The Afrotropical Terrapin

Explanation: Basking on a rock in a marsh in eastern tropical Africa is this terrapin that bears a number of interesting names. More on that in a moment, though.

This is a small turtle of wetlands found widely in sub-Sahara Africa, common and generally secure. They are sold, but do not make good pets, as they are aggressive to other turtles and at times to the hand that feeds it.

In their native habitat they can be shy, and quick to move out of sight on land or in water.

Below here is the carapace, consisting of shields that contain a rayed pattern (similar to western pond turtles of western North America, although there is no close taxonomic relationship between the two species).

To the left is the plastron or ventrum (the underside of the shell), here showing a pattern of central pale coloration that appears in some specimens of this species.

The degree to which this pattern varies geographically is not well known, although the plastron of a poorly defined southern race nigra is black.

So what about their name? The species is unique in that it is the only species of this genus, Pelomedusa. In fact, it is only one of two genera and species in the entire family Pelomedusidae, although the other genus, Pelusios, might actually belong under this genus Pelomedusa (Wong et al. 2010).

But it goes by a variety of common names, including: marsh terrapin, helmeted terrapin, helmeted turtle, African helmeted turtle, crocodile turtle, and ... African side-necked turtle.

Side-necked? Check out the main photo in this episode, or the photo below. This turtle does not retract its head into the shell directly, but cranks its neck to the side.

The few specimens I've seen of this species all seem to crank the head to its right side, which might suggest a specific anatomical feature to be "side-necked" to the right, although I know of no studies on this. Maybe someone out there has found a "left-handed" side-necked marsh terrapin?

And a final word about names. The name "terrapin" itself derives from the word torope from the 1610s, derived in turn from an American Algonquian word, perhaps the Abenaki turepe, or the Munsee tolpew, for "turtle." Later, terrapin was adopted and applied much more broadly to some turtles of North to South America, Africa, and Asia.

Information:
     Scheyer, T.M., B. Brüllmann, and M.R. Sánchez-Villagra. 2008. The ontogeny of the shell in side-necked turtles, with emphasis on the homologies of costal and neural bones. Journal of Morphology 269(8):1008-1021.
     Wong, R.A., J.J. Fong, and T.J. Papenfuss. 2010. Phylogeography of the African helmeted terrapin, Pelomedusa subrufa: genetic structure, dispersal, and human introduction. Proceedings of the California Academy of Sciences 61(12):575-585.

     Thanks to the Online Etymology Dictionary for its wonderful insights into word origins!

EPOW - Ecology Picture of the Week