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Hydrophiini (“viviparous sea snakes” or "true sea snakes") is a monophyletic group, including ~90% of living marine reptiles, nested within the terrestrial Australasian elapid subfamily Hydrophiinae (Hydrophiinae is sometimes granted full family status, as Hydrophiidae). The eight species of amphibious (and oviparous) sea kraits, Laticauda, comprise a lineage that is sister to (terrestrial Australasian elapids + Hydrophiini) and represents an independent origin of a marine habit. Together, these three groups (terrestrial Australasian Hydrophiinae, Laticauda, and Hydrophiini) comprise the subfamily Hydrophiinae (Sanders et al. 2010 and references therein).
Sanders et al. (2013) reconstructed the phylogeny of the viviparous sea snakes (Hydrophiinae: Hydrophiini), a morphologically and ecologically diverse clade of 62 species distributed throughout the Indo-Pacific. Local sea snake assemblages can include up to 10 species, including fish egg eaters with vestigial venom systems, tiny-headed specialists on burrowing eels, and wide-gaped specialists on spinous catfishes or toadfishes. Unlike the sea kraits, Hydrophiini are ovoviviparous and bear fully developed live young at sea. Ovoviviparity is the inferred ancestral character state for hydrophiine sea snakes as they are phylogenetically nested within the ovoviviparous clade of ~100 terrestrial Australo-Melanasian elapids.
Substantial morphological and molecular evidence has been found for recognizing two major clades within the Hydrophiini: An "Aipysurus" lineage comprises ten species in the genera Aipysurus and Emydocephalus, which are mostly restricted to reefs in the Australo-Papuan region. Diversity and relationsjips within the Aipsyurus lineage are relatively well understood and the taxonomy has been largely stable since 1926. The second, much more species-rich "Hydrophis" lineage, includes 49 species, many of which have very wide distributions across the Indo-Pacific, and the taxonomy of the "Hydrophis" lineage has been far less stable. In addition to these two large clades, three monotypic semi-aquatic genera, Ephalophis, Parahydrophis and Hydrelaps, have had uncertain affinities to terrestrial and fully marine hydrophiines based on morphology, but mitochondrial data nest these groups within Hydrophiini, as relatively distant sister lineages to the Hydrophis group. Sanders et al. (2013) found strong support for the paraphyly of Hydrophis and polyphyly of Disteira. Furthermore, they found that six sampled monotypic genera (Acalyptophis, Astrotia, Kerilia, Lapemis, Pelamis, Thalassophina) were nested within the very recent core Hydrophis group radiation, and many had close affinities to other sampled species.To make the taxonomy match this new understanding of relationships, rather than recognizing multiple new genera, they advocated recognizing a single genus, Hydrophis Latreille 1802, for the core Hydrophis group. They note that this approach has the added benefit of creating less confusion for conservationists, medical professionals, and fishing industries/communities (as well as herpetologists) who have contact with sea snakes. (Sanders et al. 2013 and references therein)
Rasmussen (1997) and Rasmussen et al. (2011) present detailed reviews of the taxonomic history of sea snakes. (de Silva et al. 2011 and references therein; Sanders et al. 2013 and references therein).
The analysis by Sanders et al. (2013), which included multiple individuals of 39 species and used both mitochondrial and nuclear markers, shed light on a number of taxonomic issues, but was limited in its conclusions as a result of analytical challenges attributable to this radiation being relatively recent and rapid. Sanders et al. note that Hydrophis group species have variously been classified in 10–16 genera and/or subgenera, reflecting their confusing patterns of phenotypic diversity.
Lilywhite et al. (2015) studied drinking behavior and dehydration in five species of true sea snakes. They found that dehydrated individuals of Hydrophis curtus, H. elegans and H. zweifeli drank fresh water, and the mean threshold levels of dehydration that first elicited drinking were deficits of −26, −29 and −27% of body mass, respectively. Individuals of Aipysurus mosaicus and H. peronii did not drink fresh water when similarly dehydrated.