Sea snake classification, biology & ecology

Species diversity and distribution.
There are approximately 70 species of sea snakes living in our modern oceans. They account for 86% of marine reptile species alive today. Other marine reptiles include 7 species of sea turtles, the salt water crocodile and the marine iguana.

Sea snakes are cold blooded reptiles and are found primarily in warm tropical waters of the Indo-West Pacific. They are not found in the Atlantic Ocean or Caribbean Sea.

The area of highest species diversity is between Singapore and Borneo, with 27 species. Australia’s coastal waters also has a high species diversity that ranges between 17 to 21 species, and the Great Barrier Reef has 14 species of sea snakes. Although sea snakes need warm tropical waters to survive, they are occasionally blown south by storms and have been recorded in Sydney harbour.

Evolutionary history
Snakes are the most recently evolved group of reptiles. They evolved from lizards, probably in the late Jurassic around 135 million years ago. These first snakes probably occurred in the ocean, however the sea snakes that we see today have evolved from four different groups of terrestrial snakes.

Types of sea snakes
There are five major groups of sea snakes.
hydrophiids, laticaudids, acrochordids, homalopsids, natricids

Hydrophiids - ‘true’ sea snakes
The largest group of sea snakes, the hydrophiids or ‘true’ sea snakes, evolved from Australian terrestrial elapids that returned to the marine environment around 30 million years ago.

Hydrophiid sea snakes have the same toxic venom and envenomation apparatus (they are proteroglyphs meaning they have fixed front fangs) as their terrestrial ancestors.

Hydrophiid sea snakes have exploited the viviparity that exists in some of their terrestrial ancestors, and have thus freed themselves entirely from the need to return to land to breed.

There are 54 species of hydrophiid sea snakes.

Laticaudids - sea kraits
The laticaudids or sea kraits comprise five species, four of which are marine. They are strongly banded and commonly seen in large numbers on beaches in south east Asia and some Pacific Islands.

Sea kraits have also evolved from terrestrial elapids and are proteroglyphs and have highly toxic venom. However they are very placid and unlikely to bite unless provoked.

Sea kraits are the only group of sea snakes that are oviparous (egg laying) and must return to land to breed.

Acrochordids - file snakes
The acrochordids or file snakes comprise three species. One species is fully marine while the others live in estuaries and freshwater habitats.

File snakes are not venomous and they give birth to live young.

Homalopsids - mangrove snakes
Homalopsids are colubrids that are confined almost entirely to estuarine environments.

There are nine species of aquatic homalopsids that are found primarily in tropical Asian waters and northern Australian waters. Only three species are fully marine.

Homalopsids are venomous but they are rear-fanged.

Natricids - salt marsh snakes
Natricids are colubrids that are confined almost entirely to salt marsh environments.

The three species of marine natricids are confined to temperate and subtropical north America and are not venomous. It is thought that these natricids might be in the early stages of evolving marine adaptations.

Marine adaptations
There are several features of the serpentine form that have allowed sea snakes to adapt more readily to the marine environment than other reptiles. Marine adaptations include swimming, respiration, salt excretion, and marine fouling & skin shedding.

Swimming
Snakes elongate bodies are preadapted for efficient swimming, and most sea snake species have developed a paddle shaped tail that further enhances their locomotory ability in water.

Respiration
Sea snakes are air breathing reptiles and must come to the surface to breathe, however they can spend from 30 minutes to 2 hours diving between breaths. They have two major adaptations that allows them to do this.

Firstly they have one elongate cylindrical lung that extends for almost the entire length of their body which is very efficient for gas exchange. They are also able to carry out cutaneous respiration. This means that oxygen diffuses from sea water across the snake’s skin into tiny blood vessels and carbon dioxide diffuses out.

Sea snakes have nostril valves that prevent air entering the lung while underwater. Nostril valves open inwards and are held shut from behind by erectile tissue engorged with blood, like a penis

Salt excretion
Sea snakes are able to avoid excess salt accumulation from sea water using a salt excreting gland, the posterior sublingual gland that sits under the tongue.

Marine fouling and shedding of skin
All snakes shed their skins. Sea snakes shed every two to six weeks, which is more frequently than land snakes and more often than needed for growth alone. The process involves rubbing the lips against coral or other hard substrate to loosen the skin. The snake then catches the skin against something to anchor it and crawls forward leaving the skin turned inside out behind it.

Skin shedding allows sea snakes to rid themselves of fouling marine organisms such as algae, barnacles and bryozoans. Otherwise they would be covered with fouling organsims like the hull of a boat that needs to be cleaned and this would interfere with the snakes ability to swim efficiently and may also cause disease.

Courtship, mating and reproduction
Like all snakes and lizards, male sea snakes have two penises. They are called hemipenes, but each is an autonomous independently functioning penis and only one is used during mating. Mating takes place for long periods and sea snakes must surface for air during that time. The female controls breathing and as she swims to the surface the male is pulled along attached via the hemipenis. At the surface the male needs to gulp for air or he has to wait til the next time the female comes up the the surface to breathe. Males are unable to disengage until mating is finished.

In species where courtship has been studied, eg olive and turtlehead sea snakes, one or more males follow the female very closely and occasionally prod the head and neck of females.

All sea snakes except the latidcaudids give birth to live young after gestation periods that range from four to eleven months, depending on the species. Most species reproduce every year. The timing of the reproductive cycle varies enomously between species and also differs between geographical locations for the same species.

Young are born underwater and must be independent immediately to swim to the surface to breathe. There is no parental care. In some species look quite different to the adults eg juvenile olive sea snake are strongly banded while the adults are not.