S.C. Sea Grant Consortium

Coastal Heritage Magazine

Jewel of the Marsh: the Remarkable Diamondback Terrapin

All along the South Carolina coast, the diamondback terrapin is endemic to salt marshes. But many populations of the species remain in steady decline.

An older man holding a rolled up net in front of a salt marsh.
In the Marsh. College of Charleston Professor Emeritus Dave Owens has specifically studied the diamondback terrapin for more than 20 years. Photo by Grace Beahm Alford.

Jewel of the Marsh: the Remarkable Diamondback Terrapin

After scanning the horizon for a few minutes, Dave Owens points to a ripple breaking the sun-lit creek’s surface.

About 20 yards ahead of his tandem kayak, a diamondback terrapin rises, its mouth and eyes floating just above the water. In a few seconds, with a soft splash the terrapin disappears back under the water.

Owens is paddling with the rising tide on the marsh creek known as Grice Creek, an estuary surrounded by lush marsh cordgrass, black needlerush, and sea-oat patches, across the harbor from downtown Charleston. The creek is home to black skimmers, fiddler crabs, migrating American oystercatchers, and a stable population of the diamondback terrapin — a unique and vulnerable estuarine species that is the only turtle endemic strictly to brackish water environments in North America.

Two diamondback terrapins swimming in water.

On the Cover: Female diamondback terrapins are significantly larger than males and have a more squared head. Pictured here are a female and male terrapin at the College of Charleston’s Grice Marine Laboratory. Photo by Grace Beahm Alford.

A light breeze picks up. Owens makes his way around a sandy flat, paddling further into the creek, and straight ahead, another terrapin pops its head out of the water for air.

“This one’s a female,” says Owens, professor emeritus at the College of Charleston and a longtime terrapin researcher. The giveaway, Owens explains, is the terrapin’s more squared and wider head — unlike the terrapin spotted earlier, which was a male. Showing marked sexual dimorphism, females are considerably larger than the males, and mature females have larger heads than mature males.

She scoots back under the surface.

“Notice how this terrapin was up there at the surface for ten seconds,” Owens says. After a maximum of three to four hours underwater, both males and females must surface to breathe oxygen. Yet, Owens explains, the females, because of their larger size and resulting greater oxygen storage capacity, will breathe at the surface for about three times as long as a male.

“When they come up, the females will stay about 10 seconds, that is according to our research,” says Owens.

In the habitats where diamondback terrapins call home up and down the South Carolina coast, large juvenile and adult terrapins are very difficult to spot. Owens, who has studied terrapins for more than 20 years, is practiced at seeing that ripple in the water. And timing helps; the terrapins are getting more active with the rising tide this summer morning, as they swim from the creek towards partially submerged marsh grass Sporobolus alterniflorus — formerly known as Spartina alterniflora — where during high tide they will feed on a bounty of periwinkle snails, small crabs, and little mud snails.

“Some people have been kayaking this creek for nearly their whole life, and they’ve never spotted a terrapin — until they know what to look for,” Owens says.

A diamondback terrapin in a salt marsh.

One of a Kind: The diamondback terrapin lives where no other turtle species can live, says biologist Barbara Brennessel. Photo by Stephanie Chavez Nadeau, University of North Carolina.

Unique Species Adapts to Habitat

The diamondback terrapin (Malaclemys terrapin) is a small turtle endemic, or native, to coastal marshes, rivers, and estuarine bays from Cape Cod to the Gulf of Mexico. “They live where no other turtle species can live,” says Barbara Brennessel, professor emerita of biology at Wheaton College and author of Diamonds in the Marsh: A Natural History of the Diamondback Terrapin.

“It is a truly unique species,” Brennessel adds. The terrapin has found a niche in the salt marsh, developing both physiological and behavioral adaptations for life in estuarine water that varies widely in salinity. For this reason, terrapins are the only species of turtle that live their entire life in coastal salt marshes, one of the most productive ecosystems on Earth.

One of the physiological capabilities of the terrapin is the presence of a complex salt gland near their eyes, and an example of its behavioral strategies for survival in the marsh is the ability to quickly distinguish rainwater from saltwater for drinking when needed.

Although diamondbacks live in salt or brackish water, they are not sea turtles. Rather they are related to such freshwater species as sliders, painted turtles, and eastern box turtles. Turtles in this classification have streamlined shells and are called emydids. Terrapins are smaller than a snapping turtle, and much smaller than a sea turtle. The terrapin does, however, share one trait in common with sea turtles: it is the only turtle in the emydid family with a gland that allows them to secrete excess salt from their blood, and survive in salty environments. However, the terrapin’s salt gland is not as effective as that of a sea turtle.

The species’ most salient physical characteristic is the broad upper shell that varies in individual coloration from gray or black to greenish or light brown and is grooved with a natural diamond-shaped pattern that accounts for its common name. The terrapins’ skin, which also varies in coloration, is often covered in black leopard-like spots and/or streaks.

“They have this wonderful individual variation, more so than any other species of turtles,” says Owens. “I have seen some that are blonde, almost appearing albino, and others that are brown and very dark. There is so much variation even here in this population of terrapins [in Grice Creek].”

Their favorite food sources in South Carolina are periwinkle snails and fiddler crabs that are on the smaller side, although they will eat a range of different crustaceans and mollusks — including the mud snail. They will also eat small shrimp.

At a few weeks of age, a young terrapin will develop a hard, distinctive beak, which lines its mouth and helps the terrapin to undertake the task of eating small crustaceans and mollusks. This part of the upper jaw is thick and strong, giving the diamondback terrapin another unique aspect to its appearance.

“When it is lightly colored, it resembles clown lips. When it is darkly colored it resembles a moustache,” Brennessel writes in Diamonds in the Marsh.

Terrapins generally live 25 to 40 years, with documented cases of a few 60-year-old terrapins on Cape Cod, Brennessel says.

There are seven recognized subspecies of Malaclemys terrapin that have been identified by scientists. The subspecies found in South Carolina is Malaclemys terrapin centrata. “They can be found in pretty much every major estuary up and down our coast,” says Joey Ballenger, a biologist with the S.C. Department of Natural Resources (SCDNR) and manager of the agency’s Inshore Fisheries Research Section.

There are significant differences between each subspecies, including differences in size and age at maturity, and number of eggs in a nest. For instance, Malaclemys terrapin centrata (also known as the Carolina terrapin) “are the smallest in size of all the subspecies, and females have a smaller clutch compared to the other subspecies,” says Brennessel. Malaclemys terrapin centrata will lay a clutch of five to seven eggs, while diamondbacks up in Cape Cod, for instance, will lay a dozen eggs.

Two people throw a net from a boat into the salt marsh.

Shallow-Water Sampling. In South Carolina, results from a SCDNR trammel net survey are used to help keep tabs on terrapin populations. Photo by Erin Weeks, S.C. Department of Natural Resources.

Each of the seven recognized subspecies have physical (often subtle) distinguishing characteristics. The ornate diamondback (Malaclemys terrapin macrospilota) found only on Florida’s Gulf coast has orange or yellow marks on the top of its shell in the center of each diamond shape. The bright markings contrast an otherwise darkly-colored shell.

Meanwhile, the subspecies of terrapin found from Cape Cod, Massachusetts to Cape Hatteras, North Carolina — called the Northern Diamond-backed terrapin, or Malaclemys terrapin terrapin — has a slightly raised ridge on its top shell. These northern terrapins are just slightly larger than a dinner plate, while the South Carolina subspecies are about an inch smaller when measured along the linear length of their plastron, or the bottom of their shell, Brennessel says.

All along the South Carolina coast, this unique estuarine species is endemic to the salt marshes. Populations can be found in locations ranging from Cape Romain National Wildlife Refuge, the Lower Wando River, Charleston Harbor, the lower Ashley River, the Ashepoo, Combahee, and Edisto (ACE) Basin, Edisto Beach, and Hilton Head Island. But these populations remain in gradual decline, across South Carolina, due to threats, including habitat loss and degradation, sea level rise, and road mortality. Entrapment in commercial and recreational crab pots is one of the greatest threats to terrapins. In many areas of the state, there has been a steady decline in the abundance of the species, as shown in a long-term statewide trammel net survey begun in 1990 conducted by the S.C. Department of Natural Resources (SCDNR), says Ballenger, who oversees this research. The survey showed the Wando River watershed experienced one of the greatest population decreases in the relative abundance of terrapins, from 1995 to 2010, a decline that coincided with a significant increase in development along the lower Wando River, which might have impacted the amount of coastal habitat available to the terrapins.

The diamondback terrapin is listed in South Carolina and Georgia as a “high-priority” species for conservation efforts within coastal habitats, under the states’ respective Wildlife Action Plans. Such a classification establishes research and conservation funding for at-risk species that are currently not covered under federal funding programs.

In the face of increasing perils, terrapin populations are under greater pressure to survive, and researchers are working on efforts to help ensure there is a future for the species.

At SCDNR, biologists are working on improving a simple method to prevent terrapins from entering crab traps. Further, research with “headstarting” hatchlings could help enhance the populations of terrapins. Headstarting is a conservation technique for vulnerable species, in which young animals are raised artificially and subsequently released into the wild. A volunteer community-science program, launched in 2016 by SCDNR, is also providing important information to biologists about the terrapins and helping point out problem areas.

Such efforts are crucial for the long-term conservation of this unique and declining species found in our salt marshes. “We should always remain optimistic about any declining species, especially when its environmental threats have been identified and can potentially be mitigated, as is true with the diamondback terrapin,” says Whit Gibbons, a professor emeritus at the University of Georgia, who conducted the single longest study of the terrapin, on Kiawah Island starting in 1983. “In addition, this iconic reptile has another factor operating in its favor: it is the most charismatic inhabitant of America’s salt marshes.”

Once coastal communities become aware of the possible disappearance of terrapins, Gibbons believes further efforts will be put into place to come to its aid.

Following the Tides

In habitats they call home along the South Carolina coast, large juvenile or adult terrapins follow the tides. During low tides, they will usually stick to the tidal creeks or near-shore shallows. The terrapins will accumulate there in the water bodies, where it is safer for them, out of reach of many of the predators that would try to make a meal out of them, according to Owens.

During high tides, they will display wider movements, likely because less of the marsh is exposed to terrestrial predators — compared to low tide, when more of the marsh is exposed.

Terrapins will come up into the marsh to forage on their prey of periwinkle snails (Littoraria irrorata), small crabs, and mud snails (Nassarius obsoletus), as well as other mollusks, crustaceans, and the occasional small marine invertebrate periwinkle snail.

In South Carolina, “periwinkles seem to be their favorite,” says Owens. And the terrapins have a novel way of fetching these favorite snails, which
attach themselves to the upper reaches of smooth cordgrass, just above the water line.

“They will swing at the marsh cordgrass and push it down, and then grab a periwinkle [snail] as it falls into the water,” says the College of Charleston professor emeritus. It has proven to be an effective means to an end.

Periwinkle snail clinging to marsh grass.

Favorite Food. Diamondback terrapins come into the marsh to forage on periwinkle snails and other prey. Photo by Grace Beahm Alford.

Trio of Modern Threats

The only known prehistoric diamondback terrapin fossils, two tiny bones, of two different individuals, from the neck region of the shell and the lateral portion of the shell, date back to the Pleistocene epoch (approximately 1.65 million to 10,000 years ago). They were found at Edisto Beach, Brennessel writes in her book Diamonds in the Marsh.

Terrapins were once very common and provided food for Native Americans and European settlers. In colonial times, a wagon load of terrapins could be purchased for one dollar. By the early 1800s, however, food preparations made with the diamondback terrapin began to assume a higher culinary status, and terrapin soups and stews became gourmet specialties for Victorians. “The fisheries status of the terrapin changed from annoyance or by-catch to a valuable commodity,” according to Brennessel. The terrapin became the main ingredient in a sherry-laden soup.

From the mid-1800s through the early-1900s, they were nearly hunted to extinction. “Tens of thousands of adult terrapins were removed for food — as a delicacy,” says Whit Gibbons, the professor emeritus of ecology at the University of Georgia’s Savannah River Ecology Laboratory and one of the most respected authorities on amphibians and reptiles.

The epicurean status was cemented in upscale restaurants and even in the White House’s kitchen. Diamondback terrapin meat was a popular gourmet food in the 1800s and early-1900s at many elegant resort hotels and restaurants along the U.S. East Coast. Gourmet home recipe from 1881 and 1894 describe the preparation of terrapin soup and stew. All such recipes called for a heavy dose of sweet wine. Terrapin soup was a favorite White House lunch course during the presidency of William Howard Taft, Brennessel writes.

“The soup was also served during state dinners, when Mrs. Taft would hire a cook to prepare it.”

A commercial terrapin industry resulted from this epicurean status. Throughout the 1800s, fishermen “would harvest terrapins from Texas, the Keys, or South Carolina, and bring them up to a central place in Savannah, or Charleston, or Washington D.C., and keep them all in these pens,” Owens says.

This large-scale, unregulated harvesting, along with widespread coastal development during the 19th and 20th centuries, decimated the species. “They took a big hit,” says Gibbons. According to New Jersey-based Save Coastal Wildlife:

“Terrapin or ‘turtle’ soup or stew was a delicacy that could be found alongside shad, lobster, flounder and clams on many restaurant menus. Thankfully, the commercial demand for terrapins ended by the 1930s as a result of decreasing terrapin populations, which made them difficult to find, and Prohibition from 1920 to 1933, which banned the production and sale of sherry, another of the soup’s main ingredients.”

Populations partially rebounded over the past century. Although terrapins are no longer hunted on a widespread scale in states where they are found, they still face many threats, including habitat destruction, incidental drowning in crab traps, poaching, cars, and the impacts of climate change. In South Carolina and Georgia, the diamondback terrapin (Malaclemys terrapin) is listed as a “high-priority” species. Further, in 2000 the state of South Carolina changed commercial access to terrapins, passing a law prohibiting any harvesting for commercial purposes. Persons are allowed to have only a personal possession limit of two terrapins.

Brennessel lists three of the top perils that the vulnerable species faces. The number one threat used to be harvesting of terrapins as a food delicacy. Since that has been banned or is very limited, these are now the top three threats, Brennessel says:

  • Mortality in crab traps, also called “crab pots,” used by commercial and recreational fishermen to catch blue crabs. Terrapins can drown in crab traps, even in pots that have bycatch reduction devices (BRDs). BRDs can prevent the larger females from entering the pots, but adult males and young females are still vulnerable, because they are typically small enough to enter the traps and drown.
  • Fragmented, degraded, or disappearing habitat. This includes the impact of upstream development — such as increased siltation in estuaries due to more stormwater runoff.
  • Climate change, including changing temperatures and sea-level rise.

Terrapins can enter commercial and recreational crab traps and drown in the traps. These traps pose what is likely the gravest threat to the terrapin, says Gibbons, who conducted the longest-running study of diamondback terrapins.

It only takes a few hours for a terrapin to drown in a submerged crab trap. No matter what type of crab trap it is, the more often the trap is checked, the better. According to an article in the SCDNR publication South Carolina Coastal Resources, “drawn to the pungent smell of fish heads and chicken necks, the turtles frequently enter crab pots.” Terrapins may also enter out of curiosity or because they are looking for a safe resting place.

But like all reptiles, diamondback terrapins breathe air, and they can drown when they’re unable to leave the traps.

Male terrapins may also follow a female. “One thing we know that happens if a female goes in there [a trap] in the spring, the males will follow her in, and that can be responsible for a large number of captures,” Owens says.

Development and land-use changes along the coast result in the loss of terrapin habitat. Although the largescale conversion of pristine habitat in salt marshes has slowed dramatically from the twentieth century, upstream changes slightly inland can still have an impact. Seawalls, revetments, and docks can alter terrapin habitat.

Thirdly, climate change is already causing impacts for the coastal diamondback terrapin — even if the effects on populations are not measurable yet, Brennessel says. But it is certain to become “a big issue in the near future,” adds Brennessel, who taught at Wheaton College for 33 years, and focused her research on the diamondback terrapin.

Sea-level rise will likely eliminate a lot of the nesting habitat for terrapins. Female terrapins nest on marshes, sandy spits, and islands, habitats that are especially prone to increasing inundation from sea-level rise.

A sandy beach with sea oats.

Higher Ground. Diamondback terrapins nest on sandy areas that are adjacent to the salt marsh, but at slightly higher elevations above the high-tide line. Often, sea oats grow at these beaches that make suitable nesting habitat. Photo by Roger Real Droin, S.C. Sea Grant Consortium.

Brennessel writes: “ . . . residual habitat is being impacted by the effect of climate change. We are already witnessing sea level rise and an increase in the number and severity of storms that whip up waves and wind. Our marshes are in danger of disappearing and suitable terrapin nesting areas have become inundated or have shriveled in size.”

Sea-level rise is already resulting in changes. In the last two decades alone, Charleston Harbor has experienced between four to six inches of sea-level rise, according to National Oceanic and Atmospheric Administration (NOAA) tide gauge data.

Moreover, continuing warming temperatures and additional melting of land ice are anticipated to further hasten the pace. Sea level in Charleston has risen by 10 inches since 1950, with an acceleration anticipated. From 2020 to 2050, Charleston Harbor is projected to experience a sea-level rise of 1.4 feet, according to recent data from NOAA and the National Climate Assessment.

Furthermore, even relatively small temperature fluctuations and transitions due to climate change can have major implications for the hatchlings’ development, and threaten to upend the way the terrapins develop. Like most emydid turtles, the sex of diamondback terrapins is determined by temperature during incubation. Terrapins exhibit a pivotal temperature (the temperature at which an equal ratio of males and females develop) of approximately 82.4 to 84.2 degrees Fahrenheit.

Terrapins are the opposite of alligators — terrapin females develop at warmer nest temperatures and males develop in cooler nest temperatures, says Erin Levesque, a biologist with SCDNR and manager at the Waddell Mariculture Research and Development Center, in Bluffton, S.C.

“One of our concerns is that increasing temperatures could cause development of more females, potentially to the point of detriment to the population,” says Levesque. Factors such as the ability of specific nesting areas to balance out warming temperatures, through habitat cover, would play a factor in the impact on nests.

Research conducted with diamondback terrapin eggs from St. Mary’s County, Md., showed that as little as 2.7-degree Fahrenheit difference in incubation temperature altered the ratio of female-to-male hatchlings, and greater temperature swings could result in up to 100-percent female hatchlings. Significant temperature changes could also spur irregular development, terrapin researchers say.

Further research is critical, including here in South Carolina.

“We need a long data set to see what kind of changes there are” as a result of warming temperatures, Levesque says.

Giving Terrapins a Head start

In a garden overlooking the Wappoo Creek marsh, biologist Erin Levesque digs through the damp soil very carefully with her hands.

After a few more scoops of soil, Levesque stops. The biologist and Dale Aren, a community scientist and owner of this West Ashley property on the creek, notice movement and a glimpse of the first of the grayish-olive shells of newly-hatched diamondback terrapins. In a few moments, five of the terrapins, blinking their eyes and no larger than a half dollar, scamper and crawl towards the top of the evacuated nest.

Baby terrapins typically would head to the nearby marsh — in this case, the emerald smooth cordgrass bordering the meandering Coburg Creek — where they settle in for the next few weeks, mainly burrowing in the pluff mud or along the wrack line. When their yolk sacs disappearing from the ventral plastron surface on the bottom of their shells, eventually the young terrapins begin eating insects and little crustaceans, while also attempting to remain hidden from predators such as racoons, mink, herons, snakes, and birds of prey. These terrapins, though, would not make their way into the marsh. The goal was to intercept the terrapins — which were hatched as part of an enhanced breeding program — and place them in a container for transport to the Waddell Mariculture Center where they will be reared in tanks for the first year of their lives.

Two women excavating dirt.

Careful Excavation. Erin Levesque, a biologist and manager at the Waddell Mariculture Center, carefully unearths diamondback terrapin eggs in a nest overlooking Wappoo Creek. Photo by Grace Beahm Alford.

Levesque gingerly places the terrapins in a container lined with perlite. “They’re beautiful,” says Aren.

It’s part of a “headstarting” effort that could provide one important way to enhance the stock of diamondback terrapins, as the species faces continued declines across its South Carolina range.

“We have been seeing some longterm declines,” Levesque says. “We are researching the potential for headstarting terrapins, and whether it is possible to supplement wild populations that are in decline.”

The goal is to protect the terrapins and start to plan for increasing conservation efforts, which can be implemented across different geographic areas, a “more proactive approach” to species extinction.

Levesque is researching whether accelerated growth in a captive environment would translate to accelerated breeding. At the Waddell Mariculture Center laboratory, Levesque and her team will conduct growth and reproductive studies, and examine nutrition and wintering habits. “We will keep them in the lab for up to a year,” she says, “and look at different diets and growth rates.” One thing researchers do know is that the terrapins grow “exponentially faster” in a lab during that first year, Levesque says. The shells are more calcified, and the terrapins are less vulnerable to predation by certain predators.

Levesque will also research breeding and nesting habits of terrapins at Botany Bay Plantation Wildlife Management Area. The females nest here in open sand dune habitat. Often, however, a raccoon or ghost crab will get to the nests and eat the eggs. That’s why Levesque and other officials have been putting down wire cages to protect the nests.

Witnessing a Decline

Gibbons, at the University of Georgia’s Savannah River Ecology Lab, conducted the longest-running study of diamondback terrapins. Since 1983, Gibbons and other members of the Savannah River herpetology lab have made over 3,500 captures of about 1,500 individual terrapins in several salt marsh creeks at Kiawah Island.

His research has shed light on terrapin population dynamics and has demonstrated that the Kiawah population is declining, due in large part to mortality in crab traps.

Gibbons studied terrapins on two Kiawah Island creeks, witnessing steep declines in both populations. Gibbons says a change in the upland ecosystem had a significant impact on the populations of terrapins in the creeks he studied extensively, Fiddler Creek and Sandy Creek.

Kiawah Island has changed since the 1970s and 80s. However, the island and its communities have implemented salt-marsh and open-space conservation plans since early development phases. Gibbons believes another effect on the island’s terrapin population emanates not from Kiawah Island, but rather upstream changes outside of the community. Creeks that Gibbons and his children could walk through in the early 1980s have become filled with silt which has washed downstream due to increased stormwater runoff — as a result of increased development.

An increasingly silty environment may end up being a less ideal environment for some of the prey that diamondbacks feed on. Gibbons says, for instance, a silty creek may impact shrimp and periwinkle snails, food sources of the terrapins, and he believes research is needed to look into this.

But Gibbons’ research on Kiawah Island indicates that crab traps largely contributed to the decline. As tourism has increased in the region, Gibbons has seen an increase in recreational crab traps.

“The terrapins come in these traps to eat crab or fish, and they don’t come out,” Gibbons says. “They drown. They can’t breathe under water.

“This is what we thought was the main cause of the decline in Kiawah,” Gibbons adds. “I bet thousands of adult terrapins, both males and females, from Texas to New England, drown every year,” says Gibbons.

Current bycatch reduction devices can help. Although some states now require such devices, they have not been implemented on a widespread basis.

A Grim Discovery

As a graduate student in 2007, Andrew Grosse made a grim discovery. Grosse, who is the state herpetologist with SCDNR, came upon a crab trap submerged just below the water’s surface in a tidal marsh at low tide near St. Simons Island, Ga. But there were not only blue crabs in this trap. Grosse recounted what he saw in a journal article published in 2009 in Chelonian Conservation and Biology:

“The water depth was less than one meter and a mixture of mud, algal and barnacle growth, and turtle carcasses were visible within the trap. The trap was pulled from the water, revealing the carcasses of 94 dead M. t. centrata. On May 4, we observed 23 dead and one live M. t. centrata in a second trap approximately 100 meters from the first trap. By law, we were prohibited from removing the traps from the water, and during the remainder of our 2-month sampling period, we visited this site three additional times. During each visit we observed additional dead terrapins in the derelict crab pots. Over five visits between 4 April and 30 June 2008, we documented 133 dead turtles between the 2 derelict crab pots.”

A photo Grosse took of the abandoned crab pot with the 94 drowned terrapins was published in the article. After doing additional biomass research Grosse and his team determined that 91 percent of the total terrapin biomass in this particular tidal creek was lost as a result of the two derelict crab pots.

Terrapins can drown in both commercial and recreational blue crab pots (whether abandoned or not), and Grosse’s discovery has been cited by scientists as evidence of the impacts of crab traps on terrapins and the need for research on how to prevent terrapin drownings in the traps. Subsequent to the article’s publication, his photograph has been republished on numerous occasions to illustrate the unintended effect of crab traps on terrapins.

SCDNR biologist Michael Arendt and a team of researchers have spent several years working on a solution to this problem.

It would come in the form of a greatly improved bycatch reduction device (BRD) — also known as an excluder device. Think of it as a terrapin escape hatch; the BRD fits at the end of a funnel into the trap, allowing most crabs to enter but keeping many terrapins out.

“This device was invented in 1988 and is credited to Dr. Roger Wood,” says Arendt. Concerns about the local terrapin population in and around Stone Harbor, N.J., Wood, former director of The Wetlands Institute, fashioned the first BRD prototype from a wire coat hanger. Since that time, many researchers from Texas to New Jersey have tested various designs, some that they came up with on their own and others that they were just checking to see if a particular design that was used elsewhere would work in their geographic area.

What Arendt and his team did in 2018 that was novel was to incorporate the geometry of the target capture animals (crabs) and the target exclusion animals (terrapins) and find “the sweet spot” opening size.

A square-shaped metal device with loops on the corners.

Improved Design. The new bycatch reduction device (BRD) design on top includes a curvature so that oval-bodied crabs can fit through more easily as they rotate to enter. Furthermore, the shorter design improves terrapin exclusion. Photo by Erin Weeks, S.C. Department of Natural Resources.

“Mike Arendt has led the charge with [improving the design] of an excluder,” says Grosse.

BRDs are pieces of plastic that fit inside the entrances of crab traps, restricting the size and shape of the hole. Several BRD designs have made it to market over the years, but adoption has been slow, as the devices can sometimes reduce blue crab catch rates in addition to reducing bycatch, according to an article in South Carolina Coastal Resources.

After three years of research, between 2014 and 2017, and nearly 1,000 legal-sized crabs’ worth of testing, Arendt and his team designed a BRD that would keep terrapins out while still allowing large crabs to enter traps. The testing was conducted on the Ashley and the Stono rivers.

Arendt adjusted the size of a test BRD until it achieved 99 percent capture for legal blue crabs, while also keeping out twice as many diamondback terrapins as other widely promoted designs. “There was a lot of design work to figure out the best approach to keep the biggest crabs in, while keeping the terrapins out,” Arendt says.

The researchers looked at 8,196 theoretical dimension sizes for a BRD.

Unlike those prior designs, the SCDNR BRD includes a curvature in the bottom and top of the excluder so that the oval-bodied crabs can fit through more easily as they rotate to enter. Furthermore, the design improves terrapin exclusion by focusing on their width which is twice as large as their height, and in turn offers better exclusion along with crab entry.

“Most people don’t realize crabs turn sideways as they swim — like leading with their elbows,” Arendt says.

Arendt noted that some commercial and recreational crabbers were intrigued by how well the devices worked. “I had one guy tell me he was impressed at the size of the crabs that fit in [through] the BRD into the crab trap,” Arendt says. But there still needs to be more research into crab and terrapin morphology across South Carolina given preliminary data suggesting that the device so successful in the Charleston area will work as well statewide.

Arendt points to North Carolina, where state officials have mandated BRDs, built to specifications based on Arendt’s design, in two terrapin protection areas. In 2018, the North Carolina Division of Marine Fisheries (DMF) by proclamation required that all crabs traps be fitted with the devices in Masonboro Sound and the Lower Cape Fear River.

“Any crab pot that is employed in these two areas must be equipped with a BRD that has been approved by the North Carolina Marine Fisheries,” says Amanda Southward Williard, biology professor in the Department of Biology and Marine Biology at University of North Carolina Wilmington.

Williard and colleagues have started a working group with local fishers and asked them to bring their ideas for a BRD so researchers could test them. An undergraduate honors student at UNCW is currently testing one of these prototypes, and she found some “really good results” in terms of terrapins being kept out of the traps, Williard says.

More buy-in is needed. But a foundation is there. “We had the good fortune of working in this area for over 10 years and establishing relationships in that community,” Williard says.

“We can talk openly with them.” In South Carolina, there is no legislation currently on the books that would touch on BRDs. “If any changes were to be considered they would have to be turned into a bill introduced by a legislator and eventually passed by the State’s General Assembly. However, an initiative could make its way to the legislature through a grassroots effort.

“It is not uncommon for many of our wildlife- or marine resources-related laws to originate through interface between private citizens or groups and individual legislators, so that remains a viable option for interested parties to consider,” says Mel Bell, director of fisheries management at SCDNR.

In 2000, the South Carolina legislature recognized the value of science-based management in order to protect limited populations of terrapins, and changed commercial access to the South Carolina terrapin resource from a seasonally open fishery to one managed with discretion by SCDNR. Since this time, no recent permits for commercial harvest have been issued.

A hatchling escapes a plastic container.

Escape Artist. As part of the headstarting effort, newly hatched terrapins were placed in a container lined with perlite. Photo by Grace Beahm Alford.

A national Diamondback Terrapin Working Group (DTWG) was formed in September 2004 to facilitate information exchange and to set research and management priorities. This group consists of individuals from academia, scientific and regulatory agencies, and private organizations working to promote the conservation of the diamondback terrapin, as well as preservation of intact, wild terrapin populations and associated ecosystems throughout their range.

The working group this year wrote a position statement asking states to require BRDs, says Brennessel, who is a member of the group. The group suggests that properly-used devices could help reduce terrapin by-catch while having minimal effect on blue crab catch. “The position of the working group is that the scientific data are abundant and clear that commercial and recreational use of blue crab traps is a primary threat to the conservation of Diamondback Terrapins throughout the species range,” reads the statement, published in November 2020, “and that BRDs effectively reduce terrapin capture and mortality with minimal effect on crab catch.”

As Brennessel notes, efforts such as headstarting will have little effect if core threats such as incidental drownings in crab traps and habitat loss are not addressed.

Aren, who lives on Wappoo Creek, has been participating in SCDNR’s community-science volunteer terrapin program for the last several years.

Launched in 2016, the program aims to log important information about the terrapins, including specific sightings and nesting locales.

The terrapins make the approximately 400-foot trek to Aren’s garden year after year. Nesting females come from the creek and smaller “creeklet” into her garden, which it turns out is an optimal nesting site — close to the marsh but high enough out of the water. “That they come specifically to my garden, each summer, that’s amazing,” Aren says.

The terrapins will dig their nests in the soil, pushing aside brick chips first.

Many of the residents of Edgewater Park, the West Ashley neighborhood where Aren lives, now know about the terrapins. “I will post something on our neighborhood Facebook page at the start of nesting season,” Aren says. She will post a few photos she has taken of the terrapins and let neighbors know to be wary since it is nesting season for the reptiles.

Aren, a retired marketing executive, also reports detailed sighting information to SCDNR, and she microchips nesting females. In 2017, SCDNR biologists trained her how to outfit the terrapins with Passive Integrated Transponder (PIT) microchip tags, which assists them in monitoring the terrapins’ behaviors and locations. PIT tags are a noninvasive, lightweight method of recognizing individuals.

Using the PIT tags, Aren and researchers discovered that the same terrapins would return to the same locations to lay several clutches of eggs in one summer.

Arendt, the biologist who leads terrapin research at SCDNR, said input from residents such as Aren helps the agency get a better sense of overall distribution of diamondback terrapins in South Carolina, so scientists can make sure they are conducting research in the right areas. The information has even helped point out problems. As a case in point, a concerned resident let scientists know about an area where two terrapins were killed during road construction work on U.S. Highway 21 over Harbor River in Beaufort County, says Arendt.

As a result of this information, transportation officials decided to put up a Terrapin Crossing sign on the busy thoroughfare to help notify motorists of terrapins in the area. More and more residents, as Gibbons pointed out, are becoming aware of the terrapin, the remarkable turtle of the salt marsh.

The health of diamondback terrapins is intertwined with the ecosystem in which it lives. Terrapins are a sentinel species, and their optimal survival takes place in unspoiled marsh habitats. In a 2007 study by Owens, funded by the S.C. Sea Grant Consortium, the turtles were found to be good predictors of mercury pollution, thus becoming a bioindicator for assessing mercury contamination of estuarine systems. Other studies have looked at terrapins as bioindicators of organic pollutants, including polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and chlorinated pesticides. Terrapins also help keep these ecosystems healthy by eating periwinkle snails, which in high numbers can harm marsh grasses. The terrapins may never rebound to their population levels of the 1800s before trapping and widespread habitat changes resulted in severe declines. But certainly, recent declines can be reversed, Gibbons believes, and through a variety of mitigation — such as the use of bycatch reduction devices on crab traps and preserving intact salt marshes — populations of this unique species can thrive up and down the South Carolina coast.


Interesting Facts About Malaclemys terrapin

  • There are seven subspecies of diamondback terrapin:
    Northern – Malaclemys terrapin terrapin
    Carolina – Malaclemys terrapin centrata
    Texas – Malaclemys terrapin littoralis
    Ornate, FL Gulf Coast – Malaclemys terrapin macrospilota
    Mississippi – Malaclemys terrapin pileata
    Mangrove – Malaclemys terrapin rhizophorarum
    Eastern – Malaclemys terrapin tequesta
  • Diamondback terrapins may live for 25 to 40 years in the wild, perhaps longer.
  • Terrapins are sexually dimorphic, with males being much smaller, while females are considerably larger as adults. Males are about one-third smaller.
  • Terrapins have declined greatly in the Northeast and parts of Florida, and in South Carolina and Georgia, they are considered a “high priority” species for conservation efforts.
  • Diamondback terrapins get their name from the natural pattern that occurs on the scutes, or bony plates, on the top of their shell. They are diamond-shaped and filled with raised concentric circles.
  • In South Carolina, their favorite food is the periwinkle snail (Littoraria irrorata), while in Virginia, the terrapin prefers mud snails (Nassarius obsoletus).
  • As one of the few turtles that can tolerate brackish water, terrapins can be found in a variety of coastal habitats, including salt marshes, wetlands, mangrove swamps, estuaries, lagoons, and tidal creeks along the eastern seaboard and Gulf of Mexico of the United States.
  • Like sea turtles, diamondback terrapins possess salt glands around their eyes, allowing them to secrete excess salt from their blood, and survive in salty environments. But their glands are not as efficient as those of sea turtles.
  • Diamondback terrapins select their nest sites using such physical features as the slope and height of sand dunes and the extent of plant cover, and choose their timing on the basis of the tidal phase and the weather.
  • Terrapins mate in early spring. The female will float on top of the water and wait for a suitable mate to approach.
  • Nesting season runs from early May through early July. The eggs typically take anywhere from 60 – 100 days to hatch. Females can lay several clutches in one season.
  • Hatchlings often hide in the mud to seek food and safety.
Closeup of a terrapin's head.

An adult female diamondback terrapin. Photo by Dale Aren.


A Helping Hand

Report sightings online to help diamondback terrapins.

Coastal residents and visitors in South Carolina have a new way to make a difference in the long-term conservation of the diamondback terrapin.

The S.C. Department of Natural Resources recently launched a reporting form, and is asking members of the public to help by reporting terrapin sightings across the coast.

Eleanor Pierel Chosen for Knauss Fellowship

Consortium nominee Eleanor “Ellie” Pierel has been selected for the prestigious Dean John A. Knauss Marine Policy Fellowship, providing her the opportunity to spend a year living, working, and learning in the Washington, D.C., area. This competitive fellowship is offered by the National Oceanic and Atmospheric Administration’s National Sea Grant College Program. Recipients are matched with host organizations in the legislative and executive branches of government, and the fellowship begins in February 2022.

Pierel earned a B.A. in environmental studies from George Washington University, an M.S. in geography from the University of South Carolina (USC), and she is a candidate for a Ph.D. in geography from USC. During her fellowship, Ellie hopes to help integrate science into federal policies and regulations.

Consortium Staff Hires

The S.C. Sea Grant Consortium has welcomed three new staff members.

Brita Jessen, Ph.D., recently joined the Consortium in a newly created role as interdisciplinary research and partnerships lead. Brita will continue to foster existing partnerships and forge new ones as she works to build research teams and develop actionable science tailored to stakeholder needs.

Jessen earned a Ph.D. in oceanography from the University of Rhode Island and a B.A. in biology from Wellesley College. She previously was the research coordinator for the Rookery Bay National Estuarine Research Reserve in Florida.

Katie Finegan, PE, recently joined the Consortium as coastal processes program specialist, a position shared with the Burroughs and Chapin Center for Marine and Wetland Studies at Coastal Carolina University. Finegan will contribute her technical knowledge to provide a wider range of services to those requesting science-based information and assistance about coastal processes—the connection between upland watersheds and the ocean, coastal hazards, and how to enhance resilience to these hazards.

Finegan earned an M.E. and a B.S. in environmental engineering, both from North Carolina State University. Prior to joining the Consortium, Finegan was an engineer with Moffatt & Nichol where she worked on coastal-, water resource-, and environmental-engineering projects in Virginia, North Carolina, South Carolina, and Georgia. Her areas of expertise include coastal management, coastal change analysis, and soft-engineered coastal protection.

Taylor Allred is serving as a resilience graduate assistant. He is working with the Consortium and partners to compile information on what South Carolina’s local communities are doing to plan for resilience in the face of natural disasters and environmental change. Allred’s work will help inform the S.C. Office of Resilience in their development of the first statewide resilience plan.

Allred holds a B.A. from the University of Virginia, with a major in history and a minor in economics. He is currently enrolled in the College of Charleston’s Master of Public Administration program. Prior to joining the Consortium, Allred was an energy policy manager for Southern Alliance for Clean Energy, a financial analyst for Regulatory Research Associates, and a senior analyst for SNL Financial.

Consortium Revitalizes S.C. Clean Marina Program

The Consortium has taken the lead on coordinating the South Carolina Clean Marina program. Initiated in 1998 by the S.C. Marine Association, the program continues today as a partnership between the Consortium, S.C. Department of Health and Environmental Control-Office of Ocean and Coastal Resource Management (SCDHEC-OCRM), and S.C. Department of Natural Resources (SCDNR) to protect and improve water quality. Currently, 23 marinas are certified as Clean Marinas. Program coordinators have a goal of adding 10 certified Clean Marinas each year. A training workshop was held in August 2021 with participation from 11 marinas, including five that need recertification and six new marinas interested in program designation. Planning is underway for another training workshop in spring 2022.

Boats in a marina.

Photo by Shelly McComb, S.C. Sea Grant Consortium

In order to be certified in this voluntary program, marinas pay a modest fee of $250, which helps cover the cost of training, inspection, and certification material. Marinas must meet certain characteristics, including design considerations, marina management, emergency planning, fuel control, proper sewage and waste disposal, stormwater control, habitat protection, and boater education. A technical advisory committee, which provides guidance and assistance with training workshops and site inspections, is comprised of representatives from SCDHEC-OCRM, SCDNR, and industry professionals.

Contact April Turner, coastal communities program specialist, at april.turner@scseagrant.org or (843) 953-2073 for more information.

New Report Examines Coastal Economic Benefits

South Carolina’s beaches and barrier islands produce a wide variety of economic benefits, contributing to the culture, lifestyle, and well-being of residents and visitors. The state boasts an impressive 35 barrier islands which is second only to Florida in number. These barrier islands’ beaches and associated habitats, such as wetlands, salt marshes, maritime forests, and sand dunes, provide people with jobs, recreational opportunities, coastal protection, critical habitat for bird and turtle species, and aesthetic beauty.

The Economic Benefits of South Carolina’s Beaches and Barrier Islands is a recently published report by the S.C. Sea Grant Consortium that delves into the economic benefits these habitats offer. Based on available data from a variety of sources, beaches and barrier islands in the state annually provide billions of dollars in economic benefits, from recreation and tourism to consumption of seafood. Furthermore, coastal habitats supply direct or indirect economic benefits that humans get from nature, a concept known as ecosystem services. This report covers the following economic benefits derived from ecosystem services: recreation; coastal protection due to wetlands; sea turtle habitat; carbon storage; water quality protection; water supply protection; and property value enhancement.

Read the Report

Reading and Websites

Abandoned Crab Traps. S.C. Department of Natural Resources (SCDNR).

Brennessel, Barbara. Diamonds in the Marsh: A Natural History of the Diamondback Terrapin. Waltham, Massachusetts: Brandeis University Press, 2006 (reprinted: 2021).

Chavez, S., and Williard, S.A. “The effects of bycatch reduction devices on diamondback terrapin and blue crab catch in the North Carolina commercial crab fishery.” Fisheries Research, 186, no. 1 (February 2017): 94-101.

Designing a Turtle-Proof Crab Trap.” South Carolina Coastal Resources (SCDNR).

Diamondback Terrapin (Malaclemys terrapin), species profile. Savannah River Ecology Laboratory, University of Georgia.

Diamondback Terrapin In-Water Capture Rates. SCDNR.

Diamondback Terrapin Working Group.

8 Interesting Facts about Diamondback Terrapins. Discover Wildlife.

Gibbons, W.J., Lovich, J., Tucker, A., FitzSimmons, N.N., and Greene, J. “Demographic and Ecological Factors Affecting Conservation and Management of the Diamondback Terrapin (Malaclemys terrapin) in South Carolina.” Chelonian Conservation and Biology, 4, no. 4 (January 2001): 66-74.

Grice Marine Laboratory. College of Charleston.

Grosse, A.M., Daniel Van Dijk, J., Holcomb, K.L., and Maerz, J.C. “Diamondback Terrapin Mortality in Crab Pots in a Georgia Tidal Marsh.” Chelonian Conservation and Biology, 8, no. 1 (2009): 98-100.

Identifying Diamondback Terrapins.” William & Mary, Virginia Institute of Marine Science.

Northern Diamond-backed Terrapin, Malaclemys terrapin terrapin. Virginia Herpetological Society.

Orenstein, R. Turtles, Tortoises and Terrapins: Survivors in Armor. Firefly Books, 2001.

Temperature dependent sex determination in the Diamond-backed Terrapin,” Journal of Herpetology, 48, no. 4 (December 2014): 466-470.

Kiawah Island Diamondback Terrapin Project.

Waddell Mariculture Research and Development Center. SCDNR.

Williard Lab at the University of North Carolina Wilmington.