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The importance of plant or animal content in green turtle diet is related to sea surface temperature (SST). Generally, at higher latitudes and in cold-water currents where SST is < 20 °C for at least two seasons, animal matter in the diet is > 20% (black circles; mean = 51.47%; SE = 4.84; n = 14 sites); whereas at warmer sites where SST > 25 °C for at least two seasons, green turtle diet is dominated by seagrass, macroalgae and mangroves (green circles; mean = 92.97%; SE = 9.85; n = 57 sites). Both plant and animal matter are important components at a small number of sites (green circles outlined by black, n = 4 sites). SST temperatures from the year 1993 are at the midpoint of the study time series (1971–2016; Online Resource Table S1). SST (AVHRR) data source: Phillips et al. 2012
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To better understand dietary requirements, trophic shifts, and trophic interactions of the threatened green turtle (Chelonia mydas), we conducted a comprehensive global review and literature tabulation (177 studies) reporting diets of individuals > 25 cm carapace length. We analysed those studies involving natural sites and healthy animals that rep...
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Green turtles ( Chelonia mydas ) are highly dependent on neritic foraging areas throughout much of their life. Still, knowledge of recruitment dynamics, foraging habits, and habitat use in these areas is limited. Here, we evaluated how the distribution and food preferences of green sea turtles from different life stages varied within a foraging agg...
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... These data were compiled from multiple independent research endeavors over multiple decades that used varied sampling criteria (e.g., stomach only, partial GI tract, etc). As a result, we used a variety of qualitative and quantitative metrics to evaluate GI content data herein, each of which has its advantages and disadvantages (Esteban et al., 2020). ...
... Note that although we have included results from studies using various methodologies in Supplementary Table S4, we urge caution in interpretation as to the equivalence of results. Esteban et al. (2020) noted small sample size and potential selective retention of larger items by esophageal papillae in the lavage method and reported on nontrivial differences in results using different methodologies at the same study site for green sea turtles (Chelonia mydas). In Supplementary Table S4, we note the pattern of greater predominance in sponges in studies involving examination of GI contents of dead turtles as compared to either gastric or esophageal lavage. ...
... A rich body of research has shown that abiotic factors can disproportionately influence organismal behavior at cool range edges reviewed in Paquette and Hargreaves (2021). For example, at higher latitudes, animal matter features prominently in the diets of herbivorous green turtles (Esteban et al., 2020), suggesting that temperature may be an important driver of trophic plasticity in sea turtles towards higher trophic levels, as observed for the Texas hawksbills in this study. Texas coastal waters experience much stronger seasonal shifts in temperature (16-30°C) than south Florida (22-30°C). ...
Foraging habitat selection and diet quality are key factors that influence individual fitness and meta-population dynamics through effects on demographic rates. There is growing evidence that sea turtles exhibit regional differences in somatic growth linked to alternative dispersal patterns during the oceanic life stage. Yet, the role of habitat quality and diet in shaping somatic growth rates is poorly understood. Here, we evaluate whether diet variation is linked to regional growth variation in hawksbill sea turtles ( Eretmochelys imbricata ), which grow significantly slower in Texas, United States versus Florida, United States, through novel integrations of skeletal growth, gastrointestinal content (GI), and bulk tissue and amino acid (AA)-specific stable nitrogen (δ ¹⁵ N) and carbon (δ ¹³ C) isotope analyses. We also used AA δ ¹⁵ N Σ V values (heterotrophic bacterial re-synthesis index) and δ ¹³ C essential AA (δ ¹³ C EAA ) fingerprinting to test assumptions about the energy sources fueling hawksbill food webs regionally. GI content analyses, framed within a global synthesis of hawksbill dietary plasticity, revealed that relatively fast-growing hawksbills stranded in Florida conformed with assumptions of extensive spongivory for this species. In contrast, relatively slow-growing hawksbills stranded in Texas consumed considerable amounts of non-sponge invertebrate prey and appear to forage higher in the food web as indicated by isotopic niche metrics and higher AA δ ¹⁵ N-based trophic position estimates internally indexed to baseline nitrogen isotope variation. However, regional differences in estimated trophic position may also be driven by unique isotope dynamics of sponge food webs. AA δ ¹⁵ N Σ V values and δ ¹³ C EAA fingerprinting indicated minimal bacterial re-synthesis of organic matter (Σ V < 2) and that eukaryotic microalgae were the primary energy source supporting hawksbill food webs. These findings run contrary to assumptions that hawksbill diets predominantly comprise high microbial abundance sponges expected to primarily derive energy from bacterial symbionts. Our findings suggest alternative foraging patterns could underlie regional variation in hawksbill growth rates, as divergence from typical sponge prey might correspond with increased energy expenditure and reduced foraging success or diet quality. As a result, differential dispersal patterns may infer substantial individual and population fitness costs and represent a previously unrecognized challenge to the persistence and recovery of this critically endangered species.
... There are however concerns that the extensive seagrass beds of the PNBA, which largely sustain the green turtle population, may face a severe decline over the next few decades as a result of climate change (Chefaoui et al., 2021). Notably, another green turtle foraging aggregation in West Africa was found to rely mostly on red algae as a food source (Díaz-Abad et al., 2022), and green turtles are also able to shift from seagrass to macroalgae feeding (Esteban et al., 2020), thus plasticity in foraging habits may contribute to the resilience of green turtles under climate change. Future work should focus on the diet of green turtles in this major foraging site and on their interactions with the local seagrass meadows. ...
There is a remarkable paucity of estimates of the numeric importance of sea turtles at foraging grounds. The Banc d'Arguin (BA) is a vast shallow marine area off the coast of Mauritania, known as a site of world importance for coastal migratory birds and other biodiversity, including extensive seagrass beds. We sampled foraging green turtles on the BA, and extensively tracked adult female green turtles from the Bijagós archipelago, the only significant nesting aggregation within 3000 km of the BA, to estimate the abundance of this foraging aggregation. Additionally, we used a demographic simulation to support our findings. Based on satellite tracking of adult females (n = 46), we estimate that 50 % of the nesting population from the Bijagós migrate to the BA post-nesting. We combine data on numbers nesting in the Bijagós with information on proportion migrating to the BA in the same years to conservatively estimate that 8285 adult female green turtles forage at this site. We also estimate that adult females represent only 5.6 % of the green turtles in the BA, implying that the number of turtles there is of the order of 150,000 individuals. Most of the BA enjoys effective protection as part of the Parc National du Banc d'Arguin where significant fisheries regulations are well enforced by a marine surveillance program. We show that the BA is one of the major foraging sites for green turtles nesting in the Bijagós and a site of critical importance for immature and adult green turtles in a global context.
... Chelonia mydas does not feed exclusively on seagrass in the Caribbean, although T. testudinum provides the greatest contribution to its diet in most studied locations and is the preferred diet along with a few species of red algae (Bjorndal 1980;Mortimer 1981). At higher latitudes and in colder water C. mydas consumes increasing amounts of animal matter, but this is predicted to become less available as climate change causes water temperatures to increase (Esteban et al. 2020). The results of this experiment and Hearne et al. (2019) also show that declines in T. testudinum do not necessarily lead to increases in other macrophyte species or sessile animals. ...
While green turtles (Chelonia mydas) were once abundant throughout the Caribbean, over-exploitation has dramatically reduced their numbers. We conducted a 168-day simulated grazing experiment to determine how loss of this once-abundant mega-herbivore could have affected the productivity and community composition of Thalassia testudinum-dominated seagrass beds in Bocas del Toro, Panama. Simulated grazing reduced both percent cover and productivity of T. testudinum. High runoff and local pollution from industrial farming may limit light availability and reduce seagrass photosynthetic performance to replace biomass lost to simulated grazing. Other seagrass species and algae failed to colonize space opened by reductions in T. testudinum percent cover. Many plots subjected to simulated grazing were also bioturbated by stingrays. Relevance of these findings to balancing sea turtle and seagrass conservation efforts are discussed.
... Green turtles are the only herbivorous marine turtle, consuming primarily seagrasses throughout much of their global range (Bjorndal, 1980;Esteban et al., 2020). Recovery of green turtle populations offers the opportunity to understand their ecological roles as seagrass meadows are returned to a natural grazed state. ...
Populations of green turtles (Chelonia mydas), a megaherbivore that consumes seagrasses via cultivation grazing, are recovering worldwide. Information on plant-mediated effects on herbivore foraging behavior is critical to understanding plant-herbivore interactions and sustainability of grazing as ecosystems continue to change. In a Caribbean seagrass ecosystem, we use stationary cameras and benthic surveys to evaluate effects of seagrass morphology and leaf nitrogen content on green turtle grazing behavior. Thalassia testudinum leaf morphology has significant effects on forage intake (mg dry mass [DM] min-1) for green turtles, whereas leaf nitrogen content has no effect. Intake increases in grazed areas with shorter leaves and higher leaf biomass concentration (mg DM cm-3), indicating more efficient foraging under these conditions. Bite rate (bites min-1) increases in grazed areas with short leaves, a result of reduced search time. Bite size (mg DM bite-1) increases in grazed areas with short but dense canopies, because a turtle crops more shoots with each bite. Increased foraging efficiency and reduced search time in grazed areas with high biomass concentrations collectively maximize intake. Ingested leaves are shorter than the mean height of all available leaves in grazed areas, indicating herbivore selection for shorter leaves. Our estimate for daily intake is 86.1 g DM d-1 per 33-kg turtle. Our study provides a novel contribution on the effects of plant-level cues on the grazing behavior of a marine megaherbivore, and how cultivation grazing behavior optimizes the green turtle foraging strategy by maximizing foraging efficiency and intake.
... Indeed, early studies of green turtles in the Caribbean documented their near-exclusive reliance on turtlegrass (Thalassia testudinum; Bjorndal 1980), and many populations worldwide are known for their dependence on marine algae (Bjorndal 1997;Hirth 1997;Jones and Seminoff 2013). However, in recent years, it has become clear that green turtles' diets are much more diverse than previously described, and the species consumes large amounts of animal matter in several global regions (Amorocho and Reina 2007;Cardona et al. 2009;Esteban et al. 2020). ...
... For example, the aforementioned preponderance of turtlegrass in Caribbean green turtle diet is not surprising considering this marine angiosperm's once ubiquitous distribution (Larkum et al. 2006). Diet may also be influenced by a turtle's physiological capacity to digest food in the context of local temperature regimes (Esteban et al. 2020). Considering that the digestive efficiency for seagrasses in green turtles declines with lower water temperature (Bjorndal 1980), seagrass and other vegetal prey may be expected to feature less prominently in the diets of green turtles living in coolerwater foraging areas (Santos et al. 2015). ...
... In the context of developmental migration theory, typically once green turtles recruit to neritic waters they gradually shift to an herbivorous diet dominated by benthic foods such as seagrasses and/or macroalgae (Bjorndal 1997;Esteban et al. 2020); however, this does not appear to hold true in the southeastern Pacific. For example, at the cold Humboldt Current site (La Aguada) green turtle diets appear to specialize in animal matter, such as sea anemones (Paranthus sp.) and jellies (C. ...
Feeding strategies in sea turtles are among the most important aspects of their life history, influencing demographic parameters such as growth, age-at-maturity, and reproductive migrations. However, studying sea turtle diet is often challenging and knowledge about foraging ecology is lacking for most populations worldwide. We studied green turtles (Chelonia mydas) at two disparate sites in Peru: La Aguada (~ 14°S), an area with upwelling conditions, and Virrila Estuary (~ 5°S) with year-round warm conditions. We conducted (1) in-water capture to assess population size structure and (2) esophageal lavages to recover diet components from turtles at both sites. Diet composition and feeding strategy were evaluated using several analytical approaches, and environmental influence on diet was assessed in relation to the Peruvian Oscillation Index. Our results indicate substantially different life stages and diets at the two study sites. Green turtles at La Aguada were mostly juveniles consuming animal matter, whereas turtles at Virrila Estuary were mainly sub-adults with a diet dominated by vegetal matter. Our results suggest a life-history-based habitat use model for green turtles in the southeastern Pacific Ocean. We propose that juvenile green turtles initially recruit to more southern neritic habitats of Peru, feed on high-caloric animal matter, then as individuals grow, they transition northwards to feed on lower-caloric, but abundant, vegetal matter. Our data provide a framework for ontogenic-based developmental migrations by green turtles in this portion of the southeastern Pacific Ocean, helping policymakers on the need to implement management strategies.
... Despite the extensive information that is now available on the trophic ecology of green turtles (i.e., [13,40,41,61,62]), insights on their gut microbial community composition are still scarce (but see [9,63,64]). Here, we explored the microbial community composition from four GI tract compartments of green sea turtles (Tables S5 and S6) from Guinea-Bissau, and from their potential food items (Table S7) using 16S rRNA metabarcoding, while, at the same time, establishing microbial baselines for the different green turtle GI compartments and the diverse food item groups. ...
The fitness of the endangered green sea turtle (Chelonia mydas) may be strongly affected by its gut microbiome, as microbes play important roles in host nutrition and health. This study aimed at establishing environmental microbial baselines that can be used to assess turtle health under altered future conditions. We characterized the microbiome associated with the gastrointestinal tract of green turtles from Guinea Bissau in different life stages and associated with their food items, using 16S rRNA metabarcoding. We found that the most abundant (% relative abundance) bacterial phyla across the gastrointestinal sections were Proteobacteria (68.1 ± 13.9% “amplicon sequence variants”, ASVs), Bacteroidetes (15.1 ± 10.1%) and Firmicutes (14.7 ± 21.7%). Additionally, we found the presence of two red algae bacterial indicator ASVs (the Alphaproteobacteria Brucella pinnipedialis with 75 ± 0% and a Gammaproteobacteria identified as methanotrophic endosymbiont of Bathymodiolus, with <1%) in cloacal compartments, along with six bacterial ASVs shared only between cloacal and local environmental red algae samples. We corroborate previous results demonstrating that green turtles fed on red algae (but, to a lower extent, also seagrass and brown algae), thus, acquiring microbial components that potentially aid them digest these food items. This study is a foundation for better understanding the microbial composition of sea turtle digestive tracts.
... Sea turtles were found in association with reef structures or seagrass beds near the coast and islands (Fig 3B), but never in open waters. Green turtles were most often seen in and around seagrass beds, which likely constitutes the majority of their diet in this region [48]. Al-Muwayleh in particular was found to be an important foraging ground, and has extensive seagrass meadows and numerous sightings of feeding turtles. ...
The Red Sea is particularly biodiverse, hosting high levels of endemism and numerous populations whose extinction risk is heightened by their relative isolation. Elasmobranchs and sea turtles have likely suffered recent declines in this region, although data on their distribution and biology are severely lacking, especially on the eastern side of the basin in Saudi Arabian waters. Here, we present sightings of elasmobranchs and sea turtles across the north-eastern Red Sea and Gulf of Aqaba collected through a combination of survey methods. Over 455 survey hours, we recorded 407 sightings belonging to 26 elasmobranch species and two sea turtle species, more than 75% of which are of conservation concern. We identified 4 species of rays and 9 species of sharks not previously recorded in Saudi Arabia and report a range extension for the pink whipray ( Himantura fai ) and the round ribbontail ray ( Taeniurops meyeni ) into the Gulf of Aqaba. High density of sightings of conservation significance, including green and hawksbill sea turtles and halavi guitarfish were recorded in bay systems along the eastern Gulf of Aqaba and the Saudi Arabian coastline bordering the north-eastern Red Sea, and many carcharhinid species were encountered at offshore seamounts in the region. Our findings provide new insights into the distribution patterns of megafaunal assemblages over smaller spatial scales in the region, and facilitate future research and conservation efforts, amidst ongoing, large-scale coastal developments in the north-eastern Red Sea and Gulf of Aqaba.
... Over the last two decades, the use of SIA in marine turtle studies has been invaluable to elucidate their spatial, foraging, and trophic ecology (Reich et al., 2007;Pearson et al., 2017;Haywood et al., 2019) . For green turtles (Chelonia mydas, Linnaeus, 1758), regarding foraging ecology alone, SIA has helped us uncover just how heterogenous their feeding habits can be (Esteban et al., 2020). Within populations, heterogeneity of foraging habits can range from ontogenetic differences in diet (Burgett et al., 2018), to unexpected pelagic feeding of previously thought obligate herbivore adults (Hatase et al., 2006), to sexspecific trophic variation (Roche et al., 2021). ...
... In comparison with other populations, green turtles from this area are mostly herbivorous and completely specialized in marine food sources, given that their isotopic niche was lower on the d15N axis than those of both groups of carnivorous fish, and that the signatures of mangrove, consumed by other green turtle populations (Arthur et al., 2008;Esteban et al., 2020), are too distant to suggest any importance for the diet. ...
... According to the global review on green turtle diet by Esteban et al. (2020), all turtles, despite being isotopically segregated by foraging habitat, fit the category of turtles with macroalgae dominated diet (in this case rhodophytes and ochrophytes), where seagrass is relatively sparse or absent. Only the presence of a tracer species, the seagrass H. wrightii, allowed us to further discriminate the seemingly constant foraging segregation of this population. ...
Green turtles ( Chelonia mydas ) are highly dependent on neritic foraging areas throughout much of their life. Still, knowledge of recruitment dynamics, foraging habits, and habitat use in these areas is limited. Here, we evaluated how the distribution and food preferences of green sea turtles from different life stages varied within a foraging aggregation. We focused on two islands in Guinea-Bissau, Unhocomo and Unhocomozinho, using water captures and survey dives to record habitat use and characteristics, and stable isotopes to infer diet. Additionally, we used stable isotopes to infer their diet. Two habitat types were sampled: deeper (2.26 ± 0.4 m) rocky sites fringed by mangrove with macroalgae, and sandy shallows (1.37 ± 0.12 m) surrounded by rocky reefs with macroalgae and seagrass. The two benthic communities were similar isotopically and in terms of species composition, except for the presence or absence of seagrass, which had unique signatures. We captured 89 turtles ranging from 35 cm to 97 cm in curved carapace length (i.e., juvenile to adult stages). Size distribution was habitat-dependent, with most smaller turtles present in sandy shallows and larger turtles favoring slightly deeper rocky sites. Turtle isotopic signatures differed between the habitat of capture, regardless of size, revealing a marked dichotomy in foraging preference. All turtles fed primarily on macroalgae, mostly rhodophytes. However, individuals captured in sandy habitats had evident seagrass skewed isotopic signatures. Larger turtles may be unable to use the more diverse shallower foraging sites due to increased vulnerability to predation. Despite the proximity of the sampled foraging sites (2.7 km apart), the two foraging subgroups seem to maintain consistently different feeding habits. Our study highlights how heterogeneous green turtle foraging habits can be within populations, even at small geographic scales.
... Further, these models inform on temporal dietary switching, from historical diets (Conrad et al. 2018) to ontogenetic shifts (Reich et al. 2007;Vander Zanden et al. 2013;Howell et al. 2016;Burgett et al. 2018;Ferreira et al. 2018) to annual or seasonal patterns (Páez-Rosas et al. 2021). Geographically, stable isotope analysis has allowed scientists to link diet and foraging to specific and sometimes undiscovered habitats important to sea turtles (Lemons et al. 2011;Bradshaw et al. 2017;Hancock et al. 2018;Tomaszewicz et al. 2018;Fukuoka et al. 2019;Piovano et al. 2020) in addition to relating diet to oceanographic parameters such as sea surface temperature (Esteban et al. 2020). Finally, controlled studies have investigated isotopic incorporation rates and discrimination values in green turtles Vander Zanden et al. 2012). ...
... Seasonal variability in the isotopic niche in black turtles suggests that turtles captured in the non-upwelling season were consuming specialized diets where turtles captured in the upwelling season were all consuming the same range of dietary items. Dramatic seasonal upwelling events, that causes nutritional resources to be temporally transient (Stuhldreier et al. 2015) and decreases water temperature which may influence the proportion of animal vs plant material in turtle diets (López-Mendilaharsu et al. 2008;Esteban et al. 2020;Seminoff et al. 2021). In black turtles, we observed whole blood δ 15 N ratios that varied across seasons suggesting an inconsistency in the trophic level of prey ingestion throughout the year. ...
We used stable isotopes to investigate isotopic niche size, overlap, and diet composition in green (black and yellow morphotype Chelonia mydas; 50.0 to 95.0 cm curved carapace length, CCL) and hawksbill turtles (Eretmochelys imbricata; 38.5 to 83.0 cm CCL) in a recently described foraging habitat in North Pacific Costa Rica. We measured whole blood stable carbon (δ¹³C) and nitrogen (δ¹⁵N) ratios in black (n = 39; mean ± SD, − 16.54 ± 0.66‰ and 14.39 ± 0.77‰), yellow (n = 13; − 15.74 ± 0.65‰ and 12.37 ± 0.55‰) and hawksbill turtles (n = 13; − 16.23 ± 1.34‰ and 12.63 ± 0.32‰) and skin δ¹³C and δ¹⁵N values in black (n = 36; − 15.32 ± 0.79‰ and 15.16 ± 0.72‰), yellow (n = 12; − 15.38 ± 0.91‰ and 13.78 ± 0.75‰) and hawksbill turtles (n = 10; − 14.33 ± 1.49‰ and 13.77 ± 0.29‰). Isotopic niche space revealed distinctly higher δ¹⁵N area in black turtles and significant overlap between yellow and hawksbill turtles, and a recent shift in diet in yellow turtles from omnivory to herbivory. In black turtles, isotopic niche suggests individual specialization during the non-upwelling season and generalization in diet during the upwelling season. Mixing model results suggest that black turtles forage at multiple trophic levels (fish: 34.8 ± 10.1% of diet and macroalgae: 51.8 ± 12.8% of diet), while yellow and hawksbill turtles primarily forage on macroalgae (85.0 ± 6.6% in yellow turtles and 85.1 ± 5.9% in hawksbill turtles). These results add to a growing understanding that diet in sea turtles is influenced by diet items present in the environment and suggest that black turtles are potential tertiary consumers.
... Therefore, it is very likely that a potential degradation of foraging conditions due to MHWs would have an even greater impact on green turtles than the direct effects on their physiological rates (Stubbs et al. 2020). Decreased quality and abundance of seagrass habitats, which face globally widespread threats (Unsworth et al. 2019), could also influence the range of foraging areas, as in low density and fragmented seagrass meadows, sea turtles will probably have to expand their foraging home ranges (van de Merwe et al. 2009) or even shift to new foraging grounds, possibly altering their spatial distribution (Esteban et al. 2020). However, the enclosed nature of the geography of the region might act as an obstacle in possible latitudinal range shifts, potentially trapping green turtles or other marine species in South China Sea (as in Lasram et al. 2010). ...
Extreme regional ocean warming events, like marine heatwaves (MHWs), could have severe and long-lasting impacts on species and ecosystems. Extreme and persistent warming of the ocean could directly threaten survival of marine species, as exceeding their thermal tolerance often leads to massive mortality events. Similarly, MHWs could further threaten species persistence indirectly, by altering food webs, leading to cascading effects that are expected to be more pronounced for species at a lower trophic position. Green turtles, a representative species of the charismatic marine megafauna, are largely herbivorous; thus, their food availability is tightly linked to environmental conditions. Here, we explored the degree to which foraging areas of green turtles along the Southeast Asian region could be subjected to MHWs in the future. For this, we applied a series of climatic niche models to spatially delineate important foraging habitats for adult green turtles, Chelonia mydas, across the marine region of Southeast Asia. Our analysis revealed that marine sites, which could host foraging grounds for adult green turtles, cover around 37% of the Southeast Asian region, with high probability of experiencing prolonged and intense MHWs for the vast majority of these sites. The annual number of days subjected to MHWs could increase by 16-fold from the very recent past period, leading to even a permanent MHW state. These results offer some alarming messages for scientists and conservation planners, highlighting the need to improve our knowledge on the potential response of species to MHWs and design climate adaptation strategies.
