Sea snakes

from Wikipedia, the free encyclopedia
Sea snakes
Head of a sea serpent

Head of a sea serpent

Systematics
Order : Scale reptiles (Squamata)
without rank: Toxicofera
Subordination : Snakes (serpentes)
Superfamily : Adder-like and viper-like (Colubroidea)
Family : Poison Snakes (Elapidae)
Subfamily : Sea snakes
Scientific name
Hydrophiinae
Fitzinger , 1843

In addition to the sea ​​turtles , the sea ​​snakes (Hydrophiinae) are the most famous of the reptiles living in the sea today . They belong to the snakes (Serpentes) and are classified within them in the relationship of the poisonous snakes (Elapidae). Taxonomically , the terrestrial australoasiatischen Venomari to which about Taipan , Brown snakes and pseudechis are also assigned to this subfamily. About 56 species of marine sea snakes are known.

features

Striped sea serpent

Most sea snakes reach body lengths between 1.2 and 1.4 meters, but some species can also be well over 2 meters long. For example, Hydrophis cyanocinctus can reach 2.5 meters or Hydrophis spiralis up to 2.75 meters. Most of the time the females are significantly longer than the males. The weight of the animals depends on the species and sex as well as the nutritional status. The striped sea snake Laticauda colubrina weighs on average about 0.9 to 1.3 kilograms with a body length of up to 1.80 meters (Voris et al. 1998).

The sea snakes also vary in body shape: Astrotia stokesii, for example, is rather strongly built in relation to body length and appears accordingly plump; many Hydrophis species have an extremely long and narrow head and neck area, which earlier led to the assumption that they only feed on correspondingly thin eels. Today we know, however, that they are able to swallow prey animals whose body size is twice that of themselves. The narrow head is probably used to track down prey in tight hiding places.

Due to their marine way of life, sea snakes differ significantly from other snakes in some features. The most noticeable visible feature is the laterally flattened tail, which is common to all sea snakes and is used for better locomotion in the water. In addition, there is usually a reduced number of abdominal scales (ventralia, except for the Laticauda species, which can also move on land) and the salt gland located under the tongue, which is used to excrete excess salt. In addition, the right lung of the sea snake is greatly enlarged and extends to the tip of the tail of the animals. Parts of the lungs also serve as a hydrostatic organ . The animals can dive for up to two hours and up to 180 meters deep. The valve-like closures of their breathing holes also help them with this. They are probably able to absorb oxygen through the skin and thus ensure a better supply.

Some of the characteristics described are also found in other, not closely related, snake taxa . The wart snakes living in brackish water also have a salt gland, and a reduction in the ventralia can be found in various burrowing snakes, such as blind snakes .

Occurrence

Blue - the range of sea snakes, brown - the range of terrestrial poisonous snakes
Platelet sea snake ( Hydrophis platurus )

The sea snakes inhabit the tropical marine regions of the Indian and Pacific Oceans . They are found accordingly from the Persian Gulf to the Japanese coastal waters and on the coasts of the Southeast Asian islands to Australia . With the exception of the widespread platelet sea ​​snake ( Hydrophis platurus ), all sea snakes live near the coast.

The platelet sea serpent has spread to the coasts of Madagascar and south-east Africa as well as to the west coast of tropical America , in addition to the areas mentioned , where it was also found in the Panama Canal . Some scientists fear that the snake could spread via the Panama Canal into the Caribbean and, as a neozoon , could cause a serious ecological problem here .

All other species live primarily in shallow water on the coasts, often in the area of ​​estuaries ( e.g. Enhydrina schistosa ). In these rivers they can sometimes penetrate far inland, but with Hydrophis semperi only one species is known that lives permanently in fresh water . The range of this snake is limited to the approximately 270 square kilometers large Lake Taal on the Philippines island of Luzon . From Laticauda crockeri are on the Solomon Islands freshwater populations also known. In the known distribution areas, the salt content is a maximum of 3.5 percent. In the Red Sea, with its salt concentration of 4 percent, there are therefore probably no sea snakes.

Way of life

nutrition

Adder flattail ( Laticauda colubrina )

Like almost all snakes, sea snakes are predators and feed mainly on fish. Some species are downright nutrition specialists. The adder flattail ( Laticauda colubrina ) feeds primarily on certain marine eels, and Hydrophis ornatus specializes in catfish . The platelet sea snake lives and hunts as an open water specialist almost exclusively along the thermocline , i.e. the area where two layers of water lie on top of one another. Plankton , in particular , lives here and attracts young fish from the most varied of open water species. In gastric examinations of this species, representatives of 21 species of fish were found, almost exclusively young fish. In contrast, the sea snakes that hunt in the crevices of the reef usually prey on quite large prey fish.

Some sea snakes also hunt at night. They then find their prey fish through their excellent sense of smell.

In addition to the specialists, there are also sea snake species that have a very wide range of prey. Aipysurus laevis, for example, feeds not only on fish but also on their spawn and cephalopods .

Reproduction

Except for the Laticauda species ( flattail ), the sea ​​snakes are viviparous and have their young in the sea, where they spend their entire life. The flattails, on the other hand, leave the sea and lay their eggs on land, where they can also be found quite often outside of the mating and egg-laying time and take breaks. During the breeding season, the snakes colonize different islands in very large numbers, especially in the Philippines , Laticauda laticauda and Laticauda semifasciata are found in many thousands of specimens. In general, sea snakes seeking sun and warmth can often be found in large groups on the surface of the water, even at sea.

Natural enemies

In addition to humans, the sea snakes hardly have any real predators, mainly due to their very effective poison. It can be assumed that they are occasionally eaten by sharks or whales , but the evidence for this is largely lacking. The tiger shark is said to be immune to the poison of sea snakes. Larger eagles , especially white-tailed eagles , have also been observed fishing sea snakes out of the sea when they came to the surface to breathe and sometimes also to warm up in the sun, and ate them.

Snake venom

Sea snakes produce toxins that can be actively applied by a poison apparatus. The toxins are used for the rapid immobilization of prey and for defense. Adapted to the prey, often fast fish, for example, the poison of most sea snakes is strong and quick.

As a defense, the animals only bite very rarely, except during the mating season, and instead tend to flee. In Southeast Asia in particular, coastal fishermen like to catch the animals, where they grasp them with their hands behind their heads without any major protective measures. Despite the laziness of the animals to bite, it is not uncommon for fatal bites to occur, also because antivenins (antidotes) are rarely available in the small fishing villages . More than 90 percent of all documented sea snake bites can be regarded as accidents while catching the animals. Most of the bite accidents are known from the species Enhydrina schistosa and the striped oar snake ( Hydrophis cyanocinctus ).

Poison apparatus

The apparatus consists of poison to venom glands reformed salivary glands , which via a channel with poison in the front upper jaw located, non-movable fangs (fangs, mostly in pair) are connected (proteroglyphe tooth position). The poison teeth have a deep groove as a conduit for the poison and are used to apply the poison secretion to the bite wound. The fangs of sea snakes are smaller than those of most terrestrial venomous snakes. The teeth and poison glands of Emydocephalus (turtle-headed sea snakes) are greatly reduced in adaptation to the preferred prey ( fish spawn ).

Striped row snake ( Hydrophis cyanocinctus )

toxicology

Sea snakes generally produce little venom; the yield of a venom bite (dry weight) is, for example, 8 mg for Enhydrina schistosa (Brown, 1973) and for Aipysurus duboisii (Dubois' sea snake) 0.43 mg (Minton, 1983). The toxin mixture of most sea snakes is a colorless to yellowish and viscous liquid and contains neurotoxins and myotoxins . The venom of most species is one of the strongest known snake poisons , Aipysurus duboisii is considered the most poisonous species of sea snakes. Often no poison or only a clinically irrelevant amount of poison is applied with a defensive bite.

After a poisonous bite in humans, the first symptoms appear within one to a few hours. Non-specific general symptoms (e.g. headache , nausea , vomiting , abdominal pain , diarrhea , dizziness , shock , convulsions ) can occur. Within 30 minutes to several hours after the poison bite, the myotoxic component causes general muscle pain and stiffness. After the initial general neurological symptoms (e.g. paresthesia ) and ptosis , the neurotoxic component causes progressive paralysis , which can lead to immobility of the extremities . Death can result from peripheral respiratory paralysis . In 25 percent of the fatalities, death occurs no later than eight hours after the bite, in 50 percent within eight to 24 hours and in the remaining 25 percent after up to three days. Those affected are often fully conscious until death.

The neurotoxicity is mediated by polypeptides from the group of three-finger toxins (e.g. erabutoxins ). While this family of toxins cause neurotoxic effects as well as other pharmacological effects (e.g. cytolysis or inhibition of hemostasis ) in terrestrial elapids , only five-finger toxins are known within the hydrophiinae, which mainly interact as antagonists with nicotinic acetylcholine receptors and thus through inhibition the neuronal excitation transmission at the motor end plate can cause the neurotoxic symptoms described above.

While the neurotoxins are only acutely life-threatening and their effect is limited to the exposure time of the toxins at the site of action (nicotine receptors of the motor endplate), the myotoxins lead to secondary damage to the kidneys via in part massive damage to the muscle tissue ( rhabdomyolysis ) and associated myoglobinuria up to acute kidney failure and permanent damage ( renal insufficiency of various degrees, chronic kidney failure ) are possible. In addition, the myotoxins attack the heart muscle . In this sense, they also act as cardiotoxins and, together with the hyperkalemia caused by tissue destruction, can lead to functional impairments of the heart and even cardiac arrest .

It is noteworthy that there is a strong cross-reactivity regarding the interaction between the antitoxins of the antivenin obtained by immunization with toxins from Enhydrina schistosa and toxins from numerous other sea snake species. It follows that the antivenin can be used as an antidote not only in Enhydrina , but also in many other species of sea snakes. In the closely related terrestrial elapids of Australia, the cross-reactivity to antitoxins between the genera (e.g. Oxyuranus , Pseudechis , Pseudonaja ) proves to be relatively low, which makes the use of the toxins of different genera necessary in the production of polyvalent antivenins. This cross-reactivity is, among other things, the subject of evolutionary studies of the elapids.

The so-called " Pressure / Immobilization Technique " is recommended as first aid for all sea snake bites. In addition, the possibility of artificial ventilation must be guaranteed. Further measures depend on the symptoms occurring. For most species, highly effective polyvalent antivenins are available.

Economical meaning

Sea snakes as meat suppliers

The economic use of sea snakes is relevant for two different purposes, for nutrition and for leather processing. Sea snakes serve as popular meat suppliers especially in the coastal areas of the Philippines, on the Society Islands , in southern China and in Japan . In Japan, sea snakes are imported from the Philippines because the demand exceeds the catches.

Similar to fish, the animals are prepared in a variety of ways. Sea snake meat is considered an aphrodisiac and is accordingly popular. The live snake, which is killed right at the table and eaten raw with soy sauce , is particularly valued by the Japanese . In the Philippines, however, boiled, fried, or deep-fried pieces of sea snake are more popular.

Sea snakes as leather suppliers

As a leather supplier, the sea snakes play a role almost exclusively in the Philippines. As marine organisms, these animals are not subject to the protection of the Department of Environment and Natural Resources (DENR), which regulates the trade in wild animals and their products on the islands and forbids all reptiles. The responsibility for sea snakes lies with the “Bureau of Fisheries and Aquatic Resources” (BFAR) and a ban on the use of marine resources is neither intended nor enforceable in an island state like the Philippines with a very high proportion of people who make a living from fishing . Furthermore, most sea snakes occur in their areas of distribution in such great densities of individuals that a threat cannot currently be recognized.

The catching of sea snakes for the leather industry is only worthwhile where they occur in large quantities, since the price of leather for these animals is low compared to other snake leather. The most lucrative fishing area is the coral island of Gato off the coast of Cebu , as the flattail sea snakes are particularly common here. In 1949, for example, Herre and Rabor reported that over 20,000 Laticauda fasciata were caught on the island in a single year . In 1960 the license to catch the sea snakes on and around Gato was given to a businessman, and in the following years the first company specializing entirely in the skinning of the animals was established in Tapulan on Cebu, in which the processing is still largely done by hand. In 1976 13,052 Laticauda were caught, until 1981 the catch was reduced to just 1,454 specimens; fishing for sea snakes became a sideline for individual fishermen.

In the period that followed, other sea snake species were increasingly used for leather production, in particular various Hydrophis species, Astrotia stokesi and Lapemis hardwicki , which mainly come from the Visayan Sea.

Existence risk

The capture of sea snakes as a source of food plays only a very minor role and does not pose a threat to the stocks. The situation is different with the catch for leather production. At least locally, this resulted in very strong incursions in the species Laticauda fasciata on the island of Gato, where in the early 1980s hardly any snakes could be found in the more easily accessible hiding places. The population collapse is also related to the local loyalty of the animals, which repeatedly visit the same islands for reproduction; an intensive gathering of these islands inevitably leads to a decline in the populations resident there.

A threat to the purely marine species from the leather industry is not to be assumed, since there is no targeted exploitation, the catch rates as bycatch by fishing cutters are certainly the bigger problem. According to Ward (1996), around 81,000 sea snakes were caught and killed by shrimp traffickers alone due to the close-knit nets in the waters of northern Australia in 1990. In the last 30 years, the prawn fishermen have killed several million sea snakes as bycatch, plus several thousand animals from the nets of the fish tugs. How these catch figures affect species populations and compositions has not yet been clarified.

Systematics

The systematics and phylogenetics of the Elapidae are still the subject of research. According to current knowledge, the sea snakes belong to the poisonous snakes (Elapidae) and are grouped as a subfamily Hydrophiinae within them. Fossil evidence for the ancestry of the sea snakes is not yet known, so for the time being only the first poisonous snake fossils known from the Lower Miocene of Europe provide an indication of the time of their formation .

Traditionally, a distinction is made between the real sea snakes and the flat-tailed sea snakes (genus Laticauda ). According to more recent studies, this subdivision has been abandoned and, alternatively, three or four different genus groups are combined. The following illustration follows the systematics according to Rasmussen (1997, updated by Hydrophis 2014), which is essentially based on features of the skull, such as the shape of the parietal and maxillary :

Molecular studies on the phylogenesis of sea snakes according to Keogh et. al (1998) on the basis of cytochrome b and 16S rRNA sequences suggest a further breakdown of the original systematics, since according to this, in addition to the classic subfamilies, the newly formed genus groups and even established genus as non-natural groups ( Paraphyla ).

Terrestrial Hydrophiinae

The land-dwelling poisonous snakes of Australasia are usually also placed in the subfamily of the Hydrophiinae. These include the following genera:

mythology

The sea snakes are not identical to the mythological sea ​​snake of cryptozoology known from legends and popular belief .

Individual evidence

  1. ^ Frei, Herzer & Schmidt: Poisonous and dangerous marine animals , Müller Rüschlikon, 2007.
  2. ↑ a b Spectrum Lexicon of Biology: Sea Snakes , accessed on August 25, 2015.
  3. ^ A b c O'Shea: Venomous snakes , Franckh-Kosmos-Verlag, 2006.
  4. ↑ a b WCH Clinical Toxinology Resources: Enhydrina schistosa , accessed August 25, 2015.
  5. ↑ WCH Clinical Toxinology Resources: Aipysurus duboisii , accessed on August 25, 2015.
  6. ↑ Uniprot: Erabutoxin A , accessed on August 26, 2015.
  7. ↑ a b Fry , Wuster et al .: Molecular evolution of elapid snake venom three finger Toxins , Journal of Molecular Evolution 57 (1), pages 110-129 (2003)
  8. ^ Campbell & Lamar: The Venomous Reptiles of the Western Hemisphere , Cornell University Press, 2004.
  9. ↑ Daunderer: Lexicon of Plants and Animal Poisons , Nikol Verlag, 1995.
  10. ↑ The Reptile Database: Search Results, Hydrophiinae (accessed June 30, 2018)
  11. ↑ Strickland, Carter et al .: Snake evolution in Melanesia: origin of the Hydrophiinae (Serpentes, Elapidae), and the evolutionary history of the enigmatic New Guinean elapid Toxicocalamus , Zoological Journal of the Linnean Society, Volume 178, Issue 3, November 1 2016, Pages 663–678, Published: 14 October 2016. Link to the study (accessed July 1, 2018)
  12. ↑ Strickland, Carter et al., 2016: Phylogenetic Family Tree of the Hydrophiinae (accessed July 1, 2018)

literature

General

  • JS Keogh, R. Shine, S. Donnellan: Phylogenetic Relationships of Terrestrial Australo-Papuan Elapid Snakes (Subfamily Hydrophiinae) Based on Cytochrome b and 16S rRNA Sequences. In: Molecular phylogenetics and evolution. Elsevier, San Diego Cal 10.1998, 1, 67-81. ISSN  1055-7903
  • SB McDowell: Notes on the Australian sea-snake Ephalophis greyi M. Smith (Serpentes: Elapidae: Hydrophiinae) and the origin and classification of sea-snakes. In: The journal of the Linnean Society of London. London 48.1969, 333-349. ISSN  0368-2935
  • SB McDowell: The genera of sea-snakes of the Hydrophis group (Serpentes: Elapidae). In: Transactions of the Zoological Society of London. London 32.1972, 189-247. ISSN  0084-5620
  • AR Rasmussen: Systematics of sea snakes; a critical review. In: RS Thorpe, W. Wüster, A. Malhotra (Eds.): Venomous snakes - ecology, evolution and snakebite. In: Symposia of the Zoological Society of London. Clarendon Press, London 70.1997, 15-30. ISSN  0084-5612
  • MA Smith: Monograph of the sea snakes (Hydrophiidae). British Museum of Natural History. London 1926. Wheldon & Wesley, Weinheim 1964 (repr.).
  • HK Voris: A phylogeny of the sea snakes (Hydrophiidae). In: Fieldiana. Zoology. Museum, Chicago Ill 70. 1977, 79–169. ISSN  0015-0754
  • Focus on sea snakes. In: Reptilia. Terraristic specialist magazine. Natural u. Tier-Verl., Münster 14.1998,12. ISSN  1431-8997 , is:
    • M. Gaulke: Photo report sea snakes.
    • HK Voris, HH Voris: Commuters between the tropical tides. The life of the sea cobra "Laticauda colubrina".
    • M. Gaulke: Sea snakes as a commodity.

economy

  • PT Bacolod: Notes on sea snake fishery on Gato Islet, Philippines and a proposal for a conservation and management program. In: The Philippine scientist. University of San Carlos, Cebu City 21.1984, 155-163. ISSN  0079-1466
  • PT Bacolod: The biology of some commercially important species of sea snakes (Hydrophiidae) in the Visaya Seas. In: The Philippine scientist. University of San Carlos, Cebu City 27.1990, 61-88. ISSN  0079-1466
  • Albert WCT Herre, DS Rabor: Notes on Philippine Sea Snakes of the Genus "Laticauda". In: Copeia. A journal of cold blooded vertebrates. Washington DC 1949, 182-284. ISSN  0045-8511
  • TM Ward: Sea snakes by-catch of prawn trawlers on the northern Australian continental shelf. In: Marine and Freshwater Research. CSIRO, Melbourne 1996, No. 47, pp. 631-635. ISSN  0067-1940
  • TJ Wassenberg, JP Salini, H. Heatwole, JD Kerr: Incidental capture of sea snakes (Hydrophiidae) by prawn trawlers in the Gulf of Carpentaria. In: Australian journal of marine and freshwater research. CSIRO, Melbourne 45.1994, 429-443. ISSN  0045-8511

Web links

Commons : Hydrophiinae  - collection of images, videos and audio files