Diamondback terrapin

Description

The common name refers to the diamond pattern on top of its shell (carapace), but the overall pattern and coloration vary greatly. The shell is usually wider at the back than in the front, and from above its appears wedge-shaped. The shell coloring can vary from brown to grey, and its body color can be grey, brown, yellow, or white. All have a unique pattern of wiggly, black markings or spots on their body and head. The diamondback terrapin has large webbed feet. The species is sexually dimorphic in that the males grow to approximately 13 cm (5.1 in), while the females grow to an average of around 19 cm (7.5 in), though they are capable of growing larger. The largest female on record was just over 23 cm (9.1 in) in length. Specimens from regions that are consistently warmer in temperature tend to be larger than those from cooler, more northern areas. Male diamondback terrapins weigh 300 g (11 oz) on average, while females weigh around 500 g (18 oz). The largest females can weigh up to 1,000 g (35 oz).

Adaptations to their environment

Terrapins look much like their freshwater relatives, but are well adapted to the near shore marine environment. They have several adaptations that allow them to survive in varying salinities. They can live in full strength salt water for extended periods of time, and their skin is largely impermeable to salt. Terrapins have lachrymal salt glands, not present in their relatives, which are used primarily when the turtle is dehydrated. They can distinguish between drinking water of different salinities. Terrapins also exhibit unusual and sophisticated behavior to obtain fresh water, including drinking the freshwater surface layer that can accumulate on top of salt water during rainfall and raising their heads into the air with mouths open to catch falling rain drops.

Terrapins are strong swimmers. They have strongly webbed hind feet, but not flippers as do sea turtles. Like their relatives (Graptemys), they have strong jaws for crushing shells of prey, such as clams and snails. This is especially true of females, who have larger and more muscular jaws than males.

Subspecies

Seven subspecies are recognized including the nominate race.

The isolated Bermudan population, which arrived on its own (not introduced by humans), has not been officially assigned to a subspecies, but based on mtDNA it is closely related to the population from The Carolinas.

Distribution and habitat

Diamondback terrapins live in the very narrow strip of coastal habitats on the Atlantic and Gulf coasts of the United States, from as far north as Cape Cod, Massachusetts to the southern tip of Florida and around the Gulf Coast to Texas. In most of their range terrapins live in Spartina marshes that are flooded at high tide, but in Florida they also live in mangrove swamps. This turtle can survive in freshwater as well as full-strength ocean water but adults prefer intermediate salinities. They have no competition from other turtles, although snapping turtles do occasionally make use of salty marshes. It is unclear why terrapins do not inhabit the upper reaches of rivers within their range, as in captivity they tolerate fresh water. It is possible they are limited by the distribution of their prey. Terrapins live quite close to shore, unlike sea turtles, which wander far out to sea; however, a population of terrapins on Bermuda has been determined to be self-established (not introduced by humans). Terrapins tend to live in the same areas for most or all of their lives, and do not make long distance migrations.

Life cycle

Adult diamondback terrapins mate in the early spring, and clutches of 4-22 eggs are laid in sand dunes in the early summer. They hatch in late summer or early fall. Maturity in males is reached in 2–3 years at around 4.5 inches (110 mm) in length; it takes longer for females: 6–7 years (8–10 years for northern diamondback terrapins) at a length of around 6.75 inches (171 mm).

Reproduction

Like all reptiles, terrapin fertilization occurs internally. Courtship has been seen in May and June, and is similar to that of the closely related red-eared slider (Trachemys scripta). Female terrapins can mate with multiple males and store sperm for years, resulting in some clutches of eggs with more than one father.

Like many turtles, terrapin have temperature dependent sex determination, meaning that the sex of hatchlings is the result of incubation temperature. Females can lay up to three clutches of eggs/year in the wild, and up to five clutches/year in captivity. It is not known how often they may skip reproduction, so true clutch frequency is unknown.

Females may wander considerable distances on land before nesting. Nests are usually laid in sand dunes or scrub vegetation near the ocean in June and July, but nesting may start as early as late April in Florida. Females will quickly abandon a nest attempt if they are disturbed while nesting. Clutch sizes vary latitudinally, with average clutch sizes as low as 5.8/eggs/clutch in southern Florida to 10.9 in New York. After covering the nest, terrapins quickly return to the ocean and do not return except to nest again.

The eggs usually hatch in 60–85 days, depending on the temperature and the depth of the nest. Hatchlings usually emerge from the nest in August and September, but may overwinter in the nest after hatching. Hatchlings sometimes stay on land in the nesting areas in both fall and spring, and they may remain terrestrial for much or all of the winter in some places. Hatchling terrapins are freeze tolerant, which may facilitate overwintering on land. Hatchlings have lower salt tolerance than adults, and Gibbons et al. provided strong evidence that one- and two-year-old terrapins use different habitats than do old individuals.

Growth rates, age of maturity, and maximum age are not well known for terrapins in the wild, but males reach sexual maturity before females because of their smaller adult size. In females at least, sexual maturity is dependent on size rather than age. Estimations of age based on counts of growth rings on the shell are as yet untested, so it is not clear how to determine the ages of wild terrapins.

Seasonal activities

Because nesting is the only terrapin activity that occurs on land, most other aspects of terrapin behavior are poorly known. Limited data suggest that terrapins hibernate in the colder months in most of their range, in the mud of creeks and marshes.

Diet

Diamondback terrapin diets are not generally well studied, and almost all work on diets has been done in the southeastern part of their range. They eat shrimp, clams, crabs, mussels, and other marine invertebrates, especially periwinkle snails. At high densities they may eat enough invertebrates to have ecosystem-level effects, partially because periwinkles can themselves overgraze important marsh plants.

Status

In the 1900s the species was once considered a delicacy to eat and was hunted almost to extinction. The numbers also decreased due to the development of coastal areas and, more recently, wounds from the propellors on motorboats. Another common cause of death is the trapping of the turtles under crabbing and lobster nets. Due to this, it is listed as an endangered species in Rhode Island, is considered a threatened species in Massachusetts, and is considered a "species of concern" in Georgia, Delaware, Alabama, Louisiana, North Carolina, and Virginia. In New Jersey, it was recommended to be listed as a species of Special Concern in 2001. In July 2016, the species was removed from the New Jersey game list, and is now listed as non-game with no hunting season. In Connecticut there is no open hunting season for this animal. However, it holds no federal conservation status.

Conservation Status

The species is classified as Near Threatened by the IUCN due to decreasing population numbers in much of its range. There is limited protection for terrapins on a state-by-state level throughout its range; it is listed as Endangered in Rhode Island and Threatened in Massachusetts. The Diamondback Terrapin Working Group deal with regional protection issues. There is no national protection except through the Lacey Act, and little international protection.

Diamondback terrapins are the only U.S. turtles that inhabit the brackish waters of estuaries, tidal creeks, and salt marshes. With a historic range stretching from Massachusetts to Texas, terrapin populations have been severely depleted by land development and other human impacts along the Atlantic coast.

Earthwatch Institute, a global non-profit that teams volunteers with scientists to conduct important environmental research, supports a research program called "Tagging the Terrapins of the Jersey Shore." This program allows volunteers to explore the coastal sprawl of New Jersey’s Ocean County on Barnegat Bay, one of the most extensive salt marsh ecosystems on the East Coast, in search of this ornate turtle. On this project, volunteers contribute to environmental sustainability in the face of rampant development. Veteran turtle scientists Dr. Hal Avery, Dr. Jim Spotila, Dr. Walter Bien and Dr. Ed Standora are overseeing this program and the viability of terrapin populations in the face of growing environmental change.

Threats

The major threats to diamondback terrapins are all associated with humans, and probably differ in different parts of their range. People tend to build their cities on ocean coasts near the mouths of large rivers, and in doing so they have destroyed many of the huge marshes terrapin inhabited. Nationwide, probably >75% of the salt marshes where terrapins lived have been destroyed or altered. Currently, ocean level rise threatens the remainder.

Traps used to catch crabs both commercially and privately have commonly caught and drowned many diamondback terrapins, which can result in male-biased populations and local population declines and even extinctions. When these traps are lost or abandoned (= “ghost traps”), they can kill terrapins for many years. Terrapin-excluding devices are available to retrofit crab traps, these reduce the number of terrapins captured while having little or no impact on crab capture rates. In some states (NJ, DE, MD), these devices are required by law.

Nests, hatchlings, and sometimes adults, Burke unpub. data) are commonly eaten by raccoons, foxes, rats, and many species of birds, especially crows and gulls. Density of these predators are often increased because of their association with humans. Predation rates can be extremely high; predation by raccoons on terrapin nests at Jamaica Bay Wildlife Refuge in New York varied from 92-100% each year from 1998–2008, Burke unpubl. data).

Terrapins are killed by cars when nesting females cross roads, and mortality can be high enough to seriously impact populations. Terrapins are still harvested for food in some states. Terrapins may be affected by pollutants such as metals and organic compounds, but this has not been demonstrated in wild populations.

There is an active casual and professional pet trade in terrapins, and it is unknown how many are removed from the wild for this purpose. Some people breed the species in captivity and some color variants are considered especially desirable. In Europe, Malaclemys are widely kept as pets, as are many closely related species.

Relationship with humans

In Maryland, diamondback terrapins were so plentiful in the 18th century that slaves protested the excessive use of this food source as their main protein. Late in the 19th century, demand for turtle soup claimed a harvest of 89,150 pounds from Chesapeake Bay in one year. In 1899, terrapin was offered on the dinner menu of Delmonico's Restaurant in New York City as the third most expensive item on the extensive menu. A patron could request either Maryland or Baltimore terrapin at a price of $2.50. Although demand was high, over capture was so high by 1920, the harvest of terrapins reached only 823 pounds for the year.

According to the FAA National Wildlife Strike Database, a total of 18 strikes between diamondback terrapins and civil aircraft were reported in the US from 1990 to 2007, none of which caused damage to the aircraft. On July 8, 2009, flights at John F. Kennedy Airport in New York City were delayed for up to one and a half hours as 78 diamondback terrapins had invaded one of the runways. The turtles, which according to airport authorities were believed to have entered the runway in order to nest, were removed and released back into the wild. A similar incident happened on June 29, 2011, when over 150 turtles crossed runway four, closing the runway and disrupting air traffic. Those terrapins were also relocated safely. The Port Authority of New York and New Jersey installed a turtle barrier along runway 4L at JFK to reduce the number of terrapins on the runway and encourage them to nest elsewhere. Nevertheless, on June 26, 2014, 86 terrapins made it onto the same runway, as a high tide carried them over the barrier. Their population is controlled by the racoon population; it has been shown that as the racoons decrease in number, mating terrapins increase, leading to increased turtle activity at the airport.

Many human activities threaten the safety of diamondback terrapins. The terrapins get caught and drown in crab nets that humans put out, are suffocated by pollution that humans greatly contribute to, and lose their marsh and estuarine habitats because of urban development.

History as a delicacy

Diamondback terrapins were heavily harvested for food in colonial America, and probably before that by Native Americans. Terrapins were so abundant and easily obtained that slaves and even the Continental Army ate large numbers of them. By 1917, terrapins had become a fashionable delicacy and sold for as much as $5 each. Huge numbers of terrapins were harvested from marshes and marketed in cities. By the early 1900s populations in the northern part of the range were severely depleted, and the south was greatly reduced as well. As early as 1902 the U.S. Bureau of Fisheries (which later became the U.S. Fish and Wildlife Service) recognized that terrapin populations were declining and started building large research facilities, centered at the Beaufort, North Carolina Fisheries Laboratory, to investigate methods for captive breeding terrapins for food. People tried (unsuccessfully) to establish them in many other locations, including San Francisco.

Use as a symbol

Maryland named the diamondback terrapin its official state reptile in 1994. The University of Maryland, College Park has used the species as its nickname (the Maryland Terrapins) and mascot (Testudo) since 1933, and the school newspaper has been named The Diamondback since 1921. The athletic teams are often referred to as "Terps" for short. The Terrapin turtle has also been a symbol of the Grateful Dead because of their song "Terrapin Station". Many images of the Terrapin dancing with a tambourine appear on posters, T-shirts and other places in Grateful Dead memorabilia.