Friday, March 22, 2013

[Herpetology • 2013] Recent rapid speciation and ecomorph divergence in Indo-Australian sea snakes | Shrunken Heads of Sea Snakes

Given their feeding habits, some sea snakes have developed 'shrunken heads'. An international team of scientists says this can lead to speciation (one species splitting into two). In the picture: small-headed snake foraging in waters off the Ryuku Islands, Japan.
Image: Yoshitaka Tahara

The viviparous sea snakes (Hydrophiinae) are a young radiation of at least 62 species that display spectacular morphological diversity and high levels of local sympatry. To shed light on the mechanisms underlying sea snake diversification, we investigated recent speciation and eco-morphological differentiation in a clade of four nominal species with overlapping ranges in Southeast Asia and Australia. Analyses of morphology and stomach contents identified the presence of two distinct ecomorphs: a ‘macrocephalic’ ecomorph that reaches >2 m in length, has a large head and feeds on crevice-dwelling eels and gobies; and a ‘microcephalic’ ecomorph that rarely exceeds 1 m in length, has a small head and narrow fore-body and hunts snake eels in burrows. Mitochondrial sequences show a lack of reciprocal monophyly between ecomorphs and among putative species. However, individual assignment based on newly developed microsatellites separated co-distributed specimens into four significantly differentiated clusters corresponding to morphological species designations, indicating limited recent gene flow and progress towards speciation. A coalescent species tree (based on mitochondrial and nuclear sequences) and isolation-migration model (mitochondrial and microsatellite markers) suggest between one and three transitions between ecomorphs within the last approximately 1.2 million to approximately 840 000 years. In particular, the macrocephalic ‘eastern’ population of Hydrophis cyanocinctus and microcephalic H. melanocephalus appear to have diverged very recently and rapidly, resulting in major phenotypic differences and restriction of gene flow in sympatry. These results highlight the viviparous sea snakes as a promising system for speciation studies in the marine environment.

Keywords: Australia; ecomorph evolution; Hydrophis; marine speciation; sea snake; Southeast Asia


Shrunken Heads of Sea Snakes Explained

Some sea snakes have heads that look comically small compared with the rest of their body. New research shows these shrunken heads evolved quite rapidly, allowing the snakes to hunt eels hiding in tight spaces.

If you only looked at the genes of the blue-banded sea snake and the slender-necked sea snake, the two species would seem nearly identical. But the close cousins, which are found in waters around Southeast Asia and Australia, have quite different physical looks, researchers say.

"The slender-necked sea snake is half the size, and has a much smaller head, than the blue-banded sea snake," study researcher Mike Lee, from the South Australian Museum, said in a statement. "This suggested they separated very recently from a common ancestral species and had rapidly evolved their different appearances."

The researchers believe some sea snakes developed shrunken heads to be more effective eaters since these species poke their heads into narrow eel burrows to look for food, whereas their big-headed cousins feast on crevice-sheltering eels and gobies.

"One way this could have happened is if the ancestral species was large-headed, and a population rapidly evolved small heads to probe eel burrows — and subsequently stopped interbreeding with the large-headed forms," Lee added.

Both the blue-banded sea snake and the slender-necked sea snake belong to the genus Hydrophis. With more than 30 species, this group is by far the most diverse group in the Hydrophiinae subfamily and it has a lot of tiny-headed members. In fact, microcephaly (the condition of having a small head) has evolved at least eight separate times in Hydrophis during their relatively rapid diversification over the past 3.5 million years, researchers say.

Meanwhile, no other genus in the sea snake family (not Aipysurus nor Ephalophis nor Hydrelaps) has snakes that have evolved shrunken heads. What's more, these groups tend to be much less diverse, each represented by a half-dozen species at best.

"Rapid evolution of head size variation is therefore a likely contributing factor in the explosive speciation in Hydrophis group sea snakes," the researchers wrote in an article published this week in the journal Molecular Ecology.

Kate L. Sanders, Arne R. Rasmussen, Mumpuni, Johan Elmberg, Anslem de Silva, Michael L. Guinea, Michael S. Y. Lee. 2013. Recent rapid speciation and ecomorph divergence in Indo-Australian sea snakes. Molecular Ecology, | DOI: 10.1111/mec.12291/