Dietary plasticity linked to divergent growth trajectories in a critically endangered sea turtle

Foraging habitat selection and diet quality are key factors that influence
individual fitness and metapopulation 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 versus Florida (USA), through novel integrations of
skeletal growth, gastrointestinal content (GI), and bulk tissue and amino
acid (AA)-specific stable nitrogen (δ15N) and carbon (δ13C) isotope
analyses. We also used AA δ15N ΣV values (heterotrophic bacterial
re-synthesis index) and δ13C essential AA (δ13CEAA) 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 δ15N-based trophic position
estimates internally indexed to baseline N variation. However, regional
differences in estimated trophic position may also be driven by unique
isotope dynamics of sponge food webs. AA δ15N ΣV values and
δ13CEAA fingerprinting indicated minimal bacterial resynthesis of
organic matter (ΣV < 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 conventional 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.