Nutritional Ecology of Sea Turtles

Abstract

Analysis of the nutritional ecology of sea turtles, that is, how nutrition influences their biology and determines their interactions with the environment, is necessarily restricted to the green turtle, Chelonia mydas. Our knowledge of the nutrition of the other species of sea turtles is limited to information on diet from gut content studies and a few reports on the anatomy and histology of the digestive tract. The literature on diet and gut anatomy and histology are summarized in the first two sections of this review. The remainder of this review is a discussion of the nutrition of Caribbean green turtles: their digestive efficiencies, adaptations to their major food plant Thalassia testudinum, and the effect the diet has, through nutrient limitation, on their productivity. Although Thalassia is a very abundant food source which is fairly constant in productivity and nutrient quality, few herbivores graze on it. Green turtles have two adaptations that enable them to utilize Thalassia more efficiently. First, they maintain grazing plots where, by cropping the young regrowth, they obtain blades of much higher quality because of lower lignin and higher nitrogen concentrations. Secondly, they have a hindgut microbial fermentation that digests the fiber in Thalassia and yields both an important energy source to the green turtle, in the form of volatile fatty acids, and gives the green turtle access to the highly digestible cell contents. In spite of the advantages of these adaptations-grazing plots and hindgut fermentation-they are not sufficient to prevent nutrient limitation and the resulting slow growth rates, delayed sexual maturity, and reduced reproductive output. Comparison with green turtles on high-quality, pelleted diets shows that the productivity of wild populations is well below their genetic potential. Ironically, nutrient limitation acting through delayed sexual maturity may benefit green turtles during periods of intense exploitation by man.