Great hammerhead shark
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Great hammerhead shark ( Sphyrna mokarran ) |
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| Sphyrna mokarran | ||||||||||||
| ( Rüppell , 1837) |
The great hammerhead shark ( Sphyrna mokarran ) is the largest shark within the family of hammerhead sharks (Sphyrnidae) with a maximum size between 5.5 and 6.1 meters . It lives worldwide in the area of tropical and subtropical coastal areas and the continental shelf , whereby it prefers coral reefs as habitat. It differs from other large sharks mainly in the shape of the hammer-shaped enlarged head ( cephalofoil) and its very large and sickle-shaped first dorsal fin. It is a solitary animal and feeds on a large number of invertebrates as well as bony and cartilaginous fish including small sharks and members of its own species. The main prey, however, are rays , especially stingrays , which it tracks down through the sensory organs on the cephalofoil and with the Holding head to the ground to eat.
Like all other large hammerhead sharks, this species is classified as potentially dangerous to humans, but shark accidents with hammerhead sharks are very rarely documented. Due to its large fins and his skin being hunted commercially and is on the International Union for Conservation of Nature ((IUCN) as "critically endangered" endangered ) classified.
features
The great hammerhead shark is on average 3.5 meters long and weighs over 230 kilograms. Few specimens, especially females, reach significantly greater body lengths, the maximum body lengths are 5.5 to 6.1 meters. This species is the largest representative of hammerheads, ahead of the bow-forehead hammerhead ( S. lewini ) and the smooth hammerhead ( S. zygaena ), each of which has a maximum length of around 4 meters. The heaviest great hammerhead shark recorded to date was a 4.4 meter long female weighing 580 kilograms that was caught off the coast of Boca Grande , Florida in 2006 . The specimen was pregnant with 55 pups that were about to give birth.
The shark's body, like all sharks in the family, is streamlined. The back color is dark brown, bronze or gray to gray-brown, on the flanks towards the belly it becomes lighter and the belly is colored whitish; the borderline between the color of the stomach and the back is clearly drawn. Adult specimens have no clear fin markings, with the exception of a darker edge on the underside of the pectoral fin; in juveniles, the tip of the second dorsal fin may be black. The hammer-shaped head ( cephalofoil ) has a width that corresponds to between 23 and 27% of the body length, whereby the great hammerhead has the widest head of all species of the genus Sphyrna in relation to the body ; only the head of the winged hammerhead shark ( Eusphyrna blochii ) is even wider with a width of almost 50% of the body length. The front edge of the cephalofoil is almost straight and has a clear indentation both in the middle of the head and on both sides near the edge of the head, the rear edge is slightly concave. This distinguishes the species from other large species with a curved cephalofoil. When viewed from below, the wide mouth forms a parabola , with short labial furrows . The teeth of the upper and lower jaw are obliquely triangular with a point and strongly jagged, towards the edges of the mouth they are increasingly curved. In the upper jaw there are 17 teeth in each half of the dentition and 2 to 3 teeth in the area of the symphysis , in the lower jaw there are 16 to 17 teeth in each half of the dentition and 1 to 2 teeth in the area of the symphysis. As with other sharks, there are more rows of teeth behind these teeth.
Like all species of the family also owns the Hammerhead five gill slits , a suction hole missing. It has a very distinctive and strikingly large and sickle-shaped first dorsal fin that attaches above the inner edge of the pectoral fins. The second dorsal fin and the anal fin are significantly smaller, but compared to other species they are also relatively large with deep notches on the rear edge. The pelvic fins are sickle-shaped with a concave rear edge and thus differ from the straight fin edge of the bow-forehead hammerhead. The asymmetrical caudal fin is also strongly sickle-shaped. It consists of a narrow lower lobe and a significantly longer upper lobe with a small end lobe. The skin is densely covered with tooth-like placoid scales that are diamond-shaped and each have three to five horizontal ridges in smaller individuals and five to six in larger individuals.
distribution
Great hammerhead sharks can be found almost worldwide in the tropical and subtropical coastal waters on the continental shelf between the 40th north and 37th south latitude. In the western Atlantic, the distribution area extends from North Carolina to Uruguay and includes the Gulf of Mexico , the area around the Bahamas and the Caribbean . In the eastern Atlantic it extends from Morocco to Senegal , with the shark also living in the Mediterranean ; it is rare here and has a focus in the western and southern Mediterranean. Occurrences off Gambia , Guinea , Mauritania , Sierra Leone and Western Sahara can be assumed, but could not be confirmed. It can be found almost everywhere along the coast of the Indian Ocean from the east coast of Africa along the Asian coast including the Red Sea . In the western Pacific it lives on the coasts and islands from Asia (China, Taiwan) to Australia (Northern Territories, Queensland, New South Wales), New Caledonia and French Polynesia and in the eastern Pacific from southern Baja California to Peru .
This species can be found in shallow as well as in deeper water up to about 80 meters. The animals prefer coral reefs as a habitat, but can also live on the continental shelf, in the area of island bases, in lagoons and in deep water near the coast. Migratory populations off Florida and from the South China Sea migrate towards the poles in summer.
Way of life
The great hammerhead shark is a nomadic solitary animal who usually evades other reef sharks. If it encounters other sharks of its size, it shows a threatening behavior in which it lets its pectoral fins hang down and swims stiffly and jerkily. Young animals are preyed on by larger sharks such as bull sharks ( Carcharhinus leucas ), while larger and fully grown hammerheads have no natural predators other than humans.
Occasionally schools of pilot fish ( Naucrates ductor ) accompany larger sharks, including the great hammerhead shark. Jacks of the species Carangoides bartholomaei have been observed rubbing their flanks against the skin of great hammerhead sharks, probably to rub off skin parasites. As parasites of the great hammerhead, Copepoda are known as skin parasites, including the species Alebion carchariae , A. elegans , Nesippus orientalis , N. crypturus , Eudactylina pollex , Kroyeria gemursa and Nemesis atlantica .
nutrition
The great hammerhead shark is an active hunter with a wide range of foods. The prey consists of a variety of invertebrates such as crabs , squids and octopuses as well as numerous bony fish such as tarpon , sardines , catfish , frogfish , sea bream , sweetlips , jackfish , umberfish , grouper , flatfish , boxfish and porcupine fish . It also hunts smaller sharks and rays, including members of its own species. At Rangiroa Atoll , for example, it preyes on gray reef sharks ( Carcharhinus amblyrhynchos ), which are exhausted after mating.
However, the main food of the great hammerhead sharks is rays , especially stingrays . The stingrays' poison stings are regularly found stabbed in the mouth of hammerhead sharks and do not seem to bother them. An individual caught off the coast of Florida had 96 stingray stings stuck in and around its mouth. The hunt for these sharks takes place mainly at night or during twilight, where they swing their head in large arcs over the sea floor and pick up electrical signals with the help of their Lorenzini ampoules located on the cephalofoil . At the same time, the head acts as a wing , which allows the sharks to quickly turn around and catch a ray that has just been discovered. In addition, hammerhead sharks are often the first sharks off Florida to reach freshly laid shark bait, which indicates a particularly keen sense of smell.
Another function of the widened head of the great hammerhead was derived from an observation of a hammerhead shark that captured an American stingray ( Dasyatis americana ) in the Bahamas . The shark thrust the ray onto the ocean floor first with an attack from above, then held it there with its broad head while it turned and took a large bite into each side of the enlarged pectoral fins. As a result, the ray was immobilized and then lifted off with its mouth and dismantled with the shark's quick head movements. In another sighting, a great hammerhead shark was observed attacking a spotted eagle ray ( Aetobatus narinari ) in open water by biting a large piece from one of the two pectoral fins and then pressing it with its head on the ground, where it was also took the ray head first in its mouth. Based on these observations, it is assumed that hammerhead sharks always try to prevent them from escaping with the first bite when hunting rays - a strategy also used by the great white shark ( Carcharodon carcharias ) - and that the enlarged head is used to catch the rays Dodging prey and keeping them on the ground.
Reproduction and development
Unlike most shark species, where mating usually takes place on the bottom or at least close to the bottom, great hammerhead sharks have been observed to mate near the surface of the water. For example, copulation has been observed in the Bahamas , in which the two sharks rose while swimming around each other and mated when they reached the surface of the water.
Like all hammerhead sharks, this species is viviparous (ovoviviparous), whereby the unborn young sharks are fed via a yolk sac placenta . After the yolk supply has been used up by the young sharks , the yolk sac is transformed into a placenta that is analogous to that of mammals and ensures nutrition via the maternal bloodstream in the course of further development. The females carry young animals every two years and give birth to the young sharks after a gestation period of 11 months north of the equator in spring to summer and around Australia from December to January.
Hammerheads give birth to 6 to 42 young animals, with 20 to 40 representing the normal litter. The young sharks are 50 to 70 centimeters in length at birth. Sexual maturity is reached in the males between 2.3 and 2.7 meters and about 51 kilograms of body weight, while the females are sexually mature with 2.5 to 3.0 meters and 41 kilograms. The typical life span of the great hammerhead shark is 20 to 30 years, the largest female hammerhead shark in Boca Grande was estimated to be 40 to 50 years.
Systematics
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Relationships among hammerheads according to Cavalcanti 2007
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Relationships among hammerheads according to Lim et al. 2010
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The great hammerhead was first described in 1837 by the German naturalist Eduard Rüppell as Zygaena mokarran , but later assigned to the genus Sphyrna described by Constantine S. Rafinesque-Schmaltz , to which it still belongs today. For over 200 years, however, this species was carried under the scientific name Sphyrna tudes , introduced by Achille Valenciennes in 1822 , until Enrico Tortonese discovered in 1950 that the species described by Valenciennes and depicted there was the small-eyed hammerhead shark , which had this name accordingly today, while Sphyrna carries mokarran as the oldest synonym, the official scientific name of the great hammerhead. The lectotype of the species is a 2.5 meter long animal from the Red Sea .
A total of eight species are listed in the genus Sphyrna , which together with the genus Eusphyrna formed only by the winged hammerhead shark ( Eusphyra blochii ) form the family of hammerheads (Spyrnidae). On the basis of phylogenetic studies on the basis of morphological and molecular biological characteristics ( isoenzymes and mitochondrial DNA ) it could be shown that the great hammerhead is the sister species of the smooth hammerhead ( Sphyrna zygaena ) and the bow-forehead hammerhead ( Sphyrna lewini ) and forms a taxon with them , which is compared to the other species of the genus Spyrna . The position of the large species with particularly expansive cephalofoil could also be confirmed by further investigations in 2010, whereby the results regarding the relationships between the species differ slightly from the results of 2007: Here only the great hammerhead and the smooth hammerhead form a common taxon , while the bow-fronted hammerhead shark is assigned to the group of all other hammerhead sharks as a basal species.
The position of the large species between the wingtip hammerhead ( Eusphyrna blochii ) and the smaller hammerheads with a significantly narrower head suggests that within the hammerheads a large cephalofoil represents the original state and the narrower head is derived from it. Associated with this is a change in the function of the cephalofoil, which is reflected in the way of life of hammerhead sharks: While a wide cephalofoil occurs mainly in free-swimming species of the pelagic and here primarily acts as a hydrofoil, the smaller species with smaller cephalofoil live mainly near the ground as well as in muddy coastal areas and use the equipment of the sensory organs, in particular the Lorenzini ampoules, to localize prey. Regarding height, Lim et al. 2010 due to their kinship hypothesis and the distribution of the species that the most original hammerheads were large species, from which both the small wingtip hammerhead and the smaller Sphyrna species are derived.
Relationship with people
Due to its size and sharp teeth, the great hammerhead shark is capable of inflicting severe wounds on humans. Like all other large hammerheads, especially the bow-forehead hammerhead ( S. lewini ), it is classified as potentially dangerous to humans and is generally considered to be the most aggressive and therefore “dangerous” hammerhead. Divers, on the other hand, report that the great hammerhead shark generally reacts shyly or uninterestedly to divers. However, there have also been reports that great hammerhead sharks approach divers or even attack them when entering the water. As of 2009, the International Shark Attack File recorded 34 attacks by hammerheads of the genus Sphyrna on humans, 17 of them unprovoked and one fatal. Due to the difficulty of distinguishing the sharks, it is unclear how many of these attacks were by great hammerheads; reliable assignment was only made in the event of an unprovoked accident.
Use and catch
The great hammerhead shark is caught both commercially and by sport fishermen in the tropics , using long lines, bottom nets, hook lines and trawls. Although the meat itself is rarely used, the fins are of great value for the Asian market, where they are made into shark fin soup together with those of other large sharks . In addition, their skin is processed into shark leather , the liver oil obtained from their liver is used to obtain vitamins and their carcasses are used for fish meal production. In addition, the great hammerhead shark, like other sharks, is unintentionally caught and killed as bycatch , with up to 90% of the captured sharks dying in this way, for example in the Gulf of Mexico and the north-western Atlantic. Another man-made cause of death is shark nets , which are used to protect the beaches off the coasts of Australia and South Africa and in which many sharks become entangled.
Threat and protection
The great hammerhead shark is very sensitive to overfishing due to its small number of individuals and the long development period . A reliable recording of the stocks is also very difficult, since only a few fisheries authorities separate this species from other hammerheads in their statistics. The species is globally assessed by the International Union for Conservation of Nature and Natural Resources (IUCN) as "critically endangered". In the Gulf of Mexico in particular, where the shark is mainly caught as bycatch, populations have declined by around 50% since the 1990s. In the south-western Indian Ocean, stocks declined, mainly due to the large number of longline nets used to illegally catch hammerhead sharks and the great fiddle ray ( Rhynchobatus djiddensis ). The catch for the great hammerhead shark fell by 73% from 1973 to 2003, although it is unclear whether this is related to a local or general decline in the species. The shark is "critically endangered" along the African coasts and populations are expected to have decreased by 80% over the past 25 years. The West African Sub-Regional Fishing Commission (SRFC) has classified the great hammerhead shark as one of the four most threatened species in the region if fishing continues unobserved and unregulated. Before Northern Australia, the shark was classified as a “data deficient” due to the data situation, although the high risk was emphasized. The strong increase in illegal, unreported, and unregulated (IUU) fishing due to the sharp rise in prices for shark fins is of particular importance.
The Hammerhead is on Annex I for "Highly Migratory Species" of the Convention, listed by the United Nations (UN Convention on the Law of the Sea), but no measures have been developed so far by this law. In particular, the condemnation of shark finning (a fishing method in which sharks are only caught in order to cut off their fins) by the United States , Australia and the European Union as well as internationally valid fishing regulations such as the International Commission for the Conservation of Atlantic Tunas ( ICCAT) could reduce the fishing pressure on the great hammerhead shark in the future.
In March 2013, the CITES Species Conservation Conference in Bangkok decided to regulate the trade in great hammerhead sharks; the regulation came into force on September 14, 2014.
Individual evidence
- ↑ a b c d e f g h i j k l m n Cathleen Bester: Biological Profiles: Great Hammerhead . Florida Museum of Natural History Ichthyology Department . Accessed September 5, 2010.
- ↑ a b c d e f g h i j Leonard JV Compagno: Sharks of the World. An Annotated and Illustrated Catalog of Shark Species Known to Date. Part 1: Hexanchiformes to Lamniformes (= FAO Species Catalog. 4, 1 = FAO Fisheries Synopsis. 125, 4, 1). United Nations Development Program et al., Rome 1984, ISBN 92-5-101384-5 , pp. 548-549.
- ^ A b Leonard Compagno, Marc Dando, Sarah Fowler: Sharks of the World. 2005, pp. 324-325.
- ↑ a b c Record Hammerhead Pregnant With 55 Pups . In: Discovery News , July 1, 2006. Archived from the original on June 22, 2011. Retrieved October 23, 2010.
- ↑ a b c d e f Alessandro de Maddalena, Harald Bänsch: Sharks in the Mediterranean. 2005, pp. 216-218.
- ↑ a b c d e f Sphyrna mokarran in the IUCN Red List of Endangered Species 2010. Posted by: J. Denham u. a., 2007. Retrieved June 18, 2010.
- ^ R. Aidan Martin: A review of shark agonistic displays: comparison of display features and implications for shark – human interactions. In: Marine and Freshwater Behavior and Physiology. Volume 40, No. 1, 2007, ISSN 1023-6244 , pp. 3-34, doi: 10.1080 / 10236240601154872 .
- ^ A b Jeremy Stafford-Deitsch: Red Sea Sharks. Trident Press, London 1999, ISBN 1-900724-28-6 , pp. 92-93.
- ↑ a b Great Hammerhead . Elasmodiver.com . Retrieved on October 18, 2008.
- ↑ Julia Whitty: The Fragile Edge. Diving and other Adventures in the South Pacific. Houghton Mifflin, Boston MA et al. 2007, ISBN 978-0-618-19716-3 , p. 9.
- ↑ a b Rick Hammerschlag: Sandy Plains: Great Hammerhead Shark . ReefQuest Center for Shark Research . Retrieved October 23, 2010.
- ↑ Wesley R. Strong, Jr., Franklin F. Snelson, Jr., Samuel H. Gruber: Hammerhead Shark Predation on Stingrays: An Observation of Prey Handling by Sphyrna mokarran. In: Copeia . Volume 1990, No. 3, 1990, pp. 836-840, JSTOR 1446449 .
- ↑ Demian D. Chapman, Samuel H. Gruber: A further observation of the prey-handling behavior of the great hammerhead shark, Sphyrna mokarran: Predation upon the spotted eagle ray, Aetobatus narinari. In: Bulletin of Marine Science. Volume 70, No. 3, 2002, ISSN 0007-4977 , pp. 947-952.
- ↑ a b c Mauro José Cavalcanti: A Phylogenetic Supertree of the Hammerhead Sharks (Carcharhiniformes: Sphyrnidae) . In: Zoological Studies . tape 46 , no. 1 , 2007, p. 6–11 ( digitized version [PDF; 175 kB ]).
- ↑ a b c Douglas D. Lim, Philip Motta, Kyle Mara, Andrew P. Martin: Phylogeny of hammerhead sharks (Family Sphyrnidae) inferred from mitochondrial and nuclear genes . In: Molecular Phylogenetics and Evolution . tape 55 , no. 2 , 2010, ISSN 1055-7903 , p. 572-579 , doi : 10.1016 / j.ympev.2010.01.037 .
- ^ A b Jeremy Stafford-Deitsch: Sharks of Florida, the Bahamas, the Caribbean and the Gulf of Mexico. Trident Press, London 2000, ISBN 1-900724-45-6 , pp. 90-91.
- ^ Mark Thornley, Veda Dante: Surfing Australia. A guide to the best surfing down under. New edition. Periplus, Singapore 2003, ISBN 962-593-774-9 , p. 264.
- ^ ISAF Statistics on Attacking Species of Shark . International Shark Attack File, Florida Museum of Natural History, University of Florida. Retrieved on April 24, 2009.
- ↑ Cites resolution: Endangered shark and manta ray species should be protected Spiegel Online from March 11, 2013
- ↑ Species protection: These sharks must now be better protected by humans. The time of September 12, 2014
literature
- Leonard J. V. Compagno : Sharks of the world. An Annotated and Illustrated Catalog of Shark Species Known to Date. Part 2: Carcharhiniformes (= FAO Species Catalog. 4, 2 = FAO Fisheries Synopsis. 125, 4, 2). United Nations Development Program et al., Rome 1984, ISBN 92-5-101383-7 , pp. 545-554, ( Family Sphyrnidae Gill, 1872 , species portrait ).
- Leonard Compagno, Marc Dando, Sarah Fowler: Sharks of the World. Princeton University Press, Princeton NJ et al. 2005, ISBN 0-691-12072-2 , pp. 324-325.
- Alessandro de Maddalena, Harald Bänsch: Sharks in the Mediterranean. All 49 species (= Kosmos nature guide. ). Franckh-Kosmos, Stuttgart 2005, ISBN 3-440-10458-3 , pp. 216-218.
Web links
- Great hammerhead shark on Fishbase.org (English)
- Great hammerhead shark. In: Shark Foundation.
- Sphyrna mokarran in the endangered Red List species the IUCN 2010 Posted by: J. Denham u. a., 2007. Retrieved September 5, 2010.
- Hammerhead Sharks - Aliens from the Deep ( Memento from May 2, 2010 in the Internet Archive )
