Tuesday, December 8, 2015

Sea snake diversity in the Indo-Australian Archipelago

The Indo-Australian Archipelago is a marine biodiversity hotspot centred in Southeast Asia that contains many of the extant viviparous sea snakes. Points of origin of for snake radiations are of interest  in understanding the distribution of current diversity. In an early on-line view of a new paper in the Journal of Biogeography, Ukuwela and colleagues (2015) note that previouis studies found the ancestral area for viviparous sea snakes was Australasian about 6.9 million years ago. The Aipysurus group also originated and speciated in Australasia. However, of the Aipysurus group species, only the specialist fish egg-eaters (Emydocephalus ijimae, E. szczerbaki and A. eydouxii) have colonized Southeast Asia and none have expanded into the Indian Ocean beyond the coast of Western Australia. A Beast analyses recovered a Southeast Asian origin for the core Hydrophis group, and all three methods used indicated that subsequent diversification in this rapidly speciating clade occurred primarily in Southeast Asia, with subsequent dispersals into the Indian Ocean and re-colonization of Australasia.

Ukuwela et al. (2015) found evidence for 34 divergences between lineages older than 0.5 Ma (candidate speciation events); 22 of these have > 0.7 posterior probabilities of occurring in Southeast Asia, 10 in Australasia, and 2 in the Indian Ocean. This implies most sea snake diversity in South East Asia originated from a period of rapid in situ evolution. While viviparous sea snakes originated in Australasia, Southeast Asia and its Indo-Australian Archipelago appears to be their primary centre of speciation. This is contrary to predictions of the overlap or accumulation models. Taxa are not more likely to disperse into, rather than out of, Southeast Asia and the Indo-Australian Archipelago.

The majority of sea snake diversification, including the rapid core Hydrophis radiation, occurred during major climatic and geological events that drove vicariant population and species divergence in many of the region’s marine groups. Viviparous sea snakes might be particularly influenced by ‘soft’ biogeographical barriers (such as incomplete and thus permeable land bridges) because they give birth to live young and thus lack the dispersing planktonic larval stage that is expected to promote population connectivity in most other marine groups (many fish and invertebrates). Several sea snake species accordingly show strong intraspecific genetic structure corresponding to deep-water and historical land barriers. However, biogeographical patterns and the diversification dynamics of the entire sea snake radiation have not previously been quantitatively investigated.

Citation
Ukuwela, Kanishka DB, Michael SY Lee, Arne R. Rasmussen, Anslem Silva, Bryan G. Fry, Parviz Ghezellou, Mohsen Rezaie‐Atagholipour, and Kate L. Sanders. 2015. Evaluating the drivers of Indo‐Pacific biodiversity: speciation and dispersal of sea snakes (Elapidae: Hydrophiinae). Journal of Biogeography (2015).

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