Diversity in trophic interactions of green sea turtles Chelonia mydas on a relatively pristine coastal foraging ground

Abstract

Adult green sea turtles Chelonia mydas are often the largest-bodied herbivores in their communities and may play an important role in structuring seagrass and macroalgal communities. Recent studies, however, suggest that green turtles might be more omnivorous than previously thought. We used animal-borne video and nitrogen and carbon stable isotopic analysis of skin to elucidate diets of green turtles in the relatively pristine seagrass ecosystem of Shark Bay, Australia. Stable isotope values suggested that despite the presence of abundant seagrass resources, turtles assimilated most of their energy from a combination of macroalgae and gelatinous macroplankton (cnidarians and ctenophores). Video data suggested that macroplankton might be the most commonly consumed food source. Also surprising was the considerable variation in δ 13C values, suggesting long-term dietary specialization by individual turtles. Overall, green turtle foraging under natural conditions may be less stereotyped than previously thought, and diets of green turtles inhabiting apparently similar ecosystems (e.g. seagrass-dominated ecosystems) may vary considerably across geographical regions. The apparently high degree of individual specialization in diets suggests that conservation efforts should account not only for the potential importance of non-benthic food sources for green turtle populations, but also for the possibility that subsets of the population may play different ecological roles and may be differentially vulnerable to anthropogenic impacts.KEY WORDS: Herbivory · Diet · Foraging behavior · Cnidarians · Stable isotopes · Seagrass ecosystem · Animal-borne video · Shark Bay Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 439: 277-293, 2011 278 ture (Kuiper-Linley et al. 2007, Wabnitz et al. 2010 or even result in declines in the biomass present in seagrass communities (Murdoch et al. 2007, Fourqurean et al. 2010. Excluding green turtles from a T. testudinim-dominated seagrass meadow in Bermuda for 1 yr resulted in an increase in seagrass biomass and structural complexity of the seagrass canopy when compared to grazed sites (Fourqurean et al. 2010). Understanding the reliance on seagrasses and other resources for food and how the use of these resources might vary within and among populations is of key importance in estimating the ecological impact of green turtles.Green turtles are traditionally thought to undergo abrupt ontogenetic shifts in their diets, from carnivory during the pelagic phase of their lives to nearly complete herbivory once they settle in coastal habitats at a curved carapace length (CCL) of 40−44 cm (Chaloupka & Limpus 2001, Arthur et al. 2008. Neritic green turtle diets have been investigated primarily by comparing available food sources to forage found in mouth contents, lavage samples, stomach contents of dead animals, or fecal matter (e.g. Mortimer 1981, Brand et al. 1999, Seminoff et al. 2002, Arthur et al. 2009). These studies suggest that the di...

Site fidelity, ontogenetic shift and diet composition of green turtles Chelonia mydas in Japan inferred from stable isotope analysis

“…However, inference from the stable isotope ratios of green turtle eggs suggested that some nesting females in the Ogasawara Islands consumed a substantial proportion of invertebrates prior to breeding migration (Hatase et al 2006). Consumption of animal matter by green turtles in other parts of the world has been confirmed by direct observation of episodic ingestion or inferred from stable isotope data or both (Heithaus et al 2002, Seminoff et al 2006b, Amorocho & Reina 2007, Arthur et al 2007, Cardona et al 2010, Burkholder et al 2011, Lemons et al 2011). …”

Site specialists, diet generalists? Isotopic variation, site fidelity, and foraging by loggerhead turtles in Shark Bay, Western Australia

“…Turtles fed predominantly on items on the sea floor, often using the foreflippers to clear debris and locate prey items. Unlike sympatric green turtles that frequently fed on scyphozoan jellyfish or ctenophores in the water column (Burkholder et al 2011), mid-water foraging was observed only twice for loggerhead turtles. In these cases, feeding occurred out of the field of view and was inferred from head and neck movements so prey could not be identified.…”

“…Our goal instead was to conduct a preliminary investigation of isotopic niche variation within this population and provide a foundation for more detailed behavioral and isotopic research. Isotope signatures of basal resources in our study area have been analyzed in a study of the foraging ecology of sympatric green turtles (Burkholder et al 2011). …”

Habitat selection by green turtles in a spatially heterogeneous benthic landscape in Dry Tortugas National Park, Florida

“…However, green turtle diet can vary by foraging ground (Bjorndal 1985(Bjorndal , 1997, life stage (Bolten 2003), and the availability of food sources (Garnett et al 1985). Recent studies using stable isotope analysis and animalborne videos suggested more variation in green turtle diet than previously thought (Vander Zanden et al 2010, Burkholder et al 2011.…”