Clues to the evolution of salt tolerance in mud snakes

A. The Chinese mud snake, Myrrophis chinensis. B.  The Kerala mud snake, Dieurostus dussumierii
Photo credit A.  You Chugwei; B. A. B. Kumar.

Salt tolerance has evolved a limited number of times in extant snakes: once in the file snakes (acrochordids); at least twice in the front-fanged sea snakes (once in the egg-laying sea kraits and one in the live bearing sea snakes); at least twice, possibly more in the natricids (North America and Europe); at least once in the South America dipsidids, and at least once in homalopsids. The total number of living snakes inhabiting saline environments is approximately 110 species comprising only 3% of extant snakes. Remarkable when you consider 70% of the Earth’s surface is covered with marine habitats.

In a recent paper published in Systematics and Biodiversity Kumar et al. (2012) found  the freshwater Chinese Mud Snake Myrrophis (formerly Enhydris) chinensis and the Kerala Mud Snake Dieurostus (formerly Enhydris) dussumierii are sister species. This relationship provides potential insight into the evolution of salt tolerance in the homalopsid snakes. The morphological similarity and geographical proximity of the Chinese Mud Snake and the coastal (brackish water and marine) Bennett’s mud snake (Myrrophis bennettii) in southern China strongly suggest a close relationship between these species, but its DNA was not available for inclusion in the study. Myrrophis bennettii can survive in salt water for a period of time that may range from days to weeks. It is unknown if they have salt glands or use behavioral osmoregulation (moving from salt water to freshwater when they get thirsty), but a facultative salt gland cannot be ruled out. Dunson and Dunson (1979) described a facultative salt gland in the homalopsid genus Cerberus from Pulau, Micronesia.

However, no evidence of a salt gland has been found in members of the Enhydris clade (E. enhydris E. chanardi,  E. innominata, E. jagorii, E. longicauda, and E. subtaeniata) and preliminary observations suggests they are quite sensitive to salt water and die within a few days or less when exposed to full sea water. However, Bennett’s mud snake and the Kerala mud snake are both salt tolerant, found in brackish and marine habitats, and separated by a straight line (over-water over-land) distance of more than 3300 km; following the coastline the distance is much greater, more than 15,000 km. Given the close relationship inferred between Bennett’s mud snake and the Kerala mud snake it seems unlikely that salt tolerance evolved independently in the geographically distant species.

Members of the homalopsid clade are generally thought to have relatively poor dispersal abilities with even small increases in elevation posing significant barriers to movement and thus gene flow for most species. With few exceptions homalopsids are restricted to lowland habitats and seem to be incapable of ascending small waterfalls, navigating rapids or making long-distance overland movements. Thus following the coast is likely the path of dispersal for homalopsid snakes if they are, in fact slat tolerant. Kumar et al propose a hypothetical Asian coastal lineage, the includes the ancestor to Bennett’s mud snake  and the Kerala mud snake that evolved salt tolerance in Indochina or the adjacent Sunda Shelf. The species or its descendants then followed the tropical coastline of Asia where in entered river deltas and coastal swamps and evolved into many of the homalopsid species we see today.

A map that shows the homalopsid snakes that are possible candidates for belonging to the Asian coastal clade.

Previous research placed the most recent common ancestor for the Chinese mud snake and the Enhydris clade in the middle Miocene (16 Ma). The Miocene was a period of greatly fluctuating sea levels with many highstands that were 25–35 m above current levels, and lowstands in the late Miocene that were 30–40 m below current levels.Changing sea levels could have  exerted strong selection pressures on coastal and marine snakes with highstands forcing coastal snakes that were not salt tolerant or only slightly salt tolerant further inland where fresh water was more readily available. While lowstands would increase the width of the
coastal plain, increasing the area available for colonization by lowland species.

Kumar, A.B.,  Sanders, K.L., George, S. and Murphy, J.C. (2012): The status of Eurostus
dussumierii and Hypsirhina chinensis (Reptilia, Squamata, Serpentes): with comments on the origin of salt tolerance in homalopsid snakes, Systematics and Biodiversity, 10:479-489.

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