Cloacal Respiration

While most of us are familiar with what a turtle is, the idea of what constitutes a so-called “typical turtle” depends largely on geography. In North America and the rest of the northern hemisphere, the turtle fauna is exclusively cryptodires or hidden-necked turtles.  The southern hemisphere, what remains of Gondwana, is largely comprised of pleurodires or side-necked turtles.  To those of us in the northern hemisphere, these side-necked species are very unusual.  Some have extraordinarily long necks, others have strange flaps of skin to break up their outline, and some exhibit unusual behaviors such as laying their eggs in aquatic nests.

As recently as 1980, a new species of Australian side-necked turtle was described by John M. Legler and John Cann as Rheodytes luekops, creating a new monotypic genus.  This white-eyed stream diver (literal translation of the Latin name) hails from the Fitzroy River.  To avoid confusion, it should be known that there are 3 rivers by that name on the continent but this turtle lives in the one in Queensland (not Western Australia or Victoria) and also takes the name of this waterway for its common name: Fitzroy River Turtle.

It reaches 26 cm (nearly a foot) in carapace length and is primarily light to dark brown in color.  The juveniles have a carapace with extensively flaring and serrate marginal scutes giving them a “saw shell”.  In the adults, the pitted and grooved texture of the carapace gives it an appearance similar to submerged driftwood.  The shell is somewhat reduced, giving the appearance of a longer neck than other closely related short-neck turtles.  Adults can develop some striking orange, yellow, and pink coloration on the head and neck.  The neck is covered with conical tubercles which have been hypothesized to perform a sensory function.

Another chelid, Acanthochelys spixii – Spix’s Sideneck Turtle, from the other side of the world (Brazil, Argentina, Uruguay) has a very similar neck as well as a similar lifestyle.  They both forage on the bottom of bodies of water for insects though Acanthochelys feeds primarily on Odonata (dragonfly) larva while Rheodytes prefers aquatic Lepidopteran (moth) pupae.  Whether this similarity in structure relates to such a lifestyle remains to be seen.  The eyes of Rheodytes have a white ring on the inner margin of the iris, giving rise to the species epithet.   The legs are comparatively long as are the claws which suit its lifestyle of walking along the bottom of fast-flowing streams and climbing over any rocky obstacles in their way.  These clear and highly oxygenated environs have allowed the evolution of an amazing adaptation: Cloacal Respiration.

Many aquatic turtles have the capacity to supply some of their oxygen requirement from aquatic respiration by passing water over highly vascularized buccal or cloacal surfaces.  What makes the Fitzroy River Turtle so unusual is its ability to supply most if not all its oxygen requirement via cloacal respiration.  The cloaca possesses 2 muscular and highly expanded bursae, allowing for large volumes of water to enter the structures.  These are lined with fimbriae and further divided into microfimbriae, giving a structure with a huge surface area.  The bursae have been observed to pulsate, pumping water over the fimbriate as frequently as 60 times per minute.  So great is the ability to obtain oxygen from the water that these turtles rarely surface to breath air.  They do possess fully functional lungs and so they are able to respire in a more traditional fashion but apparently only use them when out of the water, which is usually only for ovoposition.

The Fitzroy River Turtle has an extremely small range and its habitat is threatened by a proposed dam in the region.

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