensitivity of bacterial fermentation and the low temperatures experienced by green turtles in the Mediterranean, which may result in a poor assimilation of the plant-derived nutrients for most of their neritic juvenile life.