Graptemys gibbonsi

Lovich and McCoy, 1992
Pascagoula map turtle

This moderate-sized (carapace length to 29.5 cm), high-domed, medially keeled map turtle has a single vertical yellow bar on the dorsal surface of each marginal scute. The medial keel is composed of a laterally compressed knob on the posterior part of each vertebral scute; those on the second and third vertebrals are most prominent. The olive-brown carapace is divided by a black medial stripe (interrupted in some individuals), and relatively wide yellow rings and vermiculations lie on each pleural scute. Each marginal has a single relatively narrow band of dark pigment on its ventral surface. The hingeless, pale-yellow plastron is characterized by seam-following dark pigment. The skin is brown to olive with light-yellow or yellowish green stripes and blotches. Head pattern consists of a large interorbital blotch connected by thin stripes to a pair of broad postorbital blotches. The anterior part of the interorbital blotch forms a three-pronged trident between the eyes and nose on most individuals.
McKown (1972) reported that Graptemys pulchra (sensu lato) has a diploid chromosome number of 52: 26 macrochromosomes, with 48 arms, and 26 microchromosomes, with 26 arms.
Adult females are about twice the size of mature males and have conspicuously enlarged heads with broad crushing jaw surfaces. Males have longer tails with the vent posterior to the rim of the carapace. Both sexes have relatively flat plastra.

Graptemys gibbonsi is restricted to the Pascagoula and Pearl rivers and their major tributaries (including the Chickasawhay, Leaf, and Bogue Chitto) in Mississippi and Louisiana.

Geographic variation
No subspecies have been described. Individuals from the Pearl River have slightly narrower yellow bands on the dorsal surface of the marginals, but slightly wider dark bands on the ventral surface of the marginals (Lovich and McCoy, 1992). This species was formerly considered a variant of Graptemys pulchra, but Lovich and McCoy (1992) demonstrated that G. pulchra (sensu lato) is composed of three distinct species.

The Pascagoula map turtle lives in the main channel of rivers, often in swift currents. Sand or gravel bottoms and an abundance of basking sites in the form of logs and brush contribute to ideal habitat.

Natural history
Cagle (1952b) listed the sizes and sexual conditions of four male Graptemys pulchra (sensu lato), including two G. gibbonsi (Tulane University specimens 13447 and 13798). Without discriminating between species he indicated that the plastron length of the smallest mature male in the series was 89 mm. Males may mature in their fourth year. Of three female G. gibbonsi from the Pearl River with plastron lengths of 13.3, 17.0, and 19.5 cm, only the two largest specimens were sexually mature. The 17.0-cm female collected on 8 June contained two eggs in the left oviduct and one in the right oviduct; egg lengths and widths were 42.7-47.3 mm and 25.0-27.0 mm, respectively. The right ovary had three ovulation scars (corpus luteum and corpus albicans), but the left had none. Three enlarged oocytes, 8, 9, and 20.5 mm in diameter, were present on the left ovary, and the right ovary contained oocytes 14, 21, and 22 mm in diameter. Cagle thought the oviducal eggs represented the first clutch of the season; the enlarged oocytes probably would have matured and been laid later the same season.
Unpublished field notes of the late Fred Cagle indicate that Graptemys gibbonsi eats insects (including trichopterans), snails, and clams.

IUCN Red List Status (1996)
Lower risk: near threatened.