DIAMONDBACK TERRAPINS ARE SENTINELS OF THE SALT MARSH

by Whit Gibbons

October 16, 2011

My admiration for diamondback terrapins began in 1983, when we initiated a terrapin study in the salt marshes surrounding Kiawah Island, S.C. Since then teams of researchers and students have measured and marked hundreds of the animals, before releasing them where they were caught. Restricted to the narrow brackish region between saltwater and fresh, from Cape Cod to Texas, terrapins are the only turtle in the world to live exclusively in coastal salt marshes. Sea turtles live in the ocean; freshwater turtles inhabit inland waters. Of the world's approximately 300 species of turtles only a few in Southeast Asia, India, and New Guinea enter brackish-water habitats and all of these also inhabit fresh water. Terrapins possess a salt gland not found in other nonmarine species that allows them to excrete salt to maintain a balanced physiology.

Diamondback terrapins are my favorite turtle, with the dreamy eyes of a baby harp seal and the unassuming face of a manatee; the docile temperament of a dolphin and the beauty of a seashell. For extra credit include perseverance in the face of natural hazards such as coastal weather and a sea full of predators. Terrapins are highly variable in color. Some are black, others milky gray, and some have splashes of yellow, green, or orange. The face and legs of diamondback terrapins are speckled or mottled. Brightly colored, diamond-shaped concentric markings are visible on the upper shell of many. Females get three times as large as males, which are little larger than a person's hand. This attractive little turtle is also tough. Unfortunately, the perils that threaten large populations of diamondback terrapins in their environmentally fragile coastal habitats continue to increase. As in so many cases of environmental peril, the terrapins' situation is exacerbated by human activities.

The Kiawah terrapin study, which has been sampling the population many days each year for almost three decades, is now under the direction of Michael Dorcas, a professor at Davidson College. Mike's students, as well as those from other colleges and universities, continue to sample the terrapin population. More than 3,400 captures and recaptures of Kiawah terrapins have now been recorded, and the terrapins' situation is nowhere near as pretty as the turtle itself. In the Kiawah salt marshes and tidal creeks, which once harbored hundreds of terrapins, these beautiful, nonthreatening animals are declining in numbers.

No single culprit has been identified in the population decline, but two human activities--overdevelopment and crab pots--are indisputable threats to the terrapins. A terrapin that cannot escape from a crab pot will drown. Dead terrapins have been found in both commercial and recreational crab traps along the Kiawah River. Sadly, one activity that causes terrapin deaths is a result of laziness--or forgetfulness. Each year, tourists abandon thousands of traps in coastal states; these "ghost traps" continue to kill terrapins and other marsh wildlife long after the tourists have returned to their homes.

Another threat to terrapins in many areas is the loss of nesting habitat. On Kiawah, a major, and avoidable, problem looms. A development company wants to put up a concrete revetment between the salt marsh and the sand dunes where terrapins nest (see www.kiawahriver.org). The construction will further aggravate the problem of terrapin population decline by reducing recruitment of young turtles, which could spell doom for the terrapins of Kiawah. Coastal development designed to attract people to an area while marring the beauty of the habitat they come for is textbook irony.

The decline of terrapins means more than merely the loss of a spectacular turtle. By eating periwinkle snails, which eat spartina grass, the dominant vegetation characterizing a salt marsh, the terrapins contribute to the overall environmental health of the marsh. A decline in terrapins indicates that the coastal marshes themselves are being environmentally stressed beyond a sustainable limit.

Last week on Kiawah Island we saw diamondback terrapins in the wild. Whether we can make such a statement in the future will depend on how this little turtle fares in the face of coastal development.

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