Sea Snakes

Sea snakes are true reptiles closely related to Australian venomous terrestrial snakes. Indeed, both groups are included in a single subfamily, Hydrophiinae, by most modern herpetologists. The nearest relatives of the hydrophiines are the Asian members of the subfamily, Elapinae. Hydrophiine and elapine snakes are united in the family Elapidae and defined by having fixed front fangs. Sea snakes differ from their terrestrial relatives by numerous marine adaptations, including smaller belly scales, a paddle-shaped tail used in swimming, nostril valves that close from the inside and keep out water while diving (Fig. 29.1D), and a salt gland that is located beneath the tongue sheath and excretes excess salt. Sea snakes are extraordinary divers, being able to descend to depths of 100 metres and remain submerged for two hours or more. They surface to breathe air but supplement their oxygen and carbon dioxide exchange by cutaneous respiration while submerged.

There are two kinds of sea snakes, sea kraits and true sea snakes (Fig. 29.1E). The sea kraits are egg-layers and come out on land to oviposit and to rest and mate, whereas all true sea snakes give birth to live young and never voluntarily leave the sea. Although sea kraits are abundant in New Guinea and the island chain of the Solomon Is, Vanuatu, New Caledonia, Tonga and Fiji, curiously they are unknown from Australia except as rare waifs. By contrast, Australia has a rich diversity of 32 species of true sea snakes, of which 14 species maintain permanent breeding populations in the GBR region. In general, courtship (Fig. 29.1F) and mating take place in winter and live young are born in summer. Clutch sizes vary considerably among true sea snake species, ranging from just two to four per clutch to around 20 per clutch. Juveniles generally have colour patterns that differ from those of adults of the same species (Fig. 29.1G).

Sea snake species occur in a variety of habitats: some are true reefal species, whereas others occur in deeper inter-reefal areas, or on rocky or muddy substrates between the GBR and the Queensland coast (Table 29.1). Of the fourteen species that occur in the GBR region, two are restricted to reefs (Aipysurus duboisii (Fig. 29.1H) and Emydocephalus annulatus (Fig. 29.1!)) and two are characteristic of reefs but also are found in other habitats (Aipysurus laevis (Fig. 29.1/) and Astrotia stokesii (Fig. 29.1K)); three are found in inter-reefal areas (Acalyptophis peronii, Disteira kingii and Disteira major); five are mainly from coastal habitats that are muddy or rocky; and one, Pelamis platurus, is a pelagic species that lives at the surface in slicks. Pelamis platurus is the only species that is not benthic. There is a progressive decrease in species richness along the GBR from north to south.

Table 29.1 Sea snake fauna of the Great Barrier Reef and Coral Sea and the habitats where they most commonly occur

Species

Habitat

Water depth

Acalyptophis peronii

Inter-reefal

Deep

(30-64 m)

Aipysurus duboisii

Coral reefs, over sea grass or sandy habitats

Variable*

Aipysurus eydouxii

Muddy bottoms, also in rivers and estuaries

Shallow

Aipysurus laevis

Coral reefs, rocky coasts and soft sediments

Variable

(1-55 m)

Astrotia stokesii

Eurytopic

Variable

Disteira kingii

Inter-reefal

Deep

Disteira major

Inter-reefal over sandy and muddy habitats

Variable

(1-43 m)

Emydocephalus annulatus

Coral reefs

Shallow

Enhydrina schistosa

Muddy & sandy habitats; estuaries and creeks

Shallow

Hydrophis elegans

Sandy & muddy habitats; also in estuaries

Shallow

(1-18 m)

Hydrophis ornatus

Deep

(18-55 m)

Hydrophis sp.**

Sandy habitats

Deep

(15-40 m)

Lapemis curtus***

Muddy and sandy habitats in turbid waters

Variable

(1-40 m)

Pelamis platurus

Pelagic

Water surface

* This species variously reported (1) as a shallow-water form occurring in water 2 m to 10 m deep, based on direct observation of this species on coral reefs and (2) as occurring at depths of 10-30 fathoms (18-55 m) based on trawling catches. ** Sometimes erroneously called Microcephalophis gracilis in the literature. *** Often listed as Lapemis hardwickii (a synonym).

* This species variously reported (1) as a shallow-water form occurring in water 2 m to 10 m deep, based on direct observation of this species on coral reefs and (2) as occurring at depths of 10-30 fathoms (18-55 m) based on trawling catches. ** Sometimes erroneously called Microcephalophis gracilis in the literature. *** Often listed as Lapemis hardwickii (a synonym).

The diets of sea snake species range from generalist to extremely specialised. Aipysurus eydouxii and E. annulatus have the most specialised diets and eat only fish eggs, while A. peronii feeds principally on goby fish, and a number of Hydrophis species are eel-specialists; H. elegans may be such a species. Other species, such as Lapemis curtus, A. duboisii, A. laevis, D. major, P. platurus, E. schistosa and probably D. kingii, are more general in their diet and eat fish from a variety of families (some even take an occasional invertebrate). The diets of the other sea snake species from the GBR are poorly known.

Most sea snake species feed on the bottom. They poke their heads into burrows and crevices and flick out the tongue to capture odours whereby they identify their prey. They do not recognise prey by sight and do not attempt to catch fish in open water. At least three species from the GBR are exceptions.

Enhydrina schistosa lives in turbid water where visibility is low and it feeds by snapping at fish that bump into it in the muddy water. Pelamis platurus catches its prey while floating on the surface. It slowly bends its head toward vibrations of the water made by small fish and captures its prey using a fast lateral strike, sometimes accompanied by backward swimming. Lapemis curtus may feed in the water column as well as on the bottom. All species manipulate their prey until they reach the head and prey items are swallowed headfirst.

Sea snakes elicit fear in many people and indeed in Asia fishermen are bitten and die. Most bites are sustained, however, either by people using hand seines in muddy estuaries where they tread, bare-footed, on E. schistosa or when attempting to extract snakes by hand from their nets. About 90% of serious or fatal bites suffered by humans are attributable to E. schistosa and the rest are mostly inflicted by Hydrophis cyanocinctus, a species not present in Australia. Sea snakes do not constitute a significant risk on reefs where most sport divers concentrate their activities, as long as people exercise good sense and caution; that is, do not molest the snakes, and do not approach courting pairs. There are very few verified examples of unprovoked sea snake attacks. Although a wet suit is good protection against most species, A. stokesii (Fig. 29.1K) and larger female A. laevis (Fig. 29.1/) are exceptions; their fangs are long enough to penetrate the thickness of neoprene used in tropical wet suits. An anti-venom specific for the treatment of bites by E. schistosa is available and, in combination with anti-venom against tiger snakes, Notechis scutatus, is effective against other species of sea snakes.

Threatening processes affecting sea snakes are less well understood than those affecting sea turtles. In South-East Asia, many sea snake species are harvested for food and leather whereas, in Australia, all sea snake species are protected under the Environment Protection and Biodiversity Conservation Act, 1999 and direct harvesting is banned. There have been some reports of declining sea snake numbers on some reefs in Australian waters, possibly attributable to the degradation of tropical, shallow-water marine habitats due to overfishing, pollution, coral bleaching and disease. In addition, inter-reefal species comprise a significant component of commercial trawl bycatch. Considerable effort is being made to reduce this impact by developing and evaluating the effectiveness of bycatch reduction devices for sea snakes.

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