Sea Snakes

Sea Snakes

Sea
snakes come from the Family Hydrophiidae, living most or all of their lives in
usually aquatic, marine environments.  They are very closely related to the
Family Elapidae which contains “cobra” type snakes, and this is most
evident in their fang structure.  The right image shows the characteristic
small sized fangs towards the front of the mouth, which are used to envenom
their prey.  Sea snake fangs are fixed, and unlike many land based snakes,
they do not make lightning fast strikes, instead they tend to hang on and
chew.  Mmm.

Sea
snakes are typically found on tropical shores of the Pacific and Indian oceans
(luckily not the Atlantic).  This close proximity to human activity means
that there have been a range of attacks recorded although the species as a whole
is not aggressive and will usually shy away.   These snakes have evolved
special salivary glands that produce venom which functions to immobilise and
digest prey.  Their venom is one of the most deadly in the world,
containing a lethal cocktail of proteins and neurotoxins.  Sea snake venom
has been found to be 2-10 times as venomous as any terrestrial snake making them
extremely deadly, but their docile nature and relatively extreme environment
make them less dangerous than their land dwelling relatives.

Venom

The venom contains a series of proteins and
neurotoxins.

The neurotoxins are by far the most active
constituent of the venom and work by acting on the acetylcholine receptor.  
This causes paralysis of skeletal muscle and death results by respiratory
arrest.  The venom is potent, but only small amounts are usually injected
so fatalities are rare.  This is coupled with an observed reluctance to
deliver venom when they bite.

Structure

The most active component of the venom is
erabutoxin b, a short chain protein that consists of 62 amino acids.  The
primary structure can be illustrated as follows:

N-terminal-ARG-ILE-CYS-PHE-ASN-GLN-HIS-SER-SER-GLN-

PRO-GLN-THR-THR-LYS-THR-CYS-PRO-SER-GLY-SER-GLU-

SER-CYS-TYR-HIS-LYS-GLN-TRP-SER-ASP-PHE-ARG-GLY-

THR-ILE-ILE-GLU-ARG-GLY-CYS-GLY-CYS-PRO-THR-VAL

-LYS-PRO-GLY-ILE-LYS-LEU-SER-CYS-CYS-GLU-SER-GLU-

VAL-CYS-ASN-ASN-C-Terminal

Like so many other neurotoxins erabutoxin
b contains 4 disulfide bridges, which are known to be incredibly important in
the toxicity of the venom.   The neurotoxin has an anti-parallel Beta sheet
structure containing no alpha helix structures.

Erabutoxin b

Mechanism

The mechanism that erabutoxin b exerts is not yet fully
understood but it is believed to act at acetylcholine receptors.  Upon
envenomation the toxin binds to the nicotinic acetlycholine receptor on the
motor end plate blocking it irreversibly.  Acetlycholine binds to an
unaffected receptor and opens an ion channel, this results in the depolarization
of the end plate through the influx of Na⁺ ions.  If the
depolarization then causes an action potential a skeletal muscle contraction
occurs.  But in the presence of erabutoxin b there is a neuromuscular
blockade between the phrenic nerve and the diaphragm.  The diaphragm is
paralysed and death results from respiratory arrest.

Biosynthesis

The most amazing aspect of this toxin is its
biosynthesis, in that it is synthesized extremely quickly.  The rate of
biosynthesis was experimentally found by injecting labeled isoleucine into the
venom glands.  This showed the synthesis starting 30 seconds after
injection and finishing 1 minute after injection, i.e. between 30 seconds – 1
minute!  For more info.

Yellow-bellied Sea Snake – The Australian Museum

Scientific name: Pelamis platurus

Similar species:
This species is unlikely to be confused with any other sea snake, due to its highly unique appearance.

• Author(s)

  Cecilie Beatson

• Updated

  19/11/20

• Read time

  2 minutes

Introduction

The Yellow-bellied Sea Snake has the distinction of being the most widely ranging snake in the world, as well as the most aquatic, never having to set scale on land or sea floor its entire pelagic life.

Identification

A moderately-built sea snake, with an elongated head distinct from the body. The upper half of the body is black to dark blueish-brown in colour, and sharply delineated from the yellowish lower half. The tail is paddle-shaped and yellow with dark spots or bars. Body scales are small, smooth and hexagonal in shape; the head scales are large and regular. The large eye has a blueish-black iris.

Midbody scales in 47-69 rows, ventrals 264-406.

Habitat

The Yellow-bellied Sea Snake is the most pelagic of all the sea snakes, occurring in the open ocean well away from coasts and reefs. A small individual (total length = 230mm) found in a mangrove swamp suggests that the species may occasionally occur in inter-tidal habitats.

Distribution

Yellow-bellied Sea Snakes are widespread in the tropical parts of the Pacific and Indian Oceans between the 18-20º C isotherms. Currents occasionally carry the snakes into temperate waters, but these are almost certainly far from their breeding and feeding waters. In the western Pacific, the species has been found as far north as Possiet Bay (= Zaliv Pos’yeta), Russia (latitude = 42º 39’ N) and as far south as Tasmania and the coast just south of Wellington in New Zealand (latitude = c. 41º 18’ S). In the eastern Pacific, the species has been found as far north as San Clemente, California (latitude = 33º 35’ N).

In New South Wales, the species occurs occasionally as both living and dead strandings all along the east coast. These strandings have often coincided with either strong onshore winds or storms.

The residency status of Yellow-bellied Sea Snakes along the New South Wales coast is unclear. The vast majority of sightings have been of specimens in poor condition, most likely carried down passively by currents from warmer waters. However, the observation in earlier times of individuals in Port Jackson and gravid females in Botany Bay suggests that they may be, or at least might have been, resident.

Biomaps map of Yellow-bellied Sea Snake specimens in the Australian Museum collection. http://www.biomaps.net.au/biomaps2/mapam.jsp?cqn=Pelamis%20platurus&cql=sn&csy=Square

Feeding and diet

In the wild, the Yellow-bellied Sea Snake eats only fish. It hunts by stealthily approaching its prey or by waiting motionless at the surface and ambushing fish that come to shelter underneath it (small fish are often attracted to inanimate objects such as floating debris). With its mouth agape the snake makes a rapid sideways swipe to snare any fish that comes too close. This snake can even ambush small fish behind its head by smoothly swimming backwards so that the prey then comes within range of its mouth.

In captivity, the snake will feed on whole fish (both alive and dead) or pieces of fish, and may also accept frogs (although frogs would not have been in the diet of this lineage of snakes for possibly several million years). When feeding, the snakes will lunge and bite at anything, including other snakes in the tank, and is known to stick its head out of water to take prey dangled above it.

Other behaviours and adaptations

Yellow-bellied Sea Snakes swim by lateral undulation of the body, and can move both forwards and backwards. They are capable of bursts of speed of up to 1m/sec when diving, fleeing and feeding. When swimming rapidly, they sometime carry their head out of water. On land however the snakes are unable to stay upright and move effectively because their compressed shape makes them roll onto their side.

In the open ocean, Yellow-bellied Sea Snakes often occur in large numbers in association with long lines of debris. These “slicks” form in calm seas and consist variously of debris, foam and scum brought together by converging water currents. In some areas, such as the Gulf of Panama in the eastern Pacific Ocean, the slicks can vary in width from 1 to 300m and stretch for many kilometres. Several thousand snakes may be associated with a single slick. It is not clear whether the snakes actively swim to the slicks or whether they are carried into them passively. Snakes in these slicks have been observed feeding; however mating behavior in these large aggregations has not been recorded.

Being a pelagic species the Yellow-bellied Sea Snake has limited access to hard objects, such as coral, to rub against when the skin is due to be shed. Instead the snake uses a knotting behavior whereby it coils and twists upon itself, sometimes for hours on end, to loosen the old skin. The skin is shed frequently, and in captivity may be sloughed as often as every 2 to 3 weeks. The knotting behaviour also helps to detach organisms such as algae and barnacles that adhere to the skin.

Breeding behaviours

Breeding probably occurs throughout the year in warmer seas but may be restricted to the warmer months in cooler waters. In Australia, gravid females have been found washed onto Sydney beaches in winter (June-July). In the southwest Indian Ocean, females with small developing embryos have been found in late winter and females with near-term embryos have been found in early spring and mid-autumn. Females reach sexual maturity at a snout-vent length of at least 623mm.

From observations in captivity, gestation has been inferred to last at least five months. The female gives birth to between 2 and 6 young, measuring around 250mm in total length. The young are born with substantial fat-bodies, nevertheless they will feed on their first day of life.

Predators

Unlike most other species of sea snake, the Yellow-bellied Sea Snake does not seem to have many predators. In places where the snakes occur in large numbers together with potential predators (large fish, sea birds and marine mammals), no attempts at predation have been observed. The bright colouration of this species serves as a warning, not only that the snake is highly venomous, but also unpleasant and possibly even toxic to ingest. In experiments where skinned Pelamis pieces were offered to predatory marine fish, the fish refused to eat it, and those tricked into eating the meat regurgitated it soon after. In the few known records of natural predation on these snakes, both predators (a pufferfish and a leopard seal) regurgitated the snake afterwards.

Yellow-bellied Sea Snakes are fouled by a number of different marine invertebrates, including a species of barnacle that grows only on sea snakes. Most of these organisms do not directly harm the animal; however if the infestation is heavy the resulting drag can affect the snake’s performance. By frequently knotting and shedding its skin, the snake is effectively able to rid itself of these organisms.

The species’ recorded endoparasites include cestodes (tape worms) and nematodes (round worms).

Danger to humans

Most people are only likely to encounter a Yellow-bellied Sea Snake if a sick or injured animal drifts ashore. Although these specimens are usually in poor condition, they still pose a risk if they are picked up or wash against a person in the surf. If roughly handled this species is likely to bite. The fangs are quite short (~ 1. 5mm) and only a small dose of venom is usually injected, however this venom is highly toxic and contains potent neurotoxins and myotoxins. Symptoms of envenomation include muscle pain and stiffness, drooping eyelids, drowsiness and vomiting, and a serious bite can lead to total paralysis and death. Anyone suspected of being bitten by a Yellow-bellied Sea Snake should seek medical attention immediately, even if the bite appears trivial (sea snake bites are initially painless and show no sign of swelling or discolouration). This species has caused fatalities overseas, however none have been recorded in Australia.

In cases of sea snake stranding, contact your local wildlife authority or wildlife rescue service. Do not attempt to pick the snake up and return it to the sea as it is unlikely to survive. Holding some sea snakes in a tilted position out of water for a few minutes can be enough to injure or kill them, as they are unable to maintain an even blood pressure in their bodies without being supported by water.

Evolutionary relationships

True sea snakes are part of the Australian elapid radiation, and appear to have evolved from a Notechis– or Hemiaspis-type live-bearing ancestor.

References

Cogger, H. (2000) “Reptiles and Amphibians of Australia”, Reed New Holland

Greer, A.E. (2005) “Encyclopedia of Australian Reptiles : Hydrophiidae”, Australian Museum

Greer, A.E. (1997) “The Biology and Evolution of Australian Snakes”, Surrey Beatty & Sons Pty Ltd

Ehmann, H. (1992) “Encyclopedia of Australian Animals : Reptiles”, Australian Museum, Angus & Robertson

Heatwole, H. (1997) “Sea Snakes”, revised edition, UNSW Press

Wilson, S. and Swan, G. (2008) “A Complete Guide to Reptiles of Australia”, Reed New Holland

Crown of Evolution. Scientists have figured out how venomous snakes got their famous fangs.

Related video

In a new study, scientists show that the changes occurred due to modifications in the structure of the teeth, which helped to fix the fangs in the sockets. In some species of snakes, channels have developed in the teeth that run through the entire canine tooth, which began to be used to inject poison, reports theconversation.com

Of the nearly 4,000 snake species, about 600 are considered medically dangerous. This means that after a bite, a person needs urgent medical attention. But many of these snakes have small fangs and are considered not very venomous. According to scientists, the appearance of not very dangerous poisons precedes the appearance of poisonous fangs in snakes.

Taipan

Photo: wikipedia

Venomous fangs of snakes vary:

• They can be placed in the back of the mouth, as in crab-eating water snakes, cat-eyes, gray tree snakes and boomslangs
• They can be placed in the front of the mouth, like in cobras, coral snakes, kraits, taipans and sea snakes
• They may also be in the front of the mouth, but may curve backwards or sideways, as in vipers and rattlesnakes.

Location of poisonous fangs in snakes

Photo: The Conversation

The History of Fangs

“If you look at the evolution of snakes, the most recent common ancestor of all snakes with fangs probably didn’t have them,” said study authors Alessandro Palchi of Flinders University, Australia, Aaron LeBlanc of King’s College London and Olga Panagiotopoulou of Monash University, Australia.

So how did snakes evolve their syringe-like fangs that evolved from the simpler, cone-shaped teeth of their ancestors?

“To answer this question, we carefully studied snake teeth and how they develop. We examined 19 species of snakes, both venomous and common, as well as one fossil snake,” the scientists say.

Taipan skull and close-up of its left canine, in longitudinal and transverse sections, showing the relationship between the plicidentin folds and the venom channel.

Photo: The Conversation

The secret of snake teeth

“We found that almost all snakes have teeth that are strongly concave at the base and look wrinkled in cross section,” scientists say.

These folds, plicidentins, occur in the layer of the tooth called dentin. Plicidentins have been found in many extinct animals and in some species of extant fish and lizards, but their purpose is not fully understood. One theory suggests that they help the tooth not break during biting.

“When we tested this theory with computer simulations, we found that it was not,” the study authors say.

Snakes change their teeth throughout their lives and their teeth are placed in shallow holes. Scientists believe that these folds improve the anchoring of new teeth in empty sockets, providing a larger area for attachment.

Scientists say that one of the folds in poisonous snakes is much larger than the others. It occupies the entire tooth, forming a channel for the passage of poison. The researchers also found that in some species of venomous snakes, such channels may exist in teeth other than fangs, but they are not associated with venom glands.

“We found a clear relationship between the presence of plicidentins and venom channels. We concluded that at the very beginning, venomous snakes accidentally developed canals in their teeth, simply as a result of an increase in plicidentins, independent of the venom glands,” the scientists say.

Cobra

Photo: wikipedia

How ordinary teeth became venomous

The scientists then investigated how snake fangs and venom glands evolved together to become an effective means of delivering venom. In the ancestors of modern venomous snakes, the presence of venom glands was a necessary condition for the transformation of teeth with channels into enlarged venomous fangs.

Scientists believe that when a tooth appeared with a canal near the exit of the poisonous gland, natural selection contributed to the increase in this tooth in size. Also, this tooth has become more effective at injecting poison.

“This evolutionary process eventually led to the large, syringe-shaped fangs that venomous snakes have today,” the scientists say.

Poisonous snakes. Questions and Answers

There are more than 2500 species of snakes in the world. Of these, only 450 species are poisonous. These include vipers, cobras, mambas, sea snakes, etc. Even poisonous snakes only attack for defense.

Why do snakes have fangs?

With the help of fangs, snakes inject poison into the victim’s body. All venomous snakes have venom glands next to their fangs. When the snake bites the prey, the muscles press on the gland, the poison passes through the hollow fangs and is injected into the body of the victim.

The viper, one of the most venomous snakes, has only one pair of fangs. But they are longer than other types of snakes

Why do cobras have shorter fangs than vipers?

Cobras and other snakes of the same family have short fangs. Unlike vipers, these snakes cannot put their teeth in their mouths when they are not using them. If the fangs were too long, the cobra would injure itself by closing its mouth.

How many types of venom do snakes produce?

There are two types of snake venoms. Asps produce neurotoxins. They affect the nervous system of the victim and lead to rapid death. Vipers produce a hemotoxin that affects the blood and organs, but does not kill immediately.

What is the difference between a pit viper and a true viper?

Both snakes belong to the same family. However, pit vipers (rattlesnakes) have special heat-sensitive organs – pits located between the ears and nostrils. These organs allow snakes to sense temperature differences between prey and surroundings, so rattlesnakes can hunt even in the dark.

The rattle of the pit viper consists of modified scales at the end of the tail. Each time a rattlesnake changes skin, a new segment is added to the rattle.

Are rattlesnakes venomous?

Rattlesnakes are very poisonous. They are found in North America and Mexico. Most species produce a very potent hemotoxic venom. Usually snakes warn of an attack by twitching their tail, which has a rattle of several connected scaly segments. When the snake twitches its tail, the segments rub against each other and make a crackling sound that repels most animals.

How have sea snakes adapted to life in the water?

Sea snakes have a paddle-shaped tail. They have a large lung that fills almost the entire body. Because of this, snakes can stay under water for a long time. Sea snakes prey on fish and small marine animals.

Sea snakes are found in warm waters, mainly off the coast of Asia and South America. Compared to other snakes, they have flatter heads, which helps them swim better

Which snake has the longest fangs?

The Gaboon viper has the longest fangs, their length can reach 5 cm.