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The ‘beaked’ or ‘hook-nosed’ sea snakes in the genus Enhydrina are among the most distinctive and widely distributed viviparous (live-bearing) sea snakes. They are distinguished from all other sea snakes by an extremely long and narrow (rather than triangular) mental (chin) scale that is largely concealed in a deep notch between the lower jaws.
The dangerously venomous Beaked Sea Snake (Enhydrina schistosa) occurs in the Persian Gulf, Arabian Sea, Madagascar, Iran, Pakistan, India, Sri Lanka, Malaysia north to the Philippines, Singapore, west coast of Thailand, Gulf of Thailand, Vietnam, Indonesia, and Australian region (including waters of New Guinea Island, excluding Western Australian waters).
In Australia, these snakes are reportedly often found in rivers, but it is unclear whether these are brackish waters of the tidal basins or freshwater farther from the estuaries. In Thailand, these snakes are also reported to occur in estuaries and rivers as well as in coastal waters, but, again, it is not clear how far upstream they travel. Enhydrina schistosa reportedly prefer waters at a depth from less than 5 m to a maximum around 30 m. They are active during both day and night.
The “beaked sea snake” is abundant in coastal and inshore habitats throughout the Asian and Australian regions, where it is responsible for the large majority of recorded deaths and injuries from sea snake bites. Analyses by Ukuwela et al. (2013) of five independent mitochondrial and nuclear loci for populations spanning Australia, Indonesia and Sri Lanka indicate that this nominal species actually consists of two distinct lineages in Asia and Australia--and that these are not closest relatives. As a result, Ukuwela et al. elevated Australian ‘‘E. schistosa’’ to full species status, provisionally referred to as Enhydrina zweifeli. Convergence in the characteristic "beaked" morphology of these species is probably associated with the wide gape required to accommodate their spiny prey.
The recognition of cryptic species potentially has important implications for snakebite management given the medical importance of beaked sea snakes and the fact that the only sea snake anti-venom available is raised against Malaysian E. schistosa. Variation among snake venoms has a strong phylogenetic component, and the venom of the two Enhydrina populations has been shown to have a similar overall composition, but different specific isoforms--although both the Asian and Australian forms are highly toxic. However, a key aspect of sea snake venom evolution, which has prevented the confusion of these two cryptic species from having catastrophic medical impacts, is that all sea snake venoms are very streamlined due to feeding on a single higher taxon (bony fish). Consequently, all sea snake venoms tested to date have been well neutralized by the only available antivenom.
de Silva et al. (2011) reported preliminary observations on the sex, reproductive status, and reproduction of six species of hydrophiine sea snakes (including Enhydrina schistosa) collected as fishing bycatch in different coastal regions of Sri Lanka.
Ward (2000) studied sea snake bycatch by trawlers pursuing prawns off northern Australia. This bycatch included a small number of Enhydrina schistosa.
(Leviton et al. 2003; Ukuwela et al. 2013 and references therein)