Hydromantes: Salamanders in Different Places

There are times when biogeography is able to throw us some real puzzlers: organisms whose distribution seems to defy expectations. Among these mysteries, special mention must be made of the salamanders of the genus Hydromantes.

Gene's cave salamanders Hydromantes genei courting, copyright Salvatore Spano.

Hydromantes is a genus containing a dozen species from among the lungless salamanders of the family Plethodontidae. Plethodontids are the most diverse of the generally recognised families of salamanders, with over 450 known species found mostly in Central and South America. Hydromantes, however, is a geographically isolated genus in this family with its species found in two widely separated regions: California in western North America, and mainland Italy and Sardinia in Europe. Though some authors have advocated treating the species found on each continent as separate genera, both morphological and molecular studies have left little doubt that the group represents a discrete clade.

Distinctive features of Hydromantes compared to other plethodontids include feet with five, partially webbed toes and a weakly ossified, flattened skull (Wake 2013). Members of this genus capture prey with a projectile tongue which is the most extensive of any amphibian, extending up to 80% of the animal's total body length (Deban & Dicke 2004). There are some differences between North American and European species notable enough for the recognition of separate subgenera (there is something of a gigantic clusterfuck surrounding the names of said subgenera but the details are far too tedious to relate here). The three North American species of the subgenus Hydromantes have bluntly tipped tails that they use as a 'fifth leg' when navigating smooth and/or slippery surfaces, whereas the European species have unremarkable pointed tails. Historically, the North American Hydromantes species have been poorly known, being isolated to restricted ranges. Hydromantes shastae is found in limestone around Lake Shasta whereas H. brunus is found in a small area of mossy talus habitat along the Merced River in the foothills of the Sierra Nevada (Rovito 2010). The third species, H. platycephalus, is found at higher altitudes in the Sierra Nevada, well over 1000 m above sea level. Individuals found living on steep slopes are known to escape predators by tightly coiling their bodies and simply rolling down the slope (García-París & Deban 1995). A molecular analysis of H. platycephalus and H. brunus by Rovito (2010) identified the former species as derived from within the latter, and Rovito suggested that H. brunus may have originated in a remnant population from when H. platycephalus moved into lower altitudes during the Ice Age.

Mt Lyell salamander Hydromantes platycephalus, copyright Gary Nafis.

The seven or eight European species are mostly placed in the subgenus Speleomantes; a single species, Hydromantes genei, is divergent enough to be placed in its own subgenus Atylodes (though most recent studies have indicated that the European Hydromantes overall form a discrete clade). Hydromantes genei and three species of Speleomantes are found in caves on the island of Sardinia; the remaining Speleomantes species on mountains of mainland Italy. Molecular analysis suggests that H. genei became isolated on Sardinia about nine million years ago, with the ancestors of the Sardinian Speleomantes arriving later about 5.6 million years ago when the Mediterranean dried out during what is known as the Messinian Salinity Crisis (Carranza et al. 2008). The absence of any Hydromantes on neighbouring Corsica is something of a mystery, and it has been suggested that they may have been present there in the past before going extinct.

Extinction also seems the most likely explanation for Hydromantes' unusual distribution. The fossil record for the genus is minimal, and provides little information not already available from living species, but molecular dating attempts agree that the division between European and North American Hydromantes happened too recently to be related to the tectonic separation of the two continents. Such a scenario would also leave open the Hydromantes' absence in eastern North America. The description in 2005 of the Korean lungless salamander Karsenia koreana demonstrated the presence of plethodontids in eastern as well as far western Eurasia, and it seems possible that Hydromantes dispersed into Eurasia via the Bering Strait landbridge, becoming widespread across the continent before extinction reduced it to the isolated relicts it is today.

REFERENCES

Carranza, S., A. Romano, E. N. Arnold & G. Sotgiu. 2008. Biogeography and evolution of European cave salamanders, Hydromantes (Urodela: Plethodontidae), inferred from mtDNA sequences. Journal of Biogeography 35: 724–738.

Deban, S. M., & U. Dicke. 2004. Activation patterns of the tongue-projector muscle during feeding in the imperial cave salamander Hydromantes imperialis. Journal of Experimental Biology 207: 2071–2081.

García-París, M., & S. M. Deban. 1995. A novel antipredator mechanism in salamanders: rolling escape in Hydromantes platycephalus. Journal of Herpetology 29 (1): 149–151.

Rovito, S. M. 2010. Lineage divergence and speciation in the web-toed salamanders (Plethodontidae: Hydromantes) of the Sierra Nevada, California. Molecular Ecology 19: 4554–4571.

Wake, D. B. 2013. The enigmatic history of the European, Asian and American plethodontid salamanders. Amphibia-Reptilia 34: 323–336.