Dietary analysis of two sympatric marine turtle species in the eastern Mediterranean

Josie L Palmer,Mehmet F Hüseyinoğlu,Julia C Haywood,Annette C Broderick,Burak A Çiçek,Sophie Davey,Emily M Duncan,Wayne J Fuller,M Jesse Schneider,Lucy C M Omeyer,Robin T E Snape,Damla Beton,Brendan J Godley

doi:10.1007/s00227-021-03895-y

Abstract

Dietary studies provide key insights into threats and changes within ecosystems and subsequent impacts on focal species. Diet is particularly challenging to study within marine environments and therefore is often poorly understood. Here, we examined the diet of stranded and bycaught loggerhead (Caretta caretta) and green turtles (Chelonia mydas) in North Cyprus (35.33° N, 33.47° E) between 2011 and 2019. A total of 129 taxa were recorded in the diet of loggerhead turtles (n = 45), which were predominantly carnivorous (on average 72.1% of dietary biomass), foraging on a large variety of invertebrates, macroalgae, seagrasses and bony fish in low frequencies. Despite this opportunistic foraging strategy, one species was particularly dominant, the sponge Chondrosia reniformis (21.5%). Consumption of this sponge decreased with increasing turtle size. A greater degree of herbivory was found in green turtles (n = 40) which predominantly consumed seagrasses and macroalgae (88.8%) with a total of 101 taxa recorded. The most dominant species was a Lessepsian invasive seagrass, Halophila stipulacea (31.1%). This is the highest percentage recorded for this species in green turtle diet in the Mediterranean thus far. With increasing turtle size, the percentage of seagrass consumed increased with a concomitant decrease in macroalgae. Seagrass was consumed year-round. Omnivory occurred in all green turtle size classes but reduced in larger turtles (> 75 cm CCL) suggesting a slow ontogenetic dietary shift. Macroplastic ingestion was more common in green (31.6% of individuals) than loggerhead turtles (5.7%). This study provides the most complete dietary list for marine turtles in the eastern Mediterranean.

Highlights

Studies into the foraging ecology of marine organisms are logistically difficult due to the challenges of sampling animals in the marine environment (Newsome et al 2010; Komoroske et al 2017; Wildermann et al 2018; Green et al 2020)
Of the 38 species recorded, a total of 3 invasive species were identified, namely Halophila stipulacea, Caulerpa taxifolia and Caulerpa cylindracea which were found in both turtle species
The diet of loggerhead turtles was more diverse with 129 different taxa observed compared to 101 taxa in green turtles

Summary

Introduction

Studies into the foraging ecology of marine organisms are logistically difficult due to the challenges of sampling animals in the marine environment (Newsome et al 2010; Komoroske et al 2017; Wildermann et al 2018; Green et al 2020) Recent technological advancements, such as satellite tracking and stable isotope analysis, have allowed elusive life stages to be studied over extended periods of time, providing valuable information, such as the location of foraging grounds, trophic level, and timing of ontogenetic dietary shifts (Cardona et al 2010; Abascal et al 2016; Haug et al 2017; Andrews-Gof et al 2018; Haywood et al 2019, 2020a, b). Marine turtles are used as bio-indicators for monitoring plastic pollution across the Mediterranean Sea through gut contents analysis (INDICIT Consortium 2018)

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