AUSTRALIAN BIODIVERSITY RECORD - Calodema

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Australian Biodiversity Record, 2007 (8): 1-124Account of Indian Serpents. London : Shakespeare Press 15 pp.). Type locality: Tranquebar,India.Hydrophis bengalensis Gray, J.E. (1842): Monogr. Synop. Water Snakes Hydridae [p. 62].Type data: holotype BMNH 1946.1.1.73. Type locality: Bengal, India and Bangladesh.Hydrophis subfasciata Gray, J.E. (1842): Monogr. Synop. Water Snakes Hydridae [p. 62].Type data: holotype BMNH 1946.1.1.90. Type locality: Bengal, India and Bangladesh.Thalassophis werneri Schmidt, P. (1852): Abh. Geb. Naturw. Hamburg 2: 69-86 [84, pl. 6].Type data: holotype ZMH 392. Type locality: Samarang, Java, Indonesia.Description: A moderate-sized species that has a very slender forebody, and the posteriorthird of the body large and broadly compressed. The colouration and patterning seems to varyconsiderably across its range, and the Australasian population appears to be morphologydistinct from those of Asia. Kharin has applied the name Enhydrina zweifeli to this palecoloured/darkblotched population. However, it appears that both species of Enhydrina mayoccur in Australian waters, so the following description is largely composite until morematerial can be examined. In Australian waters, adults are usually a uniform grey dorsally onthe head and body, and creamish-white ventrally. In some individuals, there may be a seriesof large dark bars or blotches on the dorsum, and these can be about the same width as thepaler interspaces, or in some much larger. These large blotches extend across the dorsum, toabout mid-laterally. There is no suborbital stripe on the side of the head in E. schistosa.Juveniles have around 45-55 blackish transverse bars over the body, in marked contrast tothe greyish rings of juveniles from Asia. Some characteristic features of this species are: headsmall and barely distinct from the neck, with enlarged symmetrical shields; 3-4 maxillary teethfollowing fang; 5-6 palatine teeth; palatine teeth larger than pterygoid teeth; mental groovepresent and very distinct; mental scale long and narrow, splint or dagger-shaped, muchlonger than broad and partially hidden in the deep mental groove; anterior infralabials veryelongate; throat capable of considerable extension due to extensile skin structure; anterior ofbody slender, but posterior deep and strongly compressed; body scales imbricate, in 49-66rows at mid-body; body-scale rows at neck 40-55; body scales each with a short, low keel,with the keels being slightly larger in males; ventrals about 240-320 (females do not appear tohave a higher ventral count than that of males); ventrals not enlarged, similar in size toadjacent body scales; preanal scales slightly enlarged. The largest specimens have beenreported in Asian populations (up to 1.4m) but those from Australia are not particularly large.In Australia, this is species only attains a maximum snout-vent length of only around 85 cm.,with exceptional specimens reaching around 1.1 m in total length. Females are generallylarger than males, and males have a slightly longer tail than females.Distribution: This species as presently defined, occurs as many scattered populations acrossa vast area of tropical waters from south of the Seychelles and Madagascar, the Arabian Seaand Persian Gulf (off Oman), the seas off South Asia (Pakistan, India and Bangladesh),Southeast Asia (Myanmar (formerly Burma), Thailand, Vietnam), and through westernIndonesia to Australasia. In our region, it is largely confined to tropical seas between Australiaand New Guinea, and is generally thought to be either extremely rare or absent fromAustralian territorial waters. However, I have observed this species being commonly trawledin Queensland and Northern Territory continental shelf waters as bycatch from prawn trawlingoperations, so it may be more widespread here - at least seasonally.Habitat: Throughout its range, it mainly inhabits relatively shallow water depths from about 4to 22 metres depths over muddy or sandy bottoms. It inhabits river deltas, estuaries, bays,harbours and gulf waters, and near-shore reefs as well as the shallower continental shelfwaters. Also sometimes observed well-up tidal sections of rivers, and in parts of Asia it hasbeen found in rivers many kilometres upstream from the sea.Biology/Ecology: Although this is the commonest species of sea snake in parts of Asia, inAustralian waters it is relatively uncommon compared to Hydrophis species. It appears to bean essentially nocturnal species that is occasionally observed foraging near the surface afterdark. It feeds on a variety of small fishes, such as catfish and puffer-fishes, and also34
Australian Biodiversity Record, 2007 (8): 1-124consumes prawns. In general, the prey size is related to the growth stage of the snake, withsmaller snakes tending to feed on smaller prey such as prawns and smaller fish, and largersnakes preferring larger fish. This could explain the unique morphology of its highlyelasticised anterior throat skin that would facilitate the swallowing of the very large fishes thathave been found in the stomachs of this species. Generally, larger snakes do not eat smallprey, indicating that their foraging methods may be less effective in securing this resource.Prey is actively hunted in the dark while the snake is swimming near the bottom, and fishesare apparently detected by tactile means such as touch by the snake or perhaps pressurewaves from the swimming fish rather than by sight or smell. Once a fish is seized, it is heldfirm in the jaws until it stops moving, and the prey is not constricted. During feeding, thesnake may orientate its body in the current to reduce its effect as it manipulates the prey toease swallowing, or on occasion it may press the prey against a hard object so as to get abetter grip. Prey is usually swallowed head-first, and after swallowing, snakes tend to swimaround actively or even twist their body into a knot so as to facilitate the movement of the preyinto the stomach. During the Wet Season it can be more often found in the estuarine (tidal)parts of both large rivers and smaller creeks - particularly in shallow waters of mangroveforests - rather than in more open waters. Males are known to practice combat behaviour, butwhether this is the result of competition for mates, prey or territoriality is unclear. Thereproductive biology is poorly known in this widespread species, and there is some notablevariation both geographically and seasonally. The sex ratio for the species does not differsignificantly from parity over the species’ range. In Asia, gravid females have been found fromaround August to January (late Dry Season, through to the Wet Season), with births occurringin February-March. Depending upon the area, it would appear that a gestation period of atleast 3 or 4 months occurs, and the timing of births is correlated with the end of the Monsoonor Wet Season. In Australia, mating has been recorded in May (early Dry Season). This is anovoviviparous species, with recorded litters ranging from 3 to 34, although the averagenumber of offspring in a litter is around 7 to 18 (larger females tend to produce larger litters).Juveniles are around 230-240 mm in length at birth in Australian waters, but they appear tobe somewhat larger in Asia, at around 300-340mm when born. It has a life span of around 4years. This species is known to be occasionally infested with parasitic worms, and onoccasions may carry small colonies of bryozoans attached to its skin.Toxicity: Although it has been long regarded as a relatively inoffensive species, bites tohumans are well-known in Asia, and on occasion have resulted in fatalities - indeed, mostfatalities from all recorded sea snake bites have been from this species. I have observed thatthis species will readily try to bite if handled or harassed, so extreme caution should beexerted when approaching or handling this snake. Venom studies have indicated that this is adangerously venomous species. As there have been human fatalities arising from the bite ofthis species elsewhere, urgent medical attention should always be sought in the case of anenvenomation.Survival Status: Protected under the Qld Nature Conservation Act (1992) and the TerritoryParks and Wildlife Conservation Act (1998). Listed as a Marine Protected Species under theAustralian Environmental Protection and Biodiversity Conservation Act (1999). Probably notunder threat, but its apparently restricted distribution in Australian waters could potentiallymake it vulnerable in some parts of its range to large scale fishing operations.Etymology: The name ‘schistosa’ means very divided. The name ‘zweifeli’ honours Americanherpetologist Richard Zweifel.Additional Comment: This is a highly variable species that has sometimes been placed withinthe genus Disteira, but it is herein considered distinctive enough to warrant separate genericrecognition as has long been maintained by Cogger and other authorities. Its morphologysuggests that there may be additional species in this genus awaiting formal recognition inAsia, and that both Enhydrina schistosa and Enhydrina zweifeli may occur in Australianterritorial waters. The northern and eastern Australian population is almost certainlyEnhydrina zweifeli Kharin, 1985 [see Kharin, V.E (1985): Zool. Zh., 64(5): 785-787. Typelocality: ‘off mouth of Sepik River, PNG’] and those of the far north-west and Indian Oceanwaters Enhydrina schistosa.35
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AUSTRALIAN BIODIVERSITY RECORD - Calodema

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