Mote Marine Laboratory

Parting Ways

By: Kirsten Weir

On a moonlit beach in Sarasota, dozens of loggerhead sea turtles are bursting from their nest. The 2-inch-long hatchlings wriggle through the wet sand and scurry toward the sea. Two hundred miles away, in Boca Raton, is a mirror image: the moonlight, the hatchlings, the mad dash to the sea. To a casual observer, the west coast hatchlings are indistinguishable from their east coast cousins. But there’s more to these turtles than meets the eye. Behavioral differences suggest the western and eastern hatchlings are in the process of diverging — possible evidence, says Michael Salmon, that we might just be witnessing the first step in a behavioral evolution.

Salmon is a research professor at Florida Atlantic University. He and his graduate students Jeff Guertin and Maria Merrill, along with collaborator Tony Tucker, manager of Mote Marine Laboratory’s Sea Turtle Conservation and Research Program, are taking a closer look at turtles from Florida’s southern coasts. They hope to understand precisely how these turtles differ, and what the distinctions mean for the threatened species.

The loggerhead, Caretta caretta, is a world traveler. After hatchlings leave their nests, they head out to sea and are carried by currents to the other side of the Atlantic. “They won’t return to the nesting beach for about 30 years,” says Tucker. As they enter their third decade, female loggerheads will journey back to the beach where they were born to lay eggs of their own. “This doesn’t mean they return to nest under exactly the same palm tree, but to the same general region,” he says.

The females mostly remain faithful to their natal beaches. As a result, the nesting population is made up of separate groups, or subpopulations, of females that cluster around certain geographic regions. The turtles in each subpopulation have distinct mitochondrial DNA (mtDNA), genetic material that is passed down only from females to their offspring. The mtDNA tells us that one unique subpopulation of loggerheads exists along the east coast north of Florida, another in the Florida Panhandle, and another across the southern Florida peninsula, both east and west.

It is here, in southern Florida, that things get interesting. Using the Earth’s magnetic field to guide them, turtles from the east coast paddle furiously eastward in order to meet up with the Florida Current and Gulf Stream. These currents will carry them to their nursery grounds on the opposite side of the Atlantic. If west coast hatchlings are to follow this same route, they must swi in a different direction to make their way around the Florida peninsula. “Hatchlings leaving the Gulf coast and the Atlantic coast have entirely different problems to solve, even though they’re both members of the South Florida subpopulation,” explains Salmon. “They have to orient in two different directions.”

Orientation is only half the battle; the turtles must also paddle furiously to reach the currents. After hatching, both east and west coast turtles have a 24-hour “frenzy” period in which they swim constantly. Over the next five days, though, west-coasters are much more active than east-coasters — presumably because they have farther to travel to meet up with the currents in question. It’s not yet known, though, what’s at the root of the activity differences.

To get at that answer, FAU Master’s student Jeff Guertin transplanted turtle nests from west to east and east to west. When the hatchlings emerged from their nests, he fitted some of the turtles into tiny harnesses tethered inside a seawater-filled plastic pool. The turtles were linked to monitoring equipment that recorded their swimming activity for six days. “The point is to try to figure out if the differences have an environmental or a genetic basis,” Guertin says.

If the turtles’ behavior is genetic, west-coast turtles should continue to swim more actively, even if they hatched on the east coast. If, on the other hand, their behavior is based on location, transplanted turtles should recognize where they are and adapt to their surroundings. In other words, transplanted west coast hatchlings would behave like east coast hatchlings, and vice versa.

After a summer of late nights and lots of gas burned driving back and forth across the state, Guertin is now in the process of analyzing his results. Regardless of what the findings show, he says, he’s looking forward to filling in a piece of the puzzle. “The results will be important for finding out how females establish subpopulations, how hatchlings imprint to different regions, and maybe even how their migratory behavior is shaped by differences in geography,” he says.

Meanwhile, Maria Merrill, another FAU Master’s candidate, is coming at the east-west problem from a different direction. Using a similar tether set-up, Merrill is investigating what direction the west coast hatchlings swim in an attempt to understand how natural selection has shaped their navigation skills. Using a homemade magnetic coil system, she alters the magnetic field that the turtles experience and records how they orient themselves as a result.

“I exposed hatchlings to magnetic waypoints that they could encounter during their migration,” she says.

Merrill hypothesized that west-coast turtles would orient themselves to swim west toward the Loop Current, which would carry them through the Florida Straits and eventually link up with the Florida Current and later the Gulf Stream. “I thought they’d find themselves in the same nursery habitats in the northeast Atlantic as the turtles from the southeast coast of Florida,” she says. “My results do not support that hypothesis.”

Merrill cautions that she is still completing the analysis and that her results are preliminary. But, she says, “so far the data suggest that these turtles might possibly stay within the Gulf of Mexico,” rather than traveling the currents to the far eastern Atlantic.

For now, though, that’s just one possible explanation for the results of Merrill’s early investigations; new ideas are likely to arise. “There are still a lot of unknowns as far as how turtles and other migratory animals make their way along these huge migrations,” she says.

Adds Tucker: “What we really need is a better way to track hatchlings in the wild. Perhaps one day the satellite technology that we use to track adult females will be able to be used with hatchlings.”

Despite the unknowns, the behavioral differences between east coast and west coast hatchlings makes Salmon suspect that the turtles are in the midst of a shift in behaviors and, possibly, the beginning of a subdivision of the South Florida subpopulation. “Maybe several thousand years from now, the mtDNA of the two groups might be different,” he said

The researchers’ work could have important implications for turtle conservation. “Part of the genetic diversity of the species is the mtDNA,” says Salmon. “If we’re going to properly conserve loggerheads, we have to figure out ways of preserving all the subpopulations.”

For now, the research is keeping all of the scientists on their toes.
“You don’t really know what to expect, but it’s exciting,” says Salmon. “It’s like being on a rollercoaster and not knowing whether it will stay on the tracks.”

“I exposed hatchlings to magnetic waypoints that they could encounter during their migration,” she says.

Merrill hypothesized that west-coast turtles would orient themselves to swim west toward the Loop Current, which would carry them through the Florida Straits and eventually link up with the Florida Current and later the Gulf Stream. “I thought they’d find themselves in the same nursery habitats in the northeast Atlantic as the turtles from the southeast coast of Florida,” she says. “My results do not support that hypothesis.”

Merrill cautions that she is still completing the analysis and that her results are preliminary. But, she says, “so far the data suggest that these turtles might possibly stay within the Gulf of Mexico,” rather than traveling the currents to the far eastern Atlantic.

For now, though, that’s just one possible explanation for the results of Merrill’s early investigations; new ideas are likely to arise. “There are still a lot of unknowns as far as how turtles and other migratory animals make their way along these huge migrations,” she says.

Adds Tucker: “What we really need is a better way to track hatchlings in the wild. Perhaps one day the satellite technology that we use to track adult females will be able to be used with hatchlings.”

Despite the unknowns, the behavioral differences between east coast and west coast hatchlings makes Salmon suspect that the turtles are in the midst of a shift in behaviors and, possibly, the beginning of a subdivision of the South Florida subpopulation. “Maybe several thousand years from now, the mtDNA of the two groups might be different,” he said

The researchers’ work could have important implications for turtle conservation. “Part of the genetic diversity of the species is the mtDNA,” says Salmon. “If we’re going to properly conserve loggerheads, we have to figure out ways of preserving all the subpopulations.”

For now, the research is keeping all of the scientists on their toes.
“You don’t really know what to expect, but it’s exciting,” says Salmon. “It’s like being on a rollercoaster and not knowing whether it will stay on the tracks.”

Learn more about: Sea Turtle Conservation and Research

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