The Northern Diamondback Terrapin: Conserving a Bellwether Species on the Outer Cape

        by Don Lewis, Cape Cod Consultants

        & Massachusetts Audubon Society's Wellfleet Bay Wildlife Sanctuary 

Diamondback terrapins are elusive turtles of the Atlantic and Gulf seacoasts' rich estuarine system of rivers, creeks, coves, bays and marshes. According to Turtles of the United States and Canada (Smithsonian Institution Press, Washington and London, 1994) there are seven formally described sub-species: Malaclemys terrapin terrapin from Cape Cod to Cape Hatteras; M. terrapin centrata from Cape Hatteras to northern Florida; M. terrapin tequesta on the Atlantic coast of Florida; M. terrapin rhizophorarun in the Florida Keys; M. terrapin macrospilota from the Florida Bay to the panhandle; M. terrapin pileata from the Florida panhandle to western Louisiana; and M. terrapin littoralis from western Louisiana to western Texas.

An important part of East Coast history and lore, diamondbacks were said to have fed our impoverished minutemen during the hard times of the Revolutionary War. Nearly harvested to extinction in the last century, terrapins were thought to be making a slow recovery, but today their numbers may once again be declining as pressures intensify on their fragile habitat from development, pollution and commercial activities. Vacation and retirement homes press deeper into terrapin environment as demand escalates for waterfront property. Accidental spills, hazardous debris and "ghost" equipment from seafood farming and harvesting, all take their toll on these turtles and their habitat. And commercial activities, whether directly targeted at terrapins or affecting diamondbacks only as a by-product of fishing other species, continue to threaten their long-term viability.

Even in Maryland, where the terrapin is a state icon and is still harvested commercially, they have acknowledged the species is "declining in population and requires concerted conservation efforts." Maryland's governor recently proclaimed May 13th, 2000 as Diamondback Terrapin Day to increase "public awareness for natural resources conservation and stewardship for this species."

Massachusetts lists the diamondback terrapin as a threatened species under its Natural Heritage & Endangered Species Program. 

Sign Post at End of the Terrapin Universe

The northern diamondback terrapin, Malaclemys terrapin terrapin, occupies a unique niche on the Outer Cape, living permanently in its rich system of salt marshes. While this subspecies can be found from Cape Hatteras to Cape Cod, Wellfleet Harbor marks its absolute northern limit, subject to severe climatic variations as well as the steady advance of human activity into its once secluded habitat. Observing an animal at the extreme edge of its range serves as an important bellwether to monitor the health of the entire ecosystem. Diamondback terrapins are especially suitable for this environmental role.

First, they reflect changes in bay water quality as well as variations of the surrounding uplands. Terrapins reside year-around in the brackish waters of Cape Cod Bay's estuaries. They mate in its rivers and creeks and coves, and burrow into muddy bottoms for winter brumation. They depend on the bay's rich salt marshes and tidal flats for nourishment. For nesting, though, diamondbacks must rely on an accessible upland shoreline.

Secondly, terrapins are adaptable. They have adjusted to tremendous tidal forces in Wellfleet Harbor with up to 15-foot swings from high to low tides that drain the marshes twice daily. Formerly, sandy dunes have been covered in vegetation, pristine beaches have been sea-walled and sandbagged, and cottages have crept into their secluded habitat. Yet terrapins have coped, exploiting dirt roads and driveways and narrow bands of loose soil above the wrack line for nesting.

Thirdly, diamondbacks are extremely hardy and resilient survivors. They lose limbs to predators and debris, but soldier on. They suffer traumatic injuries in boat strikes and automobile accidents, yet snap back. I call them "Timex critters," because "they take a lickin' and keep on tickin'." Over this last research season we have rediscovered terrapins clinging to marginal marsh habitats in scattered, highly developed locations of Cape Cod and Buzzards Bay.

So, when terrapin populations decline or die-offs occur, something significant has happened within the salt marsh environment.

Terrapin Primer

In this woefully brief and inadequate primer, I will sketch some of the characteristics and behaviors of the terrapins of the Land of Ooze, the salt marshes of Outer Cape Cod. These turtles are small to medium size with distinct gender dimorphism. Mature females range from eight to nine inches straight-line carapace length, weighing up to three and a half pounds, while males grow to almost five inches and about 2/3 of a pound.

This pair captured last year well illustrates the size difference between the sexes. The female [#697 on the left] also proved a surprising success story. Marked after nesting in June 1997, she was hit by a car the next summer and given emergency treatment. Her cracked shell patched, she was returned to the creek with only a hope and a prayer for recovery. But here she appeared on 13 September 1999, fully healed and engaged in normal mating with only a few bumps and scars to recall her unfortunate encounter with people.

Terrapins have a hingeless shell. The top or carapace normally has 13 scutes ringed like diamond facets, ten around the perimeter with three in the center -- eight costals/pleurals and five vertebrals. The carapace is usually rimmed with 12 marginals on each side and a nuchal in the front. The bottom shell or plastron is symmetrically bisected with six sections on each side of the centerline.

A turtle's age can be estimated by annual growth marks, especially on the plastron. A terrapin's shell smoothes with wear and tear and usually by ten years, a turtle's age can no longer be easily determined. While we are not certain how long our Cape Cod terrapins live, we suspect they reach 40 or 50 years of age -- at least.

Terrapin #86 was first observed in 1980 when she was already recorded as an old female turtle. Recaptured again this June, nesting in the same spot as 21 seasons previous, she showed all the battle-scarred signs of a truly ancient diamondback terrapin.

Reproduction and Nesting

Reportedly, males attain sexual maturity after the third year, while females wait until at least the sixth. In Wellfleet Harbor, though, data reveal that female terrapins begin nesting after they reach 15.75 cm of carapace length and about 650 grams of weight, usually around the eighth year. 

Our nesting season, which varies in each geographical area, lasts from early June through mid July. Two clutches of pinkish white eggs, ranging in number from four to 22, are laid above the high water wrack line. The first nesting peak comes around 23 to 26 June. The period between clutches ranges from as little as eight days to as much as 25 days, with most around 16 days.

Terrapins demonstrate remarkable nesting site fidelity. Most return to the same location to lay both clutches each year. At least some turtles, though, employ a slightly different strategy, splitting sites for each clutch. Terrapin #145, for instance, has been observed since 1989 when she was 11-to-12 years old. For her first nest in mid-June, she chooses a dirt track behind the protective fore dune on northeast Lieutenant Island. Her second clutch is deposited in a dirt road along the island's south coast, about a quarter mile away. She remains faithful to each site for each successive clutch. Another terrapin, #365, has been seen nesting on either shore of Blackfish Creek.

While we don't know the reason for this alternative nesting strategy, spreading the two clutches seems to improve the chance that at least one will elude predation and the different locations may increase gender diversity of the hatchlings.

Designed for the water, terrapins encounter their greatest risk when ashore for nesting. Predators, especially fox and raccoon, have been known to specialize in hunting this protein rich prey. But today, the worst danger comes from people. Trophy vacation homes ring their nesting sites. Teetering at dune's edge, these million dollar houses demand protective seawalls to delay the inevitable advance of the bay.

Other sites are lost to decorative landscaping, locking loose dune soil in lawns and invasive ground cover. Turtles are forced to march inland and search for less suitable locations, extending their vulnerability and decreasing the viability of hatchlings. Dirt roads and driveways, free of vegetation, offer a risky, but attractive alternative, and each year more and more turtles are hit either while nesting in roads or in transit to upland sites.

Not content with just any nest, turtles have been seen digging as many as ten test holes before finally depositing their offspring. Yet others have been considerably less fussy. Some dropped eggs in the hands of observers who interrupted their nesting a bit too prematurely. 

And then there was turtle #765. She became so impatient during processing last June that she laid a clutch of eggs on the Sahara colored rug of my jeep. A surprised surrogate mom, I located a secluded south-facing site several feet above the high water mark with ample loose soil surrounded by protective vegetation. I imitated a nesting female, dug a narrow vertical shaft, scooped out a larger chamber beneath, gently stacked the four eggs together, and covered and smoothed the hole. Having witnessed wholesale predation of terrapin nests throughout the summer, I decided to paint the site with human scent as a warning.

As luck would have it, this nest escaped predators and on 27 September I became the proud virtual father of four infant hatchlings, one still sporting its egg tooth and another its yolk sac.Eggs take about three months to hatch on the Outer Cape. Yet, nest monitoring over the last decade indicates that most hatchlings seem to emerge in the spring after over-wintering in the ground. These tiny critters are a little over an inch long in carapace length and hit the scales at about four grams.

But most nests and hatchlings succumb to predation. Raccoons, skunks, foxes, birds, and many other land predators patrol nesting grounds for the scent of terrapin eggs; crabs, fish, shorebirds and a host of marine predators wait at the water's edge. In many secluded locations, an entire day's nesting can be wiped out overnight by voracious predators. Rain seems to disguise the scent, and nests which remain undiscovered until the first downpour usually escape excavation. Ironically, preliminary observations indicate that terrapin nests located on dirt roads and in driveways may not be as susceptible to predation. Disguising odors of cars and human activities may mask the nests, and human disturbance may keep predators away from nests until rains obscure their location. So, while nesting females and emerging hatchlings are extremely vulnerable to accidents at these sites, decreased predation may more than compensate for that danger.

A Quarter Century of Research and Conservancy

Scratching out an existence at the absolute edge of their range, terrapins of the Outer Cape serve as a signal species within our coastal environment. Recognizing this importance, the Massachusetts Audubon Society's Wellfleet Bay Wildlife Sanctuary has conducted research efforts since 1980 with informal observations stretching back to the mid-1970s. The core of its data collection program consists of capture, mark and recapture studies. From spring to fall, turtles are captured in creeks, coves and bays by a variety of strategies: wading, canoeing, kayaking, netting, trapping and seining. During the nesting season, researchers and volunteers walk designated shorelines, observe and mark nests, and capture/recapture turtles after their eggs have been deposited. Each turtle is measured for carapace and plastron length and width, weighed, examined for general health and anomalies, individually marked and digitally photographed for follow-up study. Through this method, histories for many turtles have spanned decades. Growth models can be constructed and demographic profiles of the Outer Cape population can be developed. And, by the percentage of marked and unmarked terrapins discovered in each capture event, population estimates can be obtained and trends can be followed.

Events affecting diamondback terrapin habitat have injected new urgency into these research and conservancy efforts. Preliminary observations in 1999 suggested a marked decrease over the last decade in nesting activity at one of the prime locations within Wellfleet Harbor, as well as a deterioration of the salt marsh and tidal flat habitat from potentially dangerous debris. Last year ended with a mass die-off event of as many as a 100 turtles, from yearlings and juveniles to ancient nesting females, found dead in an isolated marsh probably drowned by "ghost" netting. 

Proposals are pending to expand commercial shellfish farming and harvesting activities into tidal flats adjacent to the principal terrapin nesting and nursery habitat on the Outer Cape. Home development ringing the once secluded salt marsh continues unabated and seawalls still are being erected along the shoreline. Gravid turtles, forced to settle for less suitable nesting sites on dirt roads or to migrate to inland sites, were hit by vehicles in record numbers in 1999.

Taken as a whole, these pressures on diamondbacks and their habitat may pose a substantial risk to their continued survival at the end of the terrapin universe.

Paludal Posse

Girl Scouts made signs to mark nesting sites and to caution motorists of turtle crossings. Boy Scouts cleared vegetation for nesting patches and built nest protectors. Homeowners formed a network of terrapin spotters abutting key nesting sites. They tracked turtles, marked nests and vectored researchers for captures. Vacationers joined the crusade. Alerted by fliers posted in cottages and the scouts' handmade signs, renters enlisted in our paludal posse. One family saw a turtle nesting at the intersection of two busy dirt roads. They quickly moved their car in front of the nest, blocking vehicular and pedestrian traffic over the spot. They posted their own painted sign on the car window to protect the location.

One teenage volunteer patrolled the island on his bike, checking for turtles and helping them safely dodge summer traffic. On 25 June he encountered terrapin #888, coming ashore with the afternoon high tide. She scaled the northern dune off Blackfish Creek and plowed through the waist high marsh grasses of Turtle Pass. Hitting the main road, #888 lumbered on to the second island and began her inland and upward climb. By now she had attracted the posse's attention; residents and tourists alike tagged along behind our pied piper intern, who delivered impromptu natural history lectures on threatened diamondback terrapins. He kept the crowd at a respectful distance as she began digging test holes and tasting soil, now almost a half mile from her initial landing site. When satisfied with location and substrate, #888 spent the next 45 minutes carving her nest and depositing her contribution to the Year 2000 hatchling class. As she trekked back to the sea, our volunteer carefully marked and protected the site for follow-on observation.

"We Never Give Up" 

One of the posse called with sad news on 5 July. A female had been struck and killed as she made a nesting run on Lieutenant Island. When he checked her broken shell, he spotted an egg. Inspired by the groundbreaking work of Roger Wood in New Jersey, I sped to the scene and harvested 12 potentially viable eggs. The resident who reported the accident exclaimed, "We never give up. Do we? We even reap hope from tragedy." While we won't know for a couple of months if these eggs hatch, he was absolutely right. We never give up.

The diamondback terrapin is a shy creature extremely sensitive to human disturbance. They are so rarely seen that most residents and visitors to Cape Cod are surprised to learn they share the same habitat with them. They are very hardy critters. We find many turtles with missing appendages and traumatic carapace scars, yet they seem to hold their own. One breeding female, whom we affectionately dub Stumpy, has two missing forelimbs. Yet twice each year she drags herself onto the beach to nest.