Malaclemys terrapin is small to medium sized (to 23 cm) with markings and concentric grooves and ridges on each large scute; the limbs and head are flecked or spotted. The oblong carapace is widest behind the bridge and slightly serrated posteriorly, and it has a medial keel that varies from low and inconspicuous to prominent and knobby. Vertebrals are all broader than long; neural bones are usually hexagonal and shortest anteriorly, but the second may be octagonal. The posterior peripheral bones may be upturned. The carapace is gray, light brown, or black; if light brown, the scutes are ringed concentrically with darker pigment. Undersides of the marginals and the bridge are often patterned with dark flecks. The oblong, hingeless plastron usually is greenish to yellow with dark blotches or flecks. Its hindlobe is slightly notched posteriorly, and the plastral formula is: abd >< an > fem > gul > pect > hum. The humero-pectoral seam does not cross the entoplastron. The bridge is wide, but its buttresses are relatively weak. The short head is narrow in males but broad in females; it is flat and of a uniform color dorsally. It has a nonprotruding snout and an unnotched or slightly notched upper jaw. The frontal bone often enters the orbit, the maxilla is not or only narrowly in contact with the quadratojugal, and a large posterior process of the pterygoid touches the exoccipital. The triturating surface of the maxillae and palatine is smooth, lacking a ridge, and wide in females but narrow in males. Head and neck are gray to black with flecks or curved markings, but never stripes. Eyes are black, large, and prominent. Jaws are light in color, the chin may be black, and the limbs gray to black.
Diploid chromosomes total 50 (Stock, 1972): 20 metacentric or submetacentric, 10 subtelocentric, and 20 acrocentric or telocentric. Collectively the diploid complement has 80 arms (Stock, 1972). McKown (1972) noted a similar diploid chromosome number with 26 macrochromosomes and 24 microchromosomes, but recorded only 72 arms (48 macro and 24 micro).
Adult males are smaller (10-14 cm) than females (15-23 cm). Females have broader, blunter heads, deeper shells, and shorter tails than males, and their vent is closer to the body and under the posterior marginals.
Malaclemys occurs along the Atlantic and Gulf coasts from Cape Cod to Texas; it is also found on the Florida Keys.
Seven subspecies are recognized. Malaclemys terrapin terrapin (Schoepff, 1793), the northern diamondback terrapin, ranges along the Atlantic coast from Cape Cod to Cape Hatteras. Its medial keel does not bear terminal knobs on each scute; sides of the carapace diverge posteriorly. Carapace varies from uniform black to light brown with distinct concentric, dark rings; plastron variable, orangish to greenish gray. M. t. centrata (Latreille, in Sonnini and Latreille, 1802), the Carolina diamondback terrapin, ranges from Cape Hatteras south along the coast to northern Florida. Its medial keel bears no terminal knobs on each scute; sides of the carapace are nearly parallel; the marginals curl upward; otherwise similar to M. t. terrapin. The Florida East Coast diamondback terrapin M. t. tequesta Schwartz, 1955 occurs along the Atlantic coast of Florida. Its medial keel bears posteriorly facing tubercles or knobs. The carapace is dark or tan with no pattern of concentric light circles. The centers of large scutes may be slightly lighter than surrounding areas. The mangrove diamondback terrapin M. t. rhizophorarum Fowler, 1906 is restricted to the Florida Keys. Its medial keel bears terminal bulbous knobs, and its shell is strongly oblong. The carapacial scutes have no light centers, and the ventral seams of the marginals and plastral scutes are often outlined with black. Spots on the neck often fuse to form a streaked pattern, and the hindlegs may be striped. M. t. macrospilota Hay, 1904a, the ornate diamondback terrapin, ranges along the Gulf coast from Florida Bay to the panhandle. Its medial keel has terminal, often bulbous, knobs, and the carapacial scutes contain orange or yellow centers. M. t. pileata (Wied-Neuweid, 1865), the Mississippi diamondback terrapin, ranges along the Gulf coast from the Florida Panhandle to western Louisiana. Its medial keel has terminal tuberculate knobs. The scutes of the oval carapace lack light centers. Top of the head, upper lip, neck, and limbs are black or dark brown, the upturned edges of marginals are orange or yellow, and the plastron is yellow and often dusky. The Texas diamondback terrapin M. t. littoralis Hay, 1904a occurs along the Gulf coast from western Louisiana to western Texas, and has a keel with terminal knobs on its deep carapace. Its carapacial scutes lack distinct light centers, the plastron is pale or white, the upper lip and top of head whitish, and the neck and legs greenish gray with heavy black spotting.
In general, geographic variation is poorly defined and may be clinal. Analysis has shown that mitochondrial DNA genotypic diversity and divergence levels are exceptionally low among putative subspecies. One restriction site polymorphism was geographically informative and clearly distinguished populations on either side of Cape Canaveral, Florida. This break is congruent with the geographic distribution of populations with and without knobby medial keels (Lamb and Avise, 1992).
A resident of brackish water, Malaclemys lives in coastal marshes, tidal flats, coves, estuaries, and the lagoons behind barrier beaches. Juveniles seem to spend the first years of their life under mats of tidal debris and flotsam (Pitler, 1985).
Size and age of sexual maturity varies geographically (Seigel, 1984). Generally, males mature at about 9 cm before the end of their third year (Cagle, 1952a; Seigel, 1984; Lovich and Gibbons, 1990). Females mature after the sixth year at about 13-17 cm (Cagle, 1952a; Montevecchi and Burger, 1975; Lovich and Gibbons, 1990). The sexual cycles have not been adequately studied.
Courtship and mating occur from late March to May during daylight hours. Seigel (1980a) reported that courtship begins with the female floating at the water's surface; the male approaches from the rear and nuzzles or nudges her cloacal region with his snout. If she remains still, he mounts immediately, and copulation occurs at the surface. If the female swims away, the male pursues her until he catches her. Females oviposit from April through July, depending on latitude.
Nesting takes place during the day, usually near high tide, but females do not nest during heavy or prolonged rains (Burger and Montevecchi, 1975). Nests are located above the high-tide mark along the sandy edges of salt marshes and rivers, in the dunes of sea beaches, and on offshore islands. They are flask-shaped, and wider than deep; depths are to 20 cm and the diameter of the egg chamber is up to 10 cm.
Females produce at least two, and possibly up to five, clutches a year. Clutches contain 4-22 eggs, with females in the southern part of the range producing fewer but larger eggs than females from the north (Montevecchi and Burger, 1975; Seigel, 1980b; Aresco, 1996; Roosenburg and Dunham, 1997). The oblong to elliptical eggs (26.0-42.0 x 15.9-27.0 mm) are pinkish white when fresh and have thin, easily dented shells; the surface is coated with minute, scattered calcareous lumps. Natural incubation in New Jersey takes 61-104 days and hatching occurs from late August to mid-October (Burger, 1977). Incubation period is shorter for clutches laid earlier in the season. Some young may hatch but not leave the nest until the next spring (Gibbons and Nelson, 1978; Lazell and Auger, 1981).
Hatchlings are patterned much like adults but usually brighter. Their shells are round, lengths are about 25.0-34.0 mm.
Diamondback terrapins probably are scavengers, but they also take live food. Females, with their broader heads and crushing jaws, are well-adapted for eating hard-shelled prey, including salt marsh periwinkles and other snails, small bivalves, barnacles, and crabs (Tucker et al., 1995a). Other prey items include marine annelid worms, plant material, fish, and carrion (Ernst et al., 1994). Generally, the smaller, narrower-headed males eat smaller prey and softer prey items. Captives feed readily on chopped and whole fish, crabs, snails, oysters, clams, insects, marine annelids, beef, and commercial trout chow. A terrapin's desire to eat is suppressed as the salinity water of its habitat is increased, it is all suppressed at lower temperatures (Davenport and Ward, 1993). This turtle's desire for crab meat has led to many being trapped and drowning in commercial crab pots (traps) set in their feeding habitats (Carl H. Ernst, pers. obs.).
IUCN Red List Status (1996)
Lower risk: near threatened.