From the moment they are born, sea turtles fight to survive. Buried alive, they dig themselves out and evade hungry crabs and birds as they crawl to the ocean, where they begin a long and treacherous migration. One out of 1,000 will survive into adulthood. And those that do will bear a toxic burden.
Scientists are discovering that sea turtles, long ignored by toxicologists who study wildlife, are highly contaminated with industrial chemicals and pesticides.
Loggerhead turtles have altered immune systems and smaller eggs that some studies have linked to contaminants. These chemicals kill turtle cells in lab experiments, and based on research in other marine life, scientists suspect that sea turtles may be vulnerable to thyroid, liver and neurological damage.
No one, however, knows the extent to which sea turtles in the wild may be harmed.
While other ocean creatures, including whales, seals and some fish, are well-studied, the chemical threats to sea turtles remain mostly hidden under a shell.
Decimated by climate change, poaching, accidental snaring and ocean trash, all U.S. species of sea turtles are protected by the Endangered Species Act, which makes studying them difficult.
"We really have just barely touched the tip of the iceberg," said Jennifer Keller, a marine biologist at the Hollings Marine Laboratory in Charleston, S.C. She is the top expert on pollution in sea turtles.
Sea turtles have some industrial compounds in their blood nearing levels that damage marine mammals. Keller's lab last year measured perfluorochemicals (PFCs) in the blood of five sea turtle species off the southeastern U.S. coast, and her calculations suggest that the turtles' potential risk for toxic effects is high.
Other long-lived chemicals also contaminate sea turtles, including polychlorinated biphenyls (PCBs), which are widely used industrial compounds banned in the late 1970s, and brominated flame retardants.
"These chemicals may not acutely poison the animals, but they can make exposed individuals a little more vulnerable to opportunistic infections or new and emerging infectious diseases," said Peter Ross, a research scientist in Canada who is one of the world’s leading experts on marine mammals and contaminants.
Some chemicals, particularly PCBs, have been shown to suppress the immune systems of wildlife, contributing to mass die-offs of seals and other marine mammals in the late 1980s and early 1990s.
This catastrophic damage hasn't been seen in turtles. Pollutants in their blood are orders of magnitude lower than levels in marine mammal blood, Keller said.
Nevertheless, Keller’s research with loggerheads caught off South Carolina has linked immune system changes to a variety of contaminants. As levels of chlordane and mirex – two pesticides banned in the United States decades ago but still in the environment – went up in the turtles, their production of some disease-fighting immune cells decreased. And as PCBs and the pesticide DDT went up, some of their immune cells increased.
"Any alteration of immune function, even enhancement, can be considered an adverse effect," the authors wrote. Enhanced immune responses can lead to autoimmune diseases and hypersensitivity.
"Exposure to even relatively low concentrations of persistent pollutants can reduce the effectiveness of immune defense against any number of pathogens," Ross said.
The chemical soup inside turtles comes from the food they eat, which varies from the crab-eating Kemp's ridleys, jellyfish-eating leatherbacks, omnivorous loggerheads, spongivorous hawksbills and herbivorous green sea turtles.