Round Island splitjaw snakes - Casarea dussumieri - Reptilenesia

Adult female Casarea dussumieri

Not to be confused with the unfortunate Round Island burrowing boa (Bolyeria multocarinata), last seen in 1975, the Round Island keel-scaled boa is still with us, although just barely. In 1996, less than 250 adult individuals remained alive, although recent captive breeding efforts have raised that to about 1000. Together, Casarea and Bolyeria made up the strange and intriguing family Bolyeriidae. Although they're sometimes called 'boas', they are distinct from the Boidae, or true boas, in not having any vestiges of a pelvic girdle. In fact, they are more closely related to the advanced snakes (Caenophidia) than to the true boas, although their phylogenetic relationship to other snakes is not quite certain. Some have advocated calling them 'split jaw snakes' instead of boas. Due to their remote range (and the extirpations they suffered), few herpetologists have been lucky enough to see a living specimen, especially a wild one. I hope to cover what is known about these two species in this short article.

Round Island is a herpetologically interesting volcanic islet, 151 ha (just over half a square mile) in size, located approximately 22.5 km NNE of Mauritius, in the Indian Ocean east of Madagascar. Round Island and Mauritius are part of the Mascarene archipelago, which formed between 35 and 2 million years ago as a result of the Réunion hotspot. Before the 16th century, no humans inhabited the islands, which were covered in unique tropical moist broadleaf forest. All of the Mascarene flora and fauna arrived by overseas dispersal, possibly using prehistoric islands of the Mascarene plateau, now submerged by the sea, as 'stepping stones. Round Island is also home to an endemic skink (Leiolopisma telfairii) and an endemic day gecko (Phelsuma guentheri). Both snakes used to be found on Mauritius and other nearby islets, from which they were first extirpated. Microhabitats include fallen palm fronds and the burrows of nesting pelagic birds such as shearwaters. Captive breeding efforts are hindered by the fact that these two endangered lizards constitute the sole natural prey of Casarea, so they must be enticed to eat mice in captivity, which is more easily said than done. A few parasites of the taxon have been described, mostly by Peter Daszak of the EcoHealth Alliance.

What certainly hasn't helped clarify their taxonomy is that relatively few specimens and tissue samples are available for study. Twenty-eight genes have been sequenced for Casarea, but (unsurprisingly) none for Bolyeria, which is represented in museums by only seven specimens. Scientists are understandably reluctant to collect fresh Casarea specimens for study (DNA is far easier to sequence from fresh tissue), and the number of snakes in captivity is relatively few. Only in 2005 did we learn, posthumously, that Bolyeria probably laid eggs, rather than give live birth like many boas.