Basking shark
| Basking shark | ||||||||||||
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Basking shark ( Cetorhinus maximus ) |
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| Systematics | ||||||||||||
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| Scientific name of the family | ||||||||||||
| Cetorhinidae | ||||||||||||
| Gill , 1862 | ||||||||||||
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| Cetorhinus | ||||||||||||
| Blainville , 1816 | ||||||||||||
| Scientific name of the species | ||||||||||||
| Cetorhinus maximus | ||||||||||||
| ( Gunnerus , 1765) |
The basking shark ( Cetorhinus maximus ) is up to ten meters long and weighs up to four tons, making it the second largest known fish on earth after the whale shark . Like the whale shark, the basking shark feeds on plankton . In contrast to the whale shark, which sucks in the water and filters it through its gill pots, the basking shark swims with its mouth open, allowing the water to flow through its gills . With this method, it is able to filter around 1800 tons of water per hour for food.
It is the only species in the family of the same name within the mackerel sharks (Lamniformes), which includes some of the largest and most well-known species of predatory sharks such as the great white shark , the maco shark and the porbeagle sharks .
features
As an adult, the basking shark usually reaches a body length of 6.70 meters to 8.80 meters and can reach a maximum length of 10 meters, according to other sources also 12 meters. After the whale shark ( Rhincodon typus ) it is the second largest shark and fish species on earth. Around 25 percent of the basking shark's body weight is made up of the oil-containing liver , which enables the animals to float in the water. The coloring is almost monochrome dark gray, gray-brown to blackish, with the back being darker than the underside. Often the animals have light or dark spots on the back and the sides of the body, also below the head. The undersides of the fins are also dark gray in color, but in young animals they are white with a sharp demarcation from the dark areas. In very rare cases, albinism can occur.
The pectoral fins are very broad. It has two dorsal fins, of which the first clearly exceeds the second and attaches behind the free rear end of the pectoral fins. It also has a pair of pelvic fins, an anal fin that is almost the same size as the second dorsal fin, and a crescent-shaped caudal fin with broad side keels on the caudal fin stalk and a very long upper lobe, while the lower one is slightly shorter (heterocercus caudal fin). The fin is clearly notched at the upper tip.
The snout is conical-pointed and elongated and protrudes far beyond the mouth; the numerous and multi-row single-pointed teeth of the upper and lower jaw are very small and curved. Both the eyes and the injection holes are very small. The most striking feature of the basking shark, besides its size, are the five pairs of huge gill slits that surround its body behind the head like a collar and almost meet on the top and bottom of the body. On the inside, the gill arches are equipped with fish traps made of modified teeth in a mucous membrane, which are used for nutrition.
distribution and habitat
Basking sharks are found in both hemispheres. They prefer cold to moderately warm waters, where they follow the respective plankton supply. On the basis of sightings, which usually took place near the coast, their distribution has so far been to the coastal and pelagic areas of the Atlantic Ocean (Newfoundland to Florida, southern Brazil and Argentina , South Africa , the Mediterranean Sea , the North Sea , Norway and Iceland ) and the Pacific ( Japan , Korea , China , the south coast of Australia , New Zealand , Tasmania , Chile , Peru , Ecuador and the Gulf of California to the Gulf of Alaska ). The sharks also penetrate bays and estuaries in their search for food .
Way of life
The way of life of basking sharks has so far been little researched, in particular very few observations have been made about reproduction, migratory behavior and the social structures within shark schools. They are diurnal and persistent, slow swimmers who sometimes go on long hikes. They occur individually or in groups (schools) of up to 100 animals. Occasionally the groups split into same-sex groups with individuals of about the same age.
Diet and walks
The basking shark, like the whale shark, feeds on zooplankton , i.e. small animal organisms that float in the open water (mainly small crustaceans, pteropods , fish eggs and larvae). To eat it often swims near the surface of the water and, with its mouth wide open and slowly swimming, filters the plankton out of the water, which gets caught in the gills and is then swallowed. The small crustaceans and other plankton organisms get caught in the mucus-covered cage teeth and are washed into the throat when the mouth is closed. With this method, the basking shark is able to filter around 1800 tons of water per hour for food, whereby a fully grown basking shark needs up to 500 liters of zooplankton for nutrition per day.
The pots are thrown off at regular intervals of around four to five months and replaced with new ones. It is unclear whether this goes hand in hand with a shark fasting period, during which the animals retreat into deeper water layers. The assumption that the animals hibernate in the phase in which they renew their gill pots could, however, be refuted.
A three-year study by the Marine Biological Association in Plymouth in 2002, in which a total of 21 basking sharks were equipped with satellite transmitters, showed that when they search for food, they also cover large distances in the open sea and in doing so a water depth of over 700 Meters. Occasionally they even cross the equator on the routes of up to 9,000 kilometers and are extremely active during the winter months, according to a study by the Massachusetts Division Of Marine Fisheries in 2009. While they look for food near the surface in summer, they follow the plankton swarms in the northern hemisphere in the months from November to March to greater depths below the continental shelf, where large amounts of zooplankton reside at this time.
Reproduction and development
During courtship, the males follow the females to mate with them. During mating, the male places his pectoral fins over the first dorsal fin of the female while he introduces the clasps into the cloaca . The rough skin of the partner rubs against each other, which in both sexes leads to abrasions of the skin in the area of the cloaca. Based on the investigation of scar formations in the vagina of the females, it is assumed that there will be multiple mating with different males in one mating season. The males' sperm are in packets, the spermatophores , with a diameter of about three centimeters. When mating, about 15 liters of sperm are transferred into the female. The females have two ovaries , of which only the right one is formed, in which the mature eggs are then produced. After fertilization, the fertilized eggs develop in the female's uterus .
The pregnant females separate from the shark groups and move away from the coastal waters. A gestation period of more than a year is assumed. As a rule, females that are not yet sexually mature and not pregnant are caught and sighted significantly more frequently than pregnant females, and the relationship between males and females is also clearly shifted in the direction of the females.
Basking sharks give birth to live cubs that hatch from eggs incubated in the womb and open before birth; they are thus ovoviviparous . Before birth, the young probably feed on the oophagus , i.e. on the mother's unfertilized eggs. Whether it's the basking shark species to the known of related Adelphophagie comes, is not known. The litter consists of a few young animals, but so far only one litter with 6 young animals has actually been documented. Newborn basking sharks are likely to be 1.5 to 1.8 meters in length.
The very slowly growing animals reach sexual maturity at a body length of 5.70 meters for the males and 8 meters for the females.
Predators and parasites
Only the killer whale is known to be a predator for basking sharks . Very few predators are believed to hunt and prey on basking sharks. Of great white sharks has been reported to the carrion eating sharks.
Among the parasites , research has so far mainly been carried out on ectoparasites , i.e. those that parasitize on the shark's outer skin. Above all play sea lamprey ( Petromyzon marinus ) play an important role, which bite into the skin of sharks, without having to pass through, however. The cigar shark ( Isistius brasiliensis ), which is known for using its teeth to bite pieces of meat from large fish and marine mammals, also attacks the basking shark.
Evolution and systematics
Fossil remains of basking sharks are relatively common; due to the anatomical nature of the animals , they usually consist of the characteristic gill arches, teeth or individual vertebrae . The oldest clear finds of basking sharks come from the Middle Eocene . For example, a find from the La Meseta Formation on Seymour Island in the Antarctic Weddell Sea from this time has been documented. Further remains came from the Tavda Formation near Malyshkina in western Siberia and from Oregon ; they date from the Eocene to the Oligocene . Originally attributed to the extinct species C. parvus , they are now part of the basal genus Kaesius . There are also fossil finds from more recent ages such as the Miocene , which are spread across North America and Eurasia. They mostly belong to the genus Cetorhinus . Several fossil species were identified within these, but their independence is controversial. As the only recognized species besides C. maximus , C. huddlestoni was described in 2014 on the basis of Middle Miocene finds from the Sharktooth Hill Bonebed in California . At around 4 mm in height, its teeth are comparable in size to those of today's basking sharks. Other finds of extinct basking sharks point to somewhat smaller animals. Using associated vertebrae and gill arches from the Upper Miocene sections of the Empire Formation in Oregon, a length of the animal of 4.5 to 5.8 meters is reconstructed. While the finds of the Paleogene and the Neogene can be safely assigned to basking sharks, Upper Triassic remains of Pseudocetorhinus , discovered in western Europe for example, are problematic in their systematic relationship to the later forms.
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Relationships within the mackerel shark species
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The basking shark was first described scientifically by the Norwegian bishop and naturalist Johan Ernst Gunnerus in 1765 under the name Squalus maximus based on a specimen off the Norwegian coast. This was followed by a number of other descriptions under different names by various naturalists, which are now considered taxonomic synonyms. Henri Marie Ducrotay de Blainville assigned the shark to its own genus in 1816, which he called Cetorhinos , derived from the Greek ketos , which means "marine animal" or "whale", and rhinos for "nose", the species name maximus comes from the Latin and means "greatest". The classification in the own family Cetorhinidae took place in 1862 by Theodore Nicholas Gill .
The basking shark is currently the only species of the monotypical genus Cetorhinus as well as the only representative of the family of the Cetorhinidae. This results from its very derived characteristics and the unique way of life within its relationship, the mackerel shark-like , with corresponding adaptations. Other genera and species of Makrelenhaiartigen are all carnivorous hunters and are among the largest and best-known carnivorous sharks, including the great white shark ( Carcharodon carcharias ), the thresher sharks (genus Alopias ), the mako sharks (genus Isurus ) and the porbeagle shark (genus Lamna ).
On the basis of morphological features, the basking shark was considered to be a sister species of a taxon formed from the genera Isurus , Carcharodon and Lamna . These relationships were also confirmed by molecular biological studies, whereby the anatomical similarities with the basking mouth shark ( Megachasma pelagios ) were identified as convergent adaptations to a comparable way of life.
Basking sharks and humans
Humans and basking sharks
As plankton eater, basking sharks do not pose a threat to humans despite their size. They are not aggressive and do not attack divers or boats. However, due to their size, they are tremendously powerful and harpooned basking sharks have been reported to attack boats. Divers can injure themselves on the very rough, sandpaper-like skin caused by the skin's teeth .
Economical meaning
Internationally, hunting is primarily of historical importance. The sharks were hunted for their meat, oil, and liver, rich in vitamins and squalene . Today the basking shark is hunted from boats with harpoons , especially in the north-east Atlantic and in the east Pacific , although this form of fishing is still widespread off China and Japan. The liver of the fish is used to produce oil, but the fins, meat and skin are also used. The fins - together with those of other shark species - form the basis for the shark fin soup , which is popular in China. A pair of fresh fins is worth about $ 1000 in the fish market, and when dried they are worth about $ 350 a pound. The liver is sold in Japan as an aphrodisiac and remedy, and the oil is used as an ingredient in cosmetics. A large basking shark about eight meters long provides about a ton of meat and about 380 liters of oil.
A modern economic "use" is practiced today, for example, on the Isle of Man in the Irish Sea as part of ecotourism . Similar to whale watching , tourist groups are driven to the basking sharks.
Hazard and protection
Harpoon hunting and fishing with nets in which the animals get caught mean that basking sharks are rapidly declining. Due to its slow growth, the long gestation period, a low rate of offspring and the late sexual maturity, the basking shark is not able to absorb the losses. Scientists consider it to be an endangered species. For this reason, the SSG (Shark Specialist Group) of the IUCN (International Union for Conservation of Nature and Natural Resources) considered an application to CITES to add the species to the Red List of Endangered Species . The basking shark has been listed there since 2005 and is currently classified as “endangered” (“vulnerable”).
In the EU the catch is quoted; no more than 400 tonnes of live weight per year may be achieved. In Norway, a non-EU member, animals are hunted regardless of this quota. The basking shark is protected all year round around the Isle of Man . However, since too little is known about the habits of the species, an application has been made in Great Britain to ban fishing in all territorial waters .
Cryptozoology
Basking sharks swim in large groups on the surface of the water, whereby the high and non-rigid dorsal fins of the body are visible. Often several animals swim in a row, so that in the past these associations were sometimes interpreted as sea snakes or other sea monsters.
The carcasses of the animals in the state of putrefaction can quickly give the impression of strange animals, because due to their anatomy the gills, the lower jaw and the lower part of the caudal fin separate from the body relatively quickly. Various studies, including the DNA, show, for example, that the carcass known as "New Nessie" that the Japanese trawler Zuiyo Maru found off the coast of New Zealand in 1977 was, despite reports to the contrary, a basking shark.
Many carcasses of supposedly unidentifiable sea monsters that have washed up in the recent past have also turned out to be the carcasses of basking sharks, precisely because the jaws and gills, which are loosely connected to the rest of the body, loosen very quickly after death and the small one, solid brain skull connected to the spine gives the impression of a small head on a long neck. This creates a certain resemblance to the extinct plesiosaurs .
literature
- Leonard Compagno , Marc Dando, Sarah Fowler: Sharks of the World. Princeton Field Guides, Princeton University Press , Princeton and Oxford 2005; P. 181, ISBN 978-0-691-12072-0
- Leonard Compagno: Sharks of the world. An annotated and illustrated catalog of shark species known to date. Volume 2. Bullhead, mackerel and carpet sharks (Heterodontiformes, Lamniformes and Orectolobiformes). FAO Species Catalog for Fishery Purposes. No. 1, Vol. 2. FAO Rome 2001 ( Cetorhinus , Cetorhinus maximus ; Complete PDF )
- Alessandro de Maddalena, Harald Bänsch: Sharks in the Mediterranean , Franckh-Kosmos Verlags-GmbH, Stuttgart 2005; Pp. 139-141, ISBN 3-440-10458-3
- David W. Sims: Sieving a Living: A review of the Biology, Ecology and Conservation Status of the Plankton-feeding Basking Shark (Cetorhinus maximus). Advances in Marine Biology 54, Academic Press, 2008. ( Google Books , most of them available)
- Ralf M. Hennemann: Sharks and rays worldwide. Jahr-Verlag, Hamburg 2001; P. 63; 64-68, ISBN 3-86132-584-5 .
Web links
- Basking Shark on Fishbase.org (English)
- Research project on the basking shark on the Isle of Man (English) .
supporting documents
- ↑ a b c d e f g h Leonard Compagno, Marc Dando, Sarah Fowler: Sharks of the World. Princeton Field Guides, Princeton University Press, Princeton and Oxford 2005; P. 181, ISBN 978-0-691-12072-0 .
- ↑ a b c d e f g C. Knickle, L. Billingsley & K. DiVittorio: Basking Shark. Florida Museum of Natural History. Accessed April 5, 2010.
- ↑ Species portrait of the basking shark on hai.ch
- ↑ a b c d e Alessandro de Maddalena, Harald Bänsch: Sharks in the Mediterranean , Franckh-Kosmos Verlags-GmbH, Stuttgart 2005; Pp. 139-141, ISBN 3-440-10458-3 .
- ↑ a b c d e f Ralf M. Hennemann: Sharks and rays worldwide. Jahr-Verlag, Hamburg 2001; P. 63; 64-68, ISBN 3-86132-584-5 .
- ↑ a b Gregory B. Skomal, Stephen I. Zeeman, John H. Chisholm, Erin L. Summers, Harvey J. Walsh, Kelton W. McMahon, Simon R. Thorrold: Transequatorial migration by Basking Sharks in the Western Atlantic Ocean. Current Biology 19, 2009.
- ↑ a b L.H. Mathews: Reproduction in the Basking Shark, Cetorhinus maximus (Gunner). Philosophical Transactions of the Royal Society London B 234, 1950; Pp. 247-316. ( Abstract ).
- ↑ Ingrid N. Visser: First Observations of Feeding on Thresher (Alopias vulpinus) and Hammerhead (Sphyrna zygaena) Sharks by Killer Whales (Orcinus orca) Specializing on Elasmobranch Prey. In: Aquatic Mammals . 31, 2005, pp. 83-88, doi : 10.1578 / AM.31.1.2005.83 ( PDF ( memento of August 9, 2017 in the Internet Archive )).
- ^ Alberto L. Cione, MA Reguero: A middle Eocene basking shark (Lamniformes, Cetorhinidae) from Antarctica. Antarctic Science 10, 1998; Pp. 83-88. ( Abstract )
- ↑ Małgorzata Bieńkowska-Wasiluk Andrzej Radwański: A new occurrence of sharks in the Menilite Formation (Lower Oligocene) from the Outer (flysch) Carpathians of Poland. (PDF; 1.9 MB) Acta Geologica Polonica 59 (2), 2009; Pp. 235-243.
- ↑ a b Bruce J. Welton: A new archaic basking shark (Lamniformes: Cetorhinidae) from the late Eocene of western Oregon, USA, and description of the dentition, gill rakers and vertebrae of the Recent basking shark Cetorhinus maximus (Gunnerus). New Mexico Museum of Natural History and Science Bulletin 58, 2013, pp. 1-48
- ↑ Michael D. Gottfried: Miocene Basking Sharks (Lamniformes, Cetorhinidae) from the Chesapeake Group of Maryland and Virginia. Journal of Vertebrate Palaeontology 15 (2), 1995; Pp. 443-447 ( JSTOR )
- ↑ Bruce J. Welton: A new fossil basking shark (Lamniformes: Cetorhinidae) from the Middle Miocene Sharktooth Hill Bonebed, Kern County, California. Contributions in Science 522, 2014, pp. 29-44
- ↑ Bruce J. Welton: Cetorhinus cf. C. maximus (Gunnerus) (Lamniformes: Cetorhinidae), A Basking Shark from the Late Miocene Empire Formation, Coos Bay, Oregon. Bulletin of the Southern California Academy of Sciences 112 (2), 2013, pp. 74-92
- ↑ a b Kenshu Shimada: Phylogeny of lamniform sharks (Chondrichthyes: Elasmobranchii) and the contribution of dental characters to lamniform systematics. Paleontological Research 9 (1), 2005; Pp. 55-72.
- ↑ a b A.P. Martin, G. J P. Naylor: Independent origin of filter-feeding in megamouth and basking sharks (order Lamniformes) inferred from phylogenetic analysis of cytochrome b gene sequences. In: K. Yano, JF Morrissey, Y. Yabumoto, K. Nakaya (Eds.): Biology of Megamouth Shark. Tokai University Press, Tohylo 1997; Pp. 39-50.
- ↑ Cetorhinus maximus in the IUCN Red List of Threatened Species 2010.1. Posted by: SL Fowler, 2005. Retrieved April 6, 2010.
- ↑ a b Glen J. Kuban: Sea-monster or Shark? An Analysis of a Supposed Plesiosaur Carcass Netted in 1977. Reports of the National Center for Science Education 17 (3), 1997; Pp. 16-28.
