Dining with the Hawksbills: Discovering their Dietary Secrets

Uncovering the mystery to Golfo Dulce’s sea turtles

Did you know that the waters surrounding the Maritimes, including our Bay of Fundy, host four different species of sea turtles? If you didn’t, you’re not alone. It wasn’t until the 1990s that researchers began to understand the range of these marine reptiles. However, many sea turtle populations around the world are currently in decline, making it crucial to continue uncovering the mysteries surrounding these magnificent creatures. The decline of sea turtles in the Maritimes is not unique, our southern friend, the Hawksbill turtle is also experiencing some trouble.

So, who are the Hawksbills? The Hawksbill turtle is one of the most important marine species in the world! Unfortunately, their habitat—Golfo Dulce in Costa Rica—is at risk and with it, so are the Hawksbills, who are considered critically endangered. These turtles play a crucial role in maintaining the balance of the oceanic ecosystem; their survival is vital for the health of their marine environment. Sea turtles maintain seagrass beds, coral reefs, beach dunes, and prevent beach erosion. What makes the Hawksbill turtle in Golfo Dulce so important and different?

A recent study led by Dr. Estefania Méndez-Salgado at the National University of Costa Rica investigated how Hawksbill turtles fit into the food web of the Golfo Dulce area. Méndez-Salgado and their colleagues combined two methods to learn more about the diet of these turtles: esophageal lavage and stable isotope analysis. 

As complex as the two methods sound, they work on some pretty simple ideas. Esophageal lavage is basically just rinsing the inside of the turtle’s mouth and esophagus with water, and then seeing what material comes out. It is not easy to simply see what they ate based on what a turtle has spat up, but with some keen skills, patience, and a microscope, the research team was able to identify a range of different species that the turtle was eating. Imagine trying to reconstruct a list of all the things you ate in a day by just having a long look at the material that you flossed out of your teeth.

The second method, stable isotopes analysis, takes a more chemistry-based approach to determining a species’ diet. This form of analysis looks at the different signatures of a chemical that make up food items and how an animal’s body incorporates them into its own tissues. Looking at nitrogen isotopes allows researchers to gauge how far up the food chain an organism is because nitrogen is an important component of proteins. Carbon isotopes provide information on what type of organism, for example, plants, sets the base of the food web where that organism feeds. 

Méndez-Salgado’s work found that the Hawksbill turtles from Golfo Dulce feed on different species of macroalgae and marine invertebrates, like snails, worms, and jellyfish, as well as mangrove, seagrasses, and sponges. This seems like quite the buffet! However, it was the stable isotope analysis that provided some of the most intriguing data. The researchers uncovered that sponge species, which are rich in both carbon and nitrogen, were a critical part in the Hawksbill’s diet, but they also secondarily fed on snails and polychaete worms. These worms were likely important nutrient-rich dietary alternatives where sponges are not abundant. 

The fact that both marine macroalgae and invertebrates were being eaten in great abundance suggests that this particular Hawksbill turtle population has adopted a more generalist foraging strategy. Dr Jeffrey Seminoff, from the National Marine Fisheries Service and co-author of this research, says that this was a particularly fascinating discovery because it “illustrates their adaptive lifestyle and ability to live in many different types of habitats.” This is good news for a species that has been suffering widespread population decline around the world. 

But what do these findings really mean? Why do they matter? 

It seems it comes down to what role these sea turtles play in their ecosystem. It turns out that this critically endangered marine reptile is a keystone species. “Their presence helps maintain balance within the ecosystem,” says Seminoff. “If removed, sponges would likely start to overtake important reef habitats and alter the ecosystem structure, which would have negative impacts on other local species.”

Essentially, if we were to lose the Hawksbill turtle, we could expect significant reef damage and possible ecosystem collapse. Not good, given how we know their numbers are dwindling. 

Research into what species like the Hawksbill turtle eat and how they fit into food webs is helping us understand the pressing need to improve our conservation efforts for these types of animals. Seminoff reinforces this idea by acknowledging that “understanding Hawksbill habitat use and diet can help managers know what areas and habitats need protection.” 

The future of Hawksbill turtles in Golfo Dulce will depend on the collective efforts of researchers, conservationists, and the public. By working together and applying the cutting-edge techniques used in this study, there is a hope that we can apply these techniques elsewhere and ensure the survival of this magnificent species and many others. We can use the knowledge learned from Golfo Dulce at home too, here in the Bay of Fundy! We can gain an understanding of the species that inhabit our waters and take steps to protect them too. The research into the Hawksbill turtle habitat use and diet can be applied to understand and preserve our own sea turtle species.  

To learn more, check out: Trophic ecology of hawksbill turtles (Eretmochelys imbricata) in Golfo Dulce, Costa Rica: integrating esophageal lavage and stable isotope (δ13C, δ15N) analysis by Méndez-Salgado et al. (2020)! https://doi.org/10.3856/vol48-issue1-fulltext-2230

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