Study Sheds New Light on Biology of Nesting in World’s Largest Sea Turtle Species

The largest-ever study of blood proteins in leatherbacks — Earth’s largest sea turtle — was released recently by Mote Marine Laboratory, revealing some of the fascinating biology behind one of nature’s most demanding life cycles.

Leatherbacks feed on jellyfish — nature’s low-calorie Jell-O — but they eat enough of them to power a migration thousands of miles long, from feeding grounds to nesting beaches. Scientists initially believed the turtles had to feed at their nesting sites too, but recent physiology research by others has suggested that leatherbacks nesting in French Guiana are fasting, living on stored energy.

This new study, led by a Mote scientist and published in the peer-reviewed journal Conservation Physiology, used the most detailed analysis of blood proteins in leatherbacks to date to confirm that another population at St. Croix in the U.S. Virgin Islands fasts while nesting, strengthening the emerging picture of the leatherback life cycle. More results also show how fasting and migration may affect the turtles’ bodies. These findings provide vital information for resource managers seeking to protect this endangered species.

“It’s important to know where and when leatherbacks are getting their energy and how they’re using it — that has bearing on which threats they face and when they are most vulnerable,” said Dr. Justin Perrault, a Mote Marine Laboratory Postdoctoral Research Fellow. Perrault led the study as a doctoral student at Florida Atlantic University with collaborators from the University of Georgia.

The researchers collected 217 blood samples from 76 leatherbacks that crawled ashore to nest during 2009 in St. Croix, after their migration from feeding grounds in the North Atlantic. The scientists separated out the serum — the liquid surrounding blood cells — and analyzed it for proteins called albumins and globulins. Their results revealed the total amount of proteins, the percentage of each type and the ratio of albumins to globulins. These measures help reveal whether the turtles are feeding, how healthy they are and whether they have problems including injuries or inflammation.

The total amount of protein in the turtles’ blood declined during nesting season, suggesting that the turtles were eating little to nothing – they were burning through fat stores and beginning to use up their bodies’ protein for energy.

“It had been debated for years whether leatherbacks needed to feed at the nesting ground, and within the past decade, evidence has started to accumulate that most sea turtle species are fasting or feeding very little while nesting — our study confirms that pattern for this population,” Perrault said.

The researchers also found that many turtles stopped nesting when their total proteins hit a certain low point (3.5-4.5 grams per deciliter).

“This protein ‘threshold’ could be an important signal for researchers to use in the future — it could allow us to know from bloodwork that nesting is ending and feeding is ready to start again,” Perrault said. “It’s exciting to find physiological changes that relate to the turtles’ behavior — it allows us to understand what they’re doing in the water even if we can only observe them for a short time on land.”

Another intriguing finding was that proteins called gamma (γ)-globulins, which signify immune system activity, started out high and decreased over the nesting season.

“We think that leatherbacks might be exposed to all kinds of pathogens on the journey to their nesting beach, and this could mean they start the nesting season with an active immune system that naturally declines as they nest,” Perrault said. “But some of the decline over the season might also relate to manmade pollutants in coastal waters.”

During the study, an oil refinery was operating in the area, producing contaminants that could have affected the turtles’ immune systems, Perrault said. The refinery has since closed, but Perrault suggests that future studies should examine the possible immune effects of contaminants more closely.

Perrault continues to study sea turtle physiology, focusing on the effects of toxic substances, through a Mote Postdoctoral Fellowship. These fellowships, supported through philanthropic donations, are offered by Mote Marine Laboratory to support young researchers doing outstanding work early in their careers. A key part of Mote’s mission is to foster the next generation of marine scientists.