AUSTRALIAN BIODIVERSITY RECORD - Calodema

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