Charles Fergus
June 01, 1984

Under a lowering sky, a brown creek meanders through soaking grass. Bill Dunson is hunting snapping turtles. He stands on the creek bank and adjusts his earphones, raises an aluminum antenna—a cross with three horizontal bars—and turns it from side to side.

Mosquitoes by the dozens ride the shoulders and back of Dunson's coveralls; they lift from his neck, flying heavily. Dunson twirls a dial on the receiver, which he carries at his waist in a blue nylon bag.

"This isn't gonna work," he says. "The transmitters won't work in salt water—the salt blocks the signal. I normally use the radio at low tide, when the water is less saline."

Dunson turns and walks along the creek, followed by his son, Billy. Dunson is 6 feet tall, clean shaven, with wavy well-combed hair to his shoulders. Billy, in his late teens, is taller. They stop at a wooden stake; Billy hauls on a rope, and a trap the size of a bushel basket comes gushing from the water. Nylon mesh on a metal frame, the trap is empty save for a punctured sardine tin. Billy replaces the tin with a new one and drops the trap in the creek.

The next trap sags. Billy opens it and, in one quick motion, grabs a snapping turtle by a hind leg and lifts it in the air.

The turtle is mossy brown. Star-shaped, bone-sheathed eyes radiate out from the tiny black dots of its pupils. It hisses explosively and opens its mouth, pale pink and guarded by a sharply hooked beak.

"We've caught this one before," Dunson says, pointing to a notch in the edge of the shell.

Billy weighs the turtle with a hand scale—10 pounds, 3 ounces. Dunson jots in a notebook. "She's gained 2 ounces in a year," he says. Billy presses the fingers of his free hand into folds of skin where the turtle's hind leg sticks out of its shell. "No eggs," he says. Dunson motions him to free the turtle. It splashes in the dark water and sinks from sight.

"At first, I was surprised to find snapping turtles in a tidal creek," Dunson says. "I was teaching in the Marine Science Center, and had my students set some traps in Little Mosquito Creek, a couple of miles north of here. We caught snappers—50, 60 pounds of them in only 300 feet of creek.

"Snapping turtles are supposed to be freshwater animals. Here they were in an estuary, a creek that in its lower reaches can get almost as salty as the ocean. I checked the literature and couldn't find any reports of snappers living in marine or brackish environments. I talked to some old turtle trappers, and they said sure, lots of snappers in a creek like that."

Dunson walks to where the creek joins the maze of sloughs and channels that riddle the estuary. East, across an expanse of marshland—band after grassy band, each a slightly different shade of green—we see the back of Chincoteague Island. To the south, beyond Chincoteague, a long, thin line: the southern tip of Assateague. Then the inlet, with open ocean beyond; and then another low line: Wallops Island. Between the islands and the marsh where we are standing lies Watts Bay, an expanse of salt water protected from the ocean by the islands.

The bay, Dunson explains, has a salinity of about 30 parts per thousand. "If you put a red-bellied turtle in the bay," he says, "it'll die in a few days. Its body can't handle the salt. Diamondback terrapins, on the other hand, live only in the bay. They can survive in fresh water, but they don't do well in it and don't inhabit it by choice. The snapping turtle fits somewhere in between."

Snappers live across the continent in lakes, streams, ponds, and rivers. They eat everything from carrion to bullfrogs. "The estuary"—Dunson sweeps his hand—"is one of the richest habitats in nature. Have you noticed how the water is cloudy? The cloudiness comes from plankton and from detritus, decomposing plant and animal matter. Ocean fish swim in here to spawn, and their young grow fat on the detritus-based food chain. The estuary is full of crabs and clams. The snapper has a motive for being here.

"What I want to know is this: How can a freshwater turtle exploit a saltwater habitat?"

Trtles are reptiles. Like all land animals, reptiles descend from creatures that originally lived in the sea. The fossil record implies that reptiles branched off from their ancestral line after it became established on land; since then, some reptiles have returned to the sea. Eons ago, the sea turtle was probably a freshwater dweller making brief forays into salt water. Through the ages its clawed feet webbed into flippers, and it grew a gland behind its eye to expel salt. The sea turtle has not sworn off land completely: It often seeks out the reduced salinity of bays and estuaries, and lumbers onto shore to lay its eggs.

Sea snakes are completely at home in the ocean. Dunson has studied these reptiles off the Great Barrier Reef in Australia, once for 12 months straight, and in Panama and the Philippines. He has concentrated on the yellow-bellied sea snake, a venomous 3-footer that preys on fish. The snake, which breathes air, spends its life floating and swimming near the water's surface. Dunson found that the sea snake's skin permits water—but not salt—to enter its bloodstream. He was the first scientist to identify a glad in the snake's head that excretes salt. He demonstrated that the creature's tight mouth seal excludes water.

Our bodies—the bodies of humans, sea snakes, snapping turtles—retain traces of the primal oceans: salt in our cells, salt in our blood, plasma, and lymph. The fluids are high in sodium and low in potassium; the cells, bathed by the fluids, are low in sodium and high in potassium. Each cell regulates itself by excluding sodium and selecting and retaining potassium from the fluids.

A cell also regulates its volume. If you take red blood cells and put them in varying salt solutions, they contract or expand, depending on solution concentration. In a strong solution, the cells shrink—they lose water, which flows out to try to equalize the difference between the cells and the solution. In a weak solution, cells take in water, swelling and sometimes bursting.

The body buffers cells against changes in the environment by osmoregulating: maintaining constant salt levels in the extracellular fluid. Properly osmoregulated, the blood, plasma, and lymph are neither too salty nor too low in salts; their constituent salts—the ratio of potassium ions to sodium—remain at normal levels. When the body fails to osmoregulate, its cells no longer function. The red-bellied turtle, thrown in the ocean, dies.

Dunson is in the first year of a three-year project, funded by the National Science Foundation, to find out how snapping turtles osmoregulate. In the laboratory, he plans to test the properties of turtle skin. To determine permeability, he will patch the skin across an opening between two chambers and measure how rapidly a radioactive tracer leaks from one side to the other. He will immerse snapping turtles in seawater, monitoring their body fluids to see how quickly they take up salts. He intends to test coastal turtles, which may differ genetically from inland individuals. Because immature turtles are notoriously hard to find in the wild, he will catch gravid females, inject them with a hormone to expel their eggs, and hatch his own study stock.

"Early results show that an adult snapper's skin is permeable to water, but very impermeable to salt," Dunson says. "the species has no apparent salt glad; it may get rid of excess salt by migrating back into fresh or brackish water and drinking. The fresh water dilutes the blood volume and lets the kidneys excrete the salt.

"All animals have an internal set point for drinking—if their body fluids fall below a certain level, they automatically drink, no matter how salty the water. The snapper may take longer to reach its set point than turtles that never go into salty water."

The smaller the snapping turtle, the more fresh water it needs: Young snappers may depend on osmoregulation by behavior even more than the adults do. During showers, Dunson has watched small snappers tuck themselves against a stream back to drink trickles of rainwater. He believes that adult snappers do not stay in a single portion of a tidal stream, but move upstream or down as salinity changes. At low tide, when the estuary is charged with fresh water from land-originating creeks, snappers may venture as far as the bigger estuarine creeks.

On the fringe between fresh and salty water, Dunson plans to structure his research around radio transmitters instead of traps. "Few physiologists have used transmitters on aquatic creatures in order to monitor changes in the body fluids. I've helped a student use transmitters on crocodiles in the Everglades, so I've gotten some experience. You can pick up a signal from a transmitter submerged in fresh water, but you've got to be right on top of it. You've got to have a good idea of where the animals are in the first place.

"Each turtle will have its own frequency. We'll trail them by radio, taking periodic salinity readings. So far, the snapping turtle droppings we've found have been packed with chitin and claws, indicating that they're feeding mainly on crabs. Next year, we'll catch turtles and run a tube down their throats and into their stomachs—pump in some water, flush out what's inside, and identify it."

In the more saline environs, Dunson plans to put turtles in cages located in different zones of salinity, feed them, and monitor blood salt concentrations and body weight.

"The final phase of the study will measure turnover of body materials in the actual environment," Dunson says. "We'll use a natural substance, not a radioactive one, as a tracer: Bromide. There's almost no naturally occurring bromide in the environment—it's the stuff you dump in an outhouse to see if it's contaminating the neighbor's well.

"We'll inject a turtle with bromide and follow it around. Periodically we'll fish it out of the water and take a blood sample. We'll take the blood back to Penn State's nuclear reactor and use neutron activation analysis to determine how much bromide the turtle is losing over time. That should tell us how rapidly the turtle's inside environment is changing relative to outside salinity.

"On Salt Creek, we've been setting one trap every 50 meters, from totally fresh water down to where the water is almost full strength seawater at high tide. We've caught turtles all the way to this point, so we'll have to set up a new trap grid extending even farther.

"I don't know how salty the water will have to be before we'll run out of snappers. I'll tell you this: We've caught dozens of snappers out on Assateague. They were living in freshwater ponds, but unless somebody put them on the island, they had to swim across Chincoteague Bay."

Last Updated June 01, 1984