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The voyages of sea turtles

Genetics help delineate populations of endangered species

Rainer von BrandisIn the water they are discrete, and on land they seem far too vulnerable as they drag themselves across the sand, in search of a place to lay eggs. In the sea, hawksbill turtles are at home. Gliding underwater, they can migrate from a rookery in the Seychelles, in the Indian Ocean, to feed in the Fernando de Noronha islands of Brazil, in the Atlantic. The two locations are at very similar latitudes, but traveling from one to the other requires circumnavigation of Africa, a task these turtles perform as naturally as going from the bedroom to the kitchen, even if only rarely. Geneticist Sarah Vargas, a professor at the Federal University of Espírito Santo (UFES), wants to better understand these trajectories in order to pinpoint areas that might help prevent the extinction of the species, which is classified as critically endangered by the International Union for Conservation of Nature (IUCN). The researcher’s work has already produced some good news: hawksbill turtles, famous for laying their eggs in the same place they were born, seem to have some flexibility in their routes, according to an article to be featured on the cover of an upcoming issue of the Journal of Heredity.

During the year she spent in Australia in the laboratory of evolutionist Nancy Fitz-Simmons, then at the University of Canberra, Vargas analyzed DNA from skin samples of hawksbill turtles collected at 13 different nesting grounds along the Indian and Pacific Oceans, a region known as the Indo-Pacific: in Iran, Saudi Arabia, the Seychelles Islands, the Chagos Islands, Malaysia, Australia, and the Solomon Islands. “The populations in this region had not been characterized,” says Vargas, which was conducting doctoral studies at the time at the Federal University of Minas Gerais, advised by geneticist Fabrício Santos. “When we sample turtles that have been trapped in fishing nets in the Atlantic, we have no way of knowing where they came from if we don’t know the largest possible number of populations.”

As studying these animals’ ecology is a task offering more challenges than satisfactions (radio tracking transmitters can detach from a turtle’s shell within a few months), genetics presents itself as the feasible option. In 2015, already working as a professor at UFES, Vargas returned to Australia as a postdoctoral grant recipient under the Science Without Borders program, run by the National Council for Scientific and Technological Development (CNPq). There, she spent 10 months at the University of Sydney analyzing data in the laboratory of computational evolutionist Simon Ho. Her results debunked the dogma that all turtles return to the beach of their birth in order to reproduce, by showing the existence of a genetic mix of maternal lineages from different origins, and also indicated that the separations between populations are more complex than once thought.

As there are no obvious geographic barriers in the ocean, the distance between rookeries (breeding sites) is considered the main element in separation between populations – 500 kilometers would be enough to guarantee genetic differentiation. But that is not quite what the data showed. Not always, at least. Turtles that lay eggs in two areas along the Iranian coast, about 200 km apart, seem to come from separate populations, but each is in reproductive contact with Saudi Arabia, on the other side of the Persian Gulf. “We believe this is due to each rookery in Iran being closer to Saudi Arabia than to each other,” Vargas explains. Surprisingly, the opposite situation can also be true: two areas located 800 km apart in Australia are genetically homogeneous. “There should not be any genetic exchange between these two populations, as one of them shows peak nesting in the summer and the other in winter and spring,” says Vargas. “This temporal difference should work as a reproductive barrier.” A possible explanation is that the separation between the two populations is recent and not yet reflected by their mitochondrial DNA, the type of genetic material analyzed in the study. Mitochondrial DNA can provide an “eyewitness account” of ancient history.

Among the 13 sampled locations, the results identify eight areas that must be represented in regional units of conservation management, if the different lineages of hawksbill turtles in the Indo-Pacific are to be maintained. Vargas is now planning to characterize the populations of the Parnaíba delta, between the states of Piauí and Maranhão in Brazil, studying both hawksbill turtles (Eretmochelys imbricata) and leatherback turtles (Dermochelys coriacea) to find out where they came from and how long they have been in the region.

Other studies, like the one published in 2014 in PLOS One by oceanologist Maira Proietti, a professor at the Federal University of Rio Grande (FURG), show that many hawksbill turtles that feed in Brazilian waters were born locally, mainly in the states of Bahia and Rio Grande do Norte – but not without exchanges with the Caribbean and Africa. The group analyzed samples from 157 young turtles collected in feeding areas in the Caribbean and Brazil – including the Brazilian coast between the states of Rio Grande do Sul and Ceará and the archipelagos of Fernando de Noronha and Saint Peter and Paul, located respectively about 350 km and 1000 km from the coast. They detected a moderate level of homogeneity, although some genetic structuring associated with marine currents could also be discerned. “The currents seem to influence how these animals disperse in the ocean,” says Proietti, who compared the genetic data to tracking data from drifting buoys released at sea by international projects.

The population of origin could not be identified for two of the samples. “Characterizing additional areas is very important to improve the resolution of the analysis,” says the researcher. Proietti does not discard the possibility that these turtles could have come from distant areas, like those studied by Vargas. She emphasizes the importance of mapping the connections between areas, as the ecological impacts suffered by one location can affect animals many miles away.

Genetics may serve as the basis to outline management plans that are important not only for the turtle’s protection, which is carefully handled in Brazil by the Tamar Project, but also due to the ecological importance of these animals. Hawksbill turtles feed on sponges, anemones, squids and shrimp, helping to maintain healthy coral reefs by controlling the populations of these organisms. Although hawksbill turtles are still killed in some places in the world – for their shells, for food, by accidental capture in fishing nets or by ingestion of garbage released into the sea –, they are protected by law. It has become illegal to manufacture combs from their shells, as was once common.

Scientific articles
VARGAS, S. M. et al. Phylogeography, genetic diversity, and management units of hawksbill turtles in the Indo-Pacific. Journal of Heredity. Online. November 27, 2015.
PROIETTI, M. C. et al. Genetic structure and natal origins of immature hawksbill turtles (Eretmochelys imbricata) in Brazilian waters. PLOS One. V. 9, No. 2, e88746. February 2014.