by Greg Mayer
Islands that have never been connected to a continent, often called oceanic islands, must receive their flora and fauna over water, by what Darwin termed “occasional means of transport”. Such means include floating (e.g. coconuts), wind (e.g. spiders), rafting (e.g. iguanas), ice floes (e.g. arctic foxes), and, of course, flying (e.g. birds and bats). Because the ability to disperse is rather unevenly distributed across a continental fauna, the animals of oceanic islands are usually a rather distinctive subset of what is found on the nearest continents. Insular faunas have bats and birds, often lizards and snakes, occasionally mice and rats, but only very rarely amphibians or larger terrestrial mammals.
Another feature of insular faunas is that they are rich in endemics (forms peculiar to the island), because of the rarity of gene flow from the mainland. Unfortunately, these endemic forms, having evolved in isolation with an unusual fauna around them (lacking in predators, for example), often succumb to the environmental changes wrought by man when their islands are discovered and colonized. There have been some cases where island species thought extinct have been rediscovered alive, most famously perhaps the case of the giant lacertid lizards of the Canary Islands. A case from the Revillagigedo Islands in the Mexican Pacific poses an interesting twist on the rediscovery story.
In a paper in PlosOne, Daniel Mulcahy and colleagues from the Instituto de Ecología in Xalapa, Veracruz, México and the U.S. National Museum (including my old friend and mentor George Zug) report the rediscovery of an extant population of the Clarion Island nightsnake (Hypsiglena ochrorhyncha unaocularus). The only previous specimen known had been collected by the intrepid naturalist William Beebe in 1936, and had been scientifically described as an endemic form on the basis of this specimen by Wilmer Tanner in 1944.
What makes this case different from the more usual rediscovery is that in 1955, because no further specimens had been found, Bayard Brattstrom suggested that the original specimen had come from the Mexican mainland, and that the locality data on Beebe’s specimen was in error. Thus the Clarion nightsnake disappeared not into the roll of the extinct, but into the roll of the never existed! So, for nearly 80 years, until 2013, no one had found a Clarion nightsnake, and for most of that time no one thought there even was such a thing.
Mulcahy and colleagues did two things. First, rereading Beebe’s writings about his Clarion expedition, it was clear to them that Beebe had not made an error in labeling where his snake was from– he was very explicit about having found the snake on Clarion, and not the mainland. Second, in 2013 they went to Clarion, and armed with Beebe’s book, they quickly found the right place, and the snakes. They found eleven in all, collected five, and took blood samples and photos of the rest. Thus, knowing the right place and time of day to look, the species proves to be locally common.
Based on their morphological and DNA analyses, Mulcahy and colleagues have raised the Clarion snake from a subspecies to a species, but as we’ve discussed before on WEIT the ranking of divergent allopatric forms is a judgment call, and not really the take home message here. Rather it’s a genuine rediscovery (not a shift in taxonomic rank) of an island endemic, which is potentially threatened by several factors, including introduced animals. Mulcahy and colleagues make several recommendations to help insure the snake’s survival.
Beebe, C.W. 1938. Zaca Venture. Harcourt, Brace, New York.
Brattstrom, B.H. 1955. Notes on the herpetology of the Revillagigedo Islands, Mexico. American Midland Naturalist 54:219-229.
Mulcahy, D.G., J.E. Martínez-Gómez, G. Aguirre-León, J.A. Cervantes-Pasqualli, and G.R. Zug. 2014. Rediscovery of an endemic vertebrate from the remote Islas Revillagigedo in the eastern Pacific Ocean: the Clarión nightsnake lost and found. Plosone 9(5): e97682 (8 pp). pdf
Tanner, W.W. 1944. A taxonomic study of the genus Hypsiglena. Great Basin Naturalist 5: 25–92. pdf
h/t Jim Ebsary