Description: The carapace of G. nigrinoda is slightly domed with the first four vertebrae possessing backward-projecting, knob-like processes, which are black in color. The second and third processes are more dominant in size compared to the first and fourth. With aging females, the knobs are reduced to small swellings.

Though similar to the nominate subspecies, the delta map turtle (Graptemys nigrinoda delticola) has brighter light colors and darker dark pigmentation than its cousin, the black-knobbed map turtle.

The carapace is dark olive-brown in color. Within each pleural, or "plate", of the shell are yellow-green circular rings which are outlined in black. Hatchlings are similar in color to adults, but the colors tend to be more vibrant and contrasting. The knob-like processes are compressed laterally.

The head is small, and is dark brown with yellow stripes, with yellow crescents behind the eye facing towards the posterior end of the turtle. These stripes continue on the legs of the turtle also, with the underside of the leg being lighter than the dorsal surface.

Sexual dimorphism is evident in this species. Females are roughly twice the size of males. Also, females' carapaces tend to be higher than those of males, though the males have longer tails than the females. Sizes (carapace lengths) have been recorded as ranging from 3 to 4 inches in males and 4 to 7.5 inches in females.

Habitat:

Habitat: Black-knobbed map turtles live strictly in aquatic areas. This is because they suffer high rates of evaporative water loss compared to other terrestrial turtles. They commonly reside in river systems with shallow to deeper channels of water. Black-knobbed map turtles are most commonly found in sand and clay substrates of rivers, with a moderate to strong river current. When out of water, these turtles bask on brush, exposed trees, and rafted plants to acquire direct sunlight.

While their overall habitat choice stays consistent, there is some microclimate variation between age groups. Younger black-knobbed turtles are observed across a wider range of aquatic habitats, especially basking sites. Older turtles are most commonly observed at the middle course of the river while juveniles are sighted at lower courses, in more shallow waters.

Range: The Delta map turtle is endemic to the southeastern United States. In Alabama, they are found in the Mobile Bay drainage.

Found in these States: AL

Diet: Black-knobbed map turtles are opportunistic omnivores, eating mollusks (Mollusca), crayfish (Astacoidea), and various aquatic vegetation. Three animal groups comprised more than 95% of the diet of black-knobbed map turtles: sponges (Porifera) and bryozoans (Bryozoa) for both sexes, plus green algae for males, and blue mussels (Mytilus edulis) for females. It was reported that small-bodied turtles and adult males feed mostly on sponges while larger-bodied turtles and adult females feed on filamentous green algae.

Reproduction: Black-knobbed map turtles mate seasonally, beginning between late winter and early spring, when the temperature starts to rise. Males seek out females and then engage in courtship activities. Male black-knobbed map turtles employ a courtship method called titillation, in which they extend a specially adapted elongated foreclaw and vibrate it in the water in front of females. When females are receptive to mating, they follow males as they swim backwards. Males then mount the carapaces of females, their tails curl together, and males inseminate females. Both sexes produce grunting noises during intercourse. These turtles are polygynandrous, which means both sexes mate with multiple partners throughout their life. Uncommonly, male to male aggressive behavior over a female turtle occurs during mating season.

Black-knobbed map turtles lay eggs that are white or cream colored. Females lay 2 to 4 clutches of eggs between March and August. Clutch sizes range from 3 to 7 eggs (5.5 on average). Larger females produce more eggs. Black-knobbed map turtles dig nests on sandbars and lay their eggs at the mouths of these nests. Although they lay eggs nocturnally, females choose nesting sites in areas that are in direct sunlight to preserve the moisture in eggs and keep the soil warm. To find an adequate nesting spot with favorable features, turtles may travel long distances along river banks. Incubation periods range from 60 to 68 days. Recorded birth masses of these map turtles is between 8 to 14 grams. Black-knobbed map turtles are independent upon hatching.

These turtles are iteroparous – they mate multiple times throughout their lives. Across the species, males typically mature after 2 to 3 years, while females mature in 6 to 7 years. In the subspecies Graptemys nigrinoda delticola, males reach sexual maturity after 3 to 4 years of age with a carapace size of 70 mm. Females reach sexual maturity after 8 to 9 years with a carapace length of 6.7 inches.

Status: Currently, this species has been petitioned and is under consideration for listing by the U.S. Fish and Wildlife Service in subcategory 3-C, but classified as Near Threatened by the IUCN Red List. The black-knobbed map turtle is most likely threatened by habitat degradation and encroachment by humans. Humans have been known to remove dead logs that line the shoreline of rivers, which the turtle uses for basking in the sun. Also, indirect disturbances of nest sites may become an issue. Additionally, the turtle population could decline due to the consumption of their eggs by humans or other predators. Fisherman, though, in most cases not purposely, can kill turtles with their trotlines, gill nets, and hoop nets.

Taxonomy: