ORIGINAL PAPER Delayed ontogenic dietary shift and high levels of omnivory in green turtles (Chelonia mydas) from the NW coast of Africa Luis Cardona Æ A. Aguilar Æ L. Pazos Received: 5 August 2008 / Accepted: 12 March 2009 / Published online: 5 April 2009 Ó Springer-Verlag 2009 Abstract Young green turtles (Chelonia mydas) spend their early lives as oceanic omnivores with a prevalence of animal prey. Once they settle into neritic habitats (recruitment), they are thought to shift rapidly to an her- bivorous diet, as revealed by studies in the Greater Caribbean. However, the precise timing of the ontogenic dietary shift and the actual relevance of animal prey in the diet of neritic green turtles are poorly known elsewhere. Stable isotopes of carbon, sulfur and nitrogen in the cara- pace scutes of 19 green turtles from Mauritania (NW Africa), ranging from 26 to 102 cm in curved carapace length (CCLmin), were analyzed to test the hypothesis of a rapid dietary shift after recruitment. Although the length of residence time in neritic habitats increased with turtle length, as revealed by a significant correlation between turtle length and the d 13 C and the d 34 S of the scutes, comparison of the d 15 N of the innermost and outermost layers of carapace scutes demonstrated that consumption of macrophytes did not always start immediately after recruitment, and turtles often resumed an animal-based diet after starting to graze on seagrasses. As a consequence, seagrass consumption did not increase gradually with turtle size and animal prey largely contributed to the diet of turtles within the range 29–59 cm CCLmin (76–99% of assimilated nutrients). Seagrass consumption by turtles larger than 59 cm CCLmin was higher, but they still relied largely on animal prey (53–76% of assimilated nutrients). Thus, throughout most of their neritic juvenile life, green turtles from NW Africa would be better classified as omnivores rather than herbivores. Introduction Seagrass meadows are among the most productive eco- systems in the biosphere (Hemminga and Duarte 2000), but currently most of the primary production is channeled as detritus (Cebrian 1999). This is probably due to the number of large herbivorous vertebrates, the most important sea- grass grazers worldwide (Valentine and Heck 1999; Hemminga and Duarte 2000; Valentine and Duffy 2006), have declined massively in the past centuries (Hemminga and Duarte 2000; Valentine and Duffy 2006). Nevertheless, it has been suggested that sea turtle grazing may still have a dramatic impact on the structure and dynamics of tropical seagrasses meadows (Bjorndal and Jackson 2003; Moran and Bjorndal 2005). Although the loggerhead sea turtle Caretta caretta and the olive ridley turtle Lepidochelys olivacea have been reported to consume small amounts of marine macro- phytes, the green turtle Chelonia mydas is the only sea turtle considered to be an herbivore for most of its life (Bjorndal 1997). Current understanding assumes that the green turtle is oceanic in its early life but typically turns to neritic habitats when it reaches a carapace length of 25– 35 cm (Bjorndal and Bolten 1988; Limpus et al. 1994; Reich et al. 2007) and an age of 3–6 years (Zug and Glor Communicated by J.P. Grassle. Electronic supplementary material The online version of this article (doi:10.1007/s00227-009-1188-z) contains supplementary material, which is available to authorized users. L. Cardona (&)  A. Aguilar Department of Animal Biology, Faculty of Biology, University of Barcelona, Avda. Diagonal 645, 08028 Barcelona, Spain e-mail: luis.cardona@ub.edu L. Pazos IUSC, C/Fontanella 19, 08010 Barcelona, Spain 123 Mar Biol (2009) 156:1487–1495 DOI 10.1007/s00227-009-1188-z