This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Decades after polychlorinated biphenyls (PCBs)—once commonly used in pesticides, electrical transformers and coolants—were banned by the U.S. Environmental Protection Agency, the toxic chemicals continue to linger in our country's soil and water. A new study shows how exposure to these PCBs can weaken turtle bone and shell density and suggests they could also continue to affect humans.
Married researchers Dawn Holliday of Westminster College in Fulton, Mo., and Casey Holiday of the University of Missouri School of Medicine came to their conclusions after studying the effect of PCB 126 on juvenile diamondback terrapins (Malaclemys terrapin). Their study was published September 21 in the journal Aquatic Toxicology and is also available on Casey Holliday's web site.
"We chose to work with PCBs because of their ubiquitous presence in the environment (including Chesapeake Bay), slow breakdown and documented effects in other species, including turtles," says Dawn Holliday. "Diamondback terrapins are a good indicator species because they are long-lived, eat snails and clams, and generally remain within the area where they hatched." The researchers also knew from a previous study that they could collect a robust sample of terrapin eggs from the Patuxent River near Chesapeake Bay in Maryland "without having a large impact on the wild population."
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The Hollidays collected clutches of terrapin eggs from the wild and then incubated the eggs at a temperature that produced only males (the gender of many reptiles is determined by temperature). They then injected some of the 44 juvenile turtles that hatched with PCB 126 and the others with a placebo. The PCB 126 dose was higher than has been found in terrapins in nature but lower than that found in the livers of common snapping turtles (Chelydra serpentina).
Six months after the injections, the PCB-injected turtles were noticeably smaller than their counterparts, with shell growth of just one millimeter in length compared with 10 millimeters in the turtles that received placebos.
The effects inside their bodies were also striking. All 44 turtles were euthanized and their bones were tested using several different imaging technologies. The ones exposed to the PCB had dramatically lower bone density and more juvenile features than they should have had at that age.
The authors say that in the wild such poor development could affect turtles' ability to dive for prey and cause females to lay eggs with thinner shells.
They also say this study suggests PCBs could still be a threat to human health. "People are high on the food chain, and thus more susceptible to accumulation of PCBs," Casey Holliday said in a prepared statement. "Smaller animals ingest PCBs. As large animals eat these smaller animals, the chemical stays in the food chain, as it is deposited in fat cells and doesn't leave the body. Even people not directly exposed to extreme amounts of PCB may see effects; this research will help us understand these effects better."
Diamondback terrapins are not a federally protected species, but they are on the decline throughout their habitat in the eastern U.S. because of a history of overhunting (they used to be prized as delicacies) and the ongoing destruction of their marshy habitats. Massachusetts protects them an as endangered species. Rhode Island lists them as threatened. They are also listed as a "species of concern" in Alabama, Delaware, Georgia, Louisiana, North Carolina and Virginia.
Photos 1: A healthy six-week-old diamondback terrapin photographed prior to the study. Photo 2: Bone-density scans of healthy and PCB-tested terrapins. Courtesy of the University of Missouri–Columbia