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Hydrophis hardwickii (sometimes referred to as Lapemis hardwickii) occurs in Myanmar, in the coastal waters of the Tanintharyi Division (Mergui Archipelago), as well as along the southeast coast of India and Straits of Malacca east to Australia and north to China, the Philippines, Taiwan, and Japan. These snakes are active both day and night, inhabiting coral reefs as well as estuaries and regions in tidal zone with sandy or muddy bottoms. They are usually found at 6-15 m depth, but sometimes deeper. (Leviton et al. 2003 and references therein)
Some authors have considered H. hardwickii to be a synonym of H. curtus or a subspecies of H. curtus (with the nominate form found in coastal waters from the Persian Gulf to to the shores of western India and hardwickii ranging from the coastal waters of Sri Lanka and eastern India east to New Guinea and Australia and north to the coast of China, the Philippines, and Japan. Leviton et al. (2003) followed earlier treatments that considered H. curtus (with specific epithet spelled by Leviton et al. apparently in error, as curtis) to range from the Persian Gulf to the west coast of India as far as Sri Lanka, but to be absent from the east coast of India, with snakes in this area recognized as a distinct species, H. hardwickii.
Ward (2000) studied sea snake bycatch by trawlers pursuing prawns off northern Australia. This bycatch included a small number of H. hardwickii.
Halstead et al. (1978) analyzed and described the venom and venom apparatus of Hydrophis hardwickii.
Leviton et al. (2003) provide a technical description of Hydrophis hardwickii: Body short, stout, neck region not less than half as thick at midbody; head large; scales squarish or hexagonal, juxtaposed, outer 3-4 rows larger than others. Scale rows: males 23-31 around neck, females 27-35 ; around midbody, males 25-27, females 33-41; ventrals small and usually distinct anteriorly (not so posteriorly): males 114-186, females 141-230; head shields entire, parietals occasionally divide; nostrils superior, nasals touching each other; prefrontal usually in contact with second upper labial; 7-8 upper labials, 3-4 bordering eye; 1 pre- and 1-2 postoculars; 2 (rarely 3) anterior temporals; greenish or yellowish olive above, whitish below; 35-50 olive to dark gray dorsal bars, tapering to a point laterally, occasionally encircling body; a narrow dark ventral stripe or broad irregular band is occasionally present; adults often lack any pattern and are uniform olive to dark gray; head pale olive to black, with or without yellow markings on snout. Total length 860 mm; tail length 85 mm.