Feeding habits and ontogenetic dietary shift of green turtle, Chelonia mydas, in the Southwestern Atlantic Ocean: what we know until now?

GONZÁLEZ CARMAN V; VÉLEZ-RUBIO GM; LÓPEZ-MENDILAHARSU M; DOMIT C; SANTOS RG

Lima

Simposio; 36th Annual Symposium on Sea Turtle Biology and Conservation; 2016

Aggregations of green turtles (Chelonia mydas) in the Southwest Atlantic (SWA, Brazil, Uruguay and Argentina) are composed by mixed stocks, originated primarily from nesting assemblages of Ascension Island, Surinam, Aves Island and Trindade Island. Early studies suggested that C. mydas shifts from an omnivorous diet −characteristic of the juvenile oceanic stage of their life cycle− to a primarily herbivorous diet after recruitment to neritic habitats. However, recent research has revealed regional differences in the timing of this process. The combination of different techniques (e.g. stable isotope analysis, esophageal lavage, stomach contents) shows that C. mydas diet can differ strongly among ontogenetic stages, by latitude (spatial variation) and during climatic events (temporal variation), consuming high percentage of animal matter in some stages and areas after recruiting to a neritic zone. Such intraspecific variability can be expected for a species inhabiting a wide diversity of habitats, from tropical to warm temperate waters, as trophic plasticity will ensure survival on a wide range of local conditions. Our aim is to summarize the available information of regional pattern and ontogenetic diet shift of juvenile C. mydas (CCL range 27.8 to 82.6 cm) in the SWA region (ca. 3ºS, Fernando de Noronha -Brazil- to 37ºS, Bahia Samborombón-Argentina-). We reviewed a total of 10 publications published between 1968 to 2015, in which diet samples were obtained from strandings, bycatch, intentional capture and underwater observation. Our analysis showed that juveniles from south Brazil and Uruguay often have omnivorous diets, but the abundance of vegetal material ingested increases with turtle size. In fact, individuals smaller than 45 cm CCL have an omnivorous diet, including plant material and gelatinous macrozooplankton as the main animal prey. Individuals larger than 45 cm CCL are primarily herbivores, occasionally consuming some animal prey. In the areas with presence of algal assemblages C. mydas consumed high frequencies of Rhodophyta (e.g. Gelidium spp, Gracilaria spp, Pterocladiella capillacea, Chondracanthus spp Grateloupia spp; Chlorophyta (e.g. Ulva spp., Caulerpa spp); Phaeophyta (e.g. Sargassum spp) and in areas with presence of seagrass assemblages, Halodule sp. Conversely, in the southern limit of their distribution (southwest portion of the Río de la Plata estuary, Argentina), C. mydas feeding behavior changed to almost exclusively carnivorous (gelatinous macrozooplankton). We concluded that a foraging plasticity -influenced by regional or local conditions- characterizes the feeding behavior of C. mydas. The high level of animal prey ingestion in colder waters could be addressed to the scarcity or lack of macroalgae and seagrass assemblages and the relatively high availability of gelatinous macrozooplankton. In addition to this, the water temperature may also play a role in the high level of ingestion of animal prey in colder waters. C. mydas is ectothermic, and the colder temperatures in the most southern areas may lower the ability to efficiently digest macrophyte; thus, favoring the ingestion of animal matter, which may be more energetic and easily digestible source for juvenile green turtles. Plasticity is therefore observed both in the diet composition and foraging strategy of green turtles in the SWA region. In addition to well recognize role as herbivorous, green turtles may also have an important role as second-order consumers in certain areas of their southernmost distribution in the SWA. A good understanding of the feeding strategies exhibited by C. mydas and its adaptation to different habitats is crucial to delineate next research steps to better understand the life cycle of this threatened species.