Black-knobbed map turtle
This is a small (to 15 cm) turtle with broad, rounded, black, knoblike vertebral projections and a strongly serrated posterior carapacial rim. Its vertebrals are broader than long; the 1st is smallest. The carapace is dark olive; each pleural and marginal has a narrow, yellow or orange, semicircular or circular mark. The yellow plastron is often tinted with red and has a black, branching pattern. The plastral hindlobe is notched and the plastral formula is: an > abd > fem >< pect >< gul > hum. The narrow head has a nonprotruding snout and an upper jaw that is neither hooked nor notched. It is black with yellow stripes; the postorbital mark is a vertical crescent connecting dorsally with that of the opposite side to form a Y-shaped mark. Usually two to four neck stripes enter the orbit; the interorbital stripe is narrower than the neck stripes. The lower jaw has longitudinal yellow stripes as wide as the black interstices. Other skin is black, but yellow stripes occur on the limbs.
The diploid chromosome number is 50: 26 macrochromosomes and 24 microchromosomes (Killebrew, 1977a).
Adult males have long thick tails, with the vent posterior to the carapacial rim; they also have elongated foreclaws. Adult females are 10-15 cm in carapace length, adult males 7.5-10.0 cm.
Graptemys nigrinoda occurs below the fall line in the Alabama, Tombigbee, and Black Warrior river systems of Alabama and Mississippi.
Two subspecies are recognized. Graptemys nigrinoda nigrinoda Cagle, 1954a, the northern black-knobbed map turtle, is restricted to the upper parts of the Tombigbee and Alabama river systems in Alabama and Mississippi. It has a poorly developed plastral figure, which never occupies more than 30% of the plastron. Its postocular mark is, typically, crescentic and strongly recurved. The light lines that reach the eye are seldom interrupted, and its soft parts are predominantly yellow. G. n. delticola Folkerts and Mount, 1969, the southern black-knobbed map turtle, occurs in the interconnecting streams and lakes of the delta of the Mobile Bay drainage, in Baldwin and Mobile counties, Alabama. It differs from the nominate race in having a plastral figure that occupies more than 60% of the plastron. Its postocular mark is neither crescentic nor strongly recurved laterally. In many individuals the light lines that reach the eye are interrupted, and the soft parts are predominantly black.
Sand- and clay-bottomed streams with moderate currents and abundant basking sites of brush, logs, and debris are favored. Graptemys nigrinoda is found in deeper waters than either G. oculifera or G. flavimaculata.
Males are mature at 6.8-7.1 cm plastron length and 3-5 years of age (Cagle, 1954a; Lahanas (1982). Females mature at a plastron length of 13.5-17.7 cm (Cagle, 1954a; Shoop, 1967). The male and female sexual cycles are essentially like those of other North American emydids (Lahanas, 1982). Male courtship involves stroking of the female's face with the elongated foreclaws.
Nesting takes place in June. Eight nests of Graptemys n. delticola studied by Lahanas (1982) contained 3-7 elliptical (37.0 x 23.8 mm), translucent pink eggs. Nine clutches incubated under both natural and artificial conditions hatched after 60-68 days (at an average of 30°C). Hatchlings are brightly marked with knobby vertebral projections, and the entire carapacial rim serrated.
Graptemys nigrinoda is a basker, and very wary. Snails and insects seem to be its preferred food.
IUCN Red List Status (1996)
Lower risk: near threatened.