When exposed to certain levels of PCBs, turtles suffer from stunted growth and weak bones, found a new study.
With low bone-density, turtles have a tougher time diving, swimming and chewing their food. And the same is probably true for fish and other wild animals that are exposed to the ubiquitous environmental pollutant.
It's too soon to know what the results mean for people, and our level of exposure is probably much lower than what some turtles experience. But there is reason to believe that the shelled creatures might be sentinels for human health.
"There have been a few published studies done in Nordic countries that correlated individuals who consume a lot of fish with increased incidence of hip fractures," said Dawn Holliday, a physiological ecologist at Westminster College in Fulton, Missouri.
"Turtles make and break bone through a very similar process that mammals and humans do," she added. “If humans were exposed at similar concentrations, it might be relevant.”
PCBs, which were once commonly used in pesticides and a wide variety of industrial fluids, have been banned for decades. But with very slow breakdown times, they are still widespread in the environment, particularly in more developed areas.
In fish, birds, minks and other creatures, PCBs have been linked with slower growth rates, particularly in young animals and developing embryos. Studies have also connected exposure to the chemical with tumors in mink jaws and deformed heads in zebrafish. Exposed adult animals tend to suffer from immune system and related problems.
Holliday and colleague Casey Holliday, who is also her husband, had previously found that PCB exposure caused the metabolisms of turtles to slow down, making them sluggish and less able to turn their food into energy.
To see if turtle bones, too, might be at risk from chemical exposure, the researchers collected diamondback terrapin eggs from the wild, hatched them in the lab, and raised them on frozen brine shrimp.
After eight months to ensure the animals were growing normally, the scientists injected some of the turtles with a dose of PCB 126 that was equivalent to what they might encounter in an urban lake or river. Other turtles got a placebo injection.
Six months later, the researchers reported in the journal Aquatic Toxicology, the shells of the PCB-injected group had grown just one millimeter (0.04-inch) in length, compared to 10 mm (0.4 inches) in the uncontaminated turtles.
"It was such a big difference that you could see it," Dawn Holliday said. "You didn't have to get out a ruler to measure them."
The PCB group also had widespread patches of bone that were not mineralized, while the pollution-free turtles had strong, hard bones. Like dioxins and BPA, PCBs seem to interfere with hormone activity, Holliday said.
The chemicals also appear to disrupt cell growth and differentiation, said Don Tillitt, an environmental toxicologist with the United States Geological Survey in Columbia, Missouri.
Linking illnesses and deformations in wild animal with specific chemicals can be extremely difficult to do, Tillit said. Lab studies like this one add important details that can eventually help scientists both solve and prevent environmental mysteries.
"Every time they make progress in better understanding the adverse effects of chemicals like PCBs, and PCBs in particular," he said, "it quite often helps with ecological risk assessments and how we set regulatory limits for cleaning up sites."
The study is also a reminder that animal deaths aren’t the only sign that something is wrong in the environment.
"When we see effects like this, we know there are things that are maybe more insidious," Tillit said. "It's a good reminder that we have to be on guard."