Hybridization in a shell: conservation and sea turtle hybrids

By Rachel Lo @rachelnlo

Good news for hybrid sea turtles: studies show that they are “not obviously disadvantaged.”

Hybridization is the crossing of two genetically distinct species.  Though the definition of a “species” is vigorously contested by biologists, in general two separate species do not interbreed under usual circumstances, a result typically of geographic isolation or reproductive incompatibility.

In 2006, scientists from Projeto TAMAR, a Brazilian sea turtle conservation organization, looked at a population of 119 hawksbill sea turtles in the state of Bahia, Brazil.  The sea turtles, which were females that came to the Brazilian beaches to nest, looked morphologically like hawksbill sea turtles at first glance.

But when the scientists analyzed the hawksbill turtles’ mitochondrial DNA, which is inherited in animals through mothers, they found evidence of loggerhead maternity in 42% of the 119 turtles studied.  That is, those hawksbill sea turtles had loggerhead mothers – they were actually hawksbill-loggerhead hybrids.  A 2010 study in Sergipe, Brazil, which neighbors Bahia, also found 14 of 51 morphologically identified loggerhead females to have olive ridley mitochondrial DNA – 27% of these loggerheads were actually olive ridley-loggerhead hybrids.

The states in Brazil where researchers found evidence of sea turtle hybridization: Bahia (in red) and Sergipe (in green).

Hybridization in sea turtles had previously been reported, but never at such high rates, in Brazil or elsewhere in the world.

The extensive occurrence of sea turtle hybrids raised significant conservation questions.  Loggerhead and olive ridley sea turtles are classified as vulnerable by the IUCN; hawksbills are critically endangered.  What are the causes for these unprecedented, high hybridization rates, and are the hybrids here to stay?

The nesting grounds and nesting seasons of loggerheads, hawksbills, and olive ridleys overlap in Bahia and Sergipe, allowing the three species to interact.  Severe population declines across the sea turtles in Brazil, prior to conservation efforts in the 1980s, may also have contributed to the surge in hybridization.

Whether the hybrids will persist in the long-term is wholly another question – and a complex one at that.  Though there is evidence that sea turtle hybrids can live through adulthood, knowledge on other parts of the hybrids’ life histories in comparison to non-hybrids – migration patterns, life expectancy, reproductive output – is lacking.

The role of hybridization as a natural evolutionary process, including how and why it occurs, is not well understood to scientists.  Hybridization can sometimes lead to benefits in the form of hybrid vigor, when hybrids receive the best traits from parent species.  Evidence of hybridization in two highly invasive species of pythons in the Everglades is perhaps a modern example of hybrid vigor.

In many other cases however, hybridization can lead to lower fitness – that is, hybrids may not be as fit to survive as their parental species.  Intersterility, loss of genetic integrity, and other such problems can lead to a negative view of hybrids.  For the Brazilian sea turtle hybrids, scientists are trying to elucidate any such effects.

Researchers from the University of Florida, in collaboration with Projeto TAMAR, genetically analyzed over 5,000 tissue samples of sea turtles in Bahia.  The samples came from nesting adult females and their progeny, including viable hatchlings (those that had hatched from the nest) and nonviable hatchlings (those that had died or else were undeveloped embryos).

A nonviable sea turtle embryo that scientists used to extract DNA to determine what species or hybrid status the sample belonged to. No sea turtle hatchlings were sacrificed in the sampling process.

The researchers found that some of the nesting adult females could successfully reproduce – their eggs were viable, capable of producing surviving hatchlings, and the number of eggs laid by loggerhead-hawksbill hybrids were similar compared to pure loggerheads and pure hawksbills.  Moreover, there were no differences of hatchling viability for hybrids versus non-hybrid hatchlings.  Put another way, hybrid hatchlings were not any less fit to survive compared to “pure” hatchlings, at this nascent stage in sea turtle life.

Though further studies need to be done on sea turtle hybrid adulthood, these results have interesting implications for conservation.  The Brazilian sea turtle hybrids don’t appear to be any worse off than their non-hybrid counterparts, but what does this mean for conservation status?

Hybrids are not included on the IUCN Red List.

In the United States, hybrids are also not addressed by the Endangered Species Act.  North American gray wolf-coyote hybrids have illuminated the need for conservation policies to focus also on hybrids.

Some conservation scientists have advocated for conserving ecological roles, rather than species.  If hybrids fill a niche formerly filled by their parent species, those hybrids should be included in a conservation approach, they argue.

There is evidence to suggest that loggerhead-hawksbill hybrids forage in areas similar to loggerheads, but not hawksbills.  Hawksbills consume mainly sea sponges.  In contrast, loggerhead diets consist primarily of mollusks and crustaceans – shelly organisms that can be crushed by loggerheads’ powerful jaws.

For now, the scientists of the University of Florida study argue that “different conservation standards for hybrids would create an imminent threat to the endangered hawksbills, loggerheads and olive ridleys.”  Only time will tell for certain if the hybrids are here to stay for good.

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