The black-knobbed map turtle (''Graptemys nigrinoda''), formerly known as the black-knobbed sawback, is a small to medium-sized aquatic

turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked tu ...

with light gray skin.Blankenship, Emmett L., Brian P. Butterfield, and James C. Goodwin. 2008. "''Grapemys nigrinoda'' Cagle 1954 - Black-Knobbed Map Turtle, Black-Knobbed Sawback." Conservation Biology of Freshwater Turtles and Tortoises: A Compilation Project of the IUCN/SSC Tortoise and Freshwater Turtle Specialist Group. Chelonian Research Monographs (ISSN 1088-7105) (5): 005.1 - 005.6. Some of the most distinguishing characteristics of the black-knobbed map turtle, and the ''

Graptemys ''Graptemys'' is a genus of freshwater turtles containing 14 species, commonly known as map turtles. Crother, B. I. (editor) (2017). Scientific and Standard English Names of Amphibians and Reptiles of North America North of Mexico, with Comments ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

, are the protruding "spikes" on the turtle's

carapace A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...

. This species inhabits mainly the

fall line A fall line (or fall zone) is the area where an upland region and a coastal plain meet and is typically prominent where rivers cross it, with resulting rapids or waterfalls. The uplands are relatively hard crystalline basement rock, and the coa ...

s of rivers in the Mobile Bay drainage, in Alabama and Mississippi. Behler, J.L., and F.W. King. 1979. ''The Audubon Society Field Guide to North American Reptiles and Amphibians''. New York: Alfred A. Knopf. 743 pp. . ("Black-knobbed Sawback", ''Graptemys nigrinoda'', pp. 462-63 + Plate 281.)

Description

The

of ''G. nigrinoda'' is slightly domed with the first four

vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic i ...

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. 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 In oviparous biology, a hatchling is a newly hatched fish, amphibian, reptile, or bird. A group of mammals called monotremes lay eggs, and their young are hatchlings as well. Fish Fish hatchlings generally do not receive parental care, similar to ...

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 Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

.Mount, R.H. 1975. ''The Reptiles and Amphibians of Alabama''. Auburn Univ. Agr. Exp. Sta. Auburn, Alabama, 347 pp.

Sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

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 7.6 to 10.2 cm (3–4 in) in males and 10.2-19.1 cm (4-7.5 in) in females.

Distribution

The black-knobbed map turtle is

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...

to the southeastern United States. In Alabama, they are found in the Mobile Bay drainage. In Mississippi, they are found in the Tombigbee River system and in the Black Warrior River as far north as Jefferson County, Alabama. They are only able to survive in

fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

, thus they are only found within freshwater river systems.

Habitat and ecology

Black-knobbed map turtles are seasonally active from April to late November. Basking is a routine part of their day, occurring in the early morning and early afternoon.

Thermoregulation Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...

is thought to be the reason for basking, along with the removal of

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

s and algal growth. When approached, the turtles jump into the nearby water. Once in the water, they seek protection between the branches of fallen trees on the river bottom."Black-knobbed Map Turtle." Graptemys.com Map Turtles. N.p., n.d. Web. 6 Apr. 2011. <> Most of the riverbeds where they live have

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...

and

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

bottoms with moderate currents. Hatchlings prefer more sluggish waters off the main channel. Little is known about foraging behavior. However, this species has been observed to consume

beetles Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...

and dragonflies that have fallen into the river.Waters, J.C. 1974. The biological significance of the basking habit in the black-knobbed sawback, ''Graptemys nigrinoda'' Cagle. Master’s Thesis, Auburn University, Auburn, Alabama. Upon examination of both female and male stomach matter, Lahanas found a distinction of food material percentages. Males had roughly 58% animal matter and 40% plant matter, while females had 70% animal matter and 29% plant matter. The three primary sources of animal matter came from freshwater

sponges Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through ...

, bryozoans, and

mollusks Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

. The only plant matter found was a freshwater alga. Males reach sexual maturity around 3–4 years and females reach it around 7–8 years. Females have a

clutch A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). ...

size of roughly five eggs and can lay three or four clutches in a year. Nesting occurs from May to August, and occurs nocturnally on a sandbank.Lahanas, P.N. 1982. Aspects of the life history of the southern black-knobbed sawback, ''Graptemys nigrinoda delticola'' Folkerts and Mount. Master’s Thesis, Auburn University, Auburn, Alabama. 243 pp. This species feeds primarily on insects.

Conservation

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 A near-threatened species is a species which has been categorized as "Near Threatened" (NT) by the International Union for Conservation of Nature as that may be vulnerable to endangerment in the near future, but it does not currently qualify fo ...

by the

IUCN Red List The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is the world's most comprehensive inventory of the global conservation status of biol ...

. 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. Most of the lands encompassed by the species habitat are protected lands, but the rivers remain vulnerable. Mobile River Basin Aquatic Ecosystem Recovery PlanU.S. Fish and Wildlife Service. 2000. The Mobile River Basin Aquatic Ecosystem Recovery Plan. Atlanta, GA, 128 pp. has been implemented to address the needs of 22 aquatic species. One of these species is the red-bellied turtle (''

Pseudemys rubriventris The northern red-bellied turtle (''Pseudemys rubriventris'') or American red-bellied turtle is a species of turtle in the Pseudemys (cooter) genus of the family Emydidae. Description A fairly large river turtle, it averages about in length and w ...

''), whose habitat overlaps with ''G. nigrinoda'', so the plan will be beneficial to the black-knobbed map turtle.

Captive breeding Captive breeding, also known as captive propagation, is the process of plants or animals in controlled environments, such as wildlife reserves, zoos, botanic gardens, and other conservation facilities. It is sometimes employed to help species that ...

has been an option for conservation efforts as well. Captive breeding is plausible for increasing population sizes in captivity. However, it remains unclear if captive-bred black-knobbed map turtles can be released into the wild and breed on their own. Black-knobbed map turtles are popular in the pet trade, but became more common because of captive breeding.

References

External links