Modern Genetics Reveals Ancient Diversity in the Loggerhead

A loggerhead turtle surfaces to take a breath in the Mediterranean Sea. © Kostas Papafitsoros

A loggerhead turtle surfaces to take a breath in the Mediterranean Sea. © Kostas Papafitsoros

By Brian J. Hutchinson and Dr. Peter Dutton

What makes a loggerhead turtle a loggerhead turtle? Over the years, “loggerhead” has meant many different things. At one time, the term “loggerhead” included the Ridley turtles. More recently, it encompassed two different subspecies (Caretta caretta caretta and Caretta caretta gigas).

Owing largely to modern genetics, the identity of loggerhead turtles is no longer ambiguous. We now know that the loggerhead turtle, Caretta caretta, is in fact a single species. But what does this mean for the loggerhead—and for its conservation and management? Life is certainly not simple, and much like the loggerhead’s genetic structure, our knowledge of loggerhead genetics is continually evolving.

Using mitochondrial DNA sequence analysis, scientists have been able to identify different genetic stocks that represent rookeries or groups of rookeries that are genetically isolated from each other, as detailed in the inset on the following page. These studies have also traced the relationships among different stocks and have proposed scenarios of how the different lineages have evolved. Today’s lineages can all be traced back to a common ancestral stock that existed approximately 3 million years ago, when the Atlantic Ocean was separated from the Indo-Pacific Ocean by the formation of the Panama Isthmus.

Since then, geographic, environmental, and biological forces (natal homing, for example) have shaped dispersal patterns, extinctions, and recolonization events. All of these events have left clues in the loggerhead’s DNA, and these continue to be revealed through ongoing studies. The global genetic diversity that we now see in loggerhead turtles has evolved over the last three million years as they adapted to changing climate and geography to colonize new rookeries, while other rookeries disappeared. This genetic diversity is reflected in the current population structure as we understand it. Although we know a great deal about loggerhead genetics, a complete understanding will require new research that targets the many genetically unknown populations.

Genetic markers have also been used as tags to map transoceanic migrations and to uncover the linkages between foraging areas— enabling researchers to identify the stock origin of turtles that are caught incidentally by fishers on the high seas and in coastal areas. These studies have revealed a great deal. Loggerheads born of the North Pacific (Japanese) stock, for example, spend part of their lives some 7,000 miles away off the coast of Baja California, Mexico. Similarly, studies suggest that loggerheads of the eastern Australian stock may inhabit waters off Peru and Chile—more than 10,000 miles from their natal shores. In the Atlantic, juvenile loggerheads born on the shores of the United States venture into the Mediterranean and eventually return to reside in coastal U.S. waters. New genetic data suggest that these larger juveniles generally take residence near their natal beaches, where the females return to nest as adults.

The most important fact that we have learned in the field of loggerhead genetics is this: conserving the loggerhead is less about protecting the species at the global scale and more about focusing on the survival of each of the many distinct parts that make up the loggerhead species. After all, genetic diversity is the key to a species’ ability to adapt and persist through environmental change—a truth that bears even greater poignancy amid today’s changing climate.