Six-gill sawsharks
| Six-gill sawsharks | ||||||||||||
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Head of Warren's six- gill sawshark ( Pliotrema warenni ), drawing from the first description, view from the side and from below |
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| Regan , 1906 |
The six-gill saw sharks ( Pliotrema ) are a type of shark from the family of saw sharks (Pristiophoridae). Like their relatives, the shark species is characterized by a long, saw-like snout. Six-gill saw sharks live off the coast of East and South Africa, near Madagascar and the Mascarene Plateau .
features
Six-gill saw sharks reach a maximum length of 98 cm or 143 cm; possibly Pliotrema warreni , the largest species, becomes a maximum of 170 cm long. The body is long, cylindrical and slim. The head is flattened and has a greatly elongated and flattened snout with the saw-like rostrum typical of saw sharks . The rostrum has a distinctive pair of barbels in front of the nostrils, serrated teeth that are close together on the side edge of the "saw", and more or less large spines on the underside. In juvenile fish, large and small rostral teeth alternate; the number of small teeth in the spaces between the large ones increases in the course of ontogenesis . The individual teeth of the saw do not sit in tooth sockets , but are only attached to the skin. Rostral teeth and spines are folded back in embryos and covered by skin. In contrast to the other saw sharks, the six-gill saw sharks also have smaller barbels at the base of the larger saw teeth and the main barbels are closer to the mouth than in the species of the genus Pristiophorus . The spines on the underside become smaller or lost with age. In some species they are black.
The eyes are on the side of the head, behind them the large suction holes . A nictitating membrane is missing. The jaw teeth, arranged in several rows, are small, have conical tips and broad bases. The species of the genus have six gill slits , while the saw shark species of the genus Pristiophorus have only five gill slits. Like these, six-gill sawsharks have two dorsal fins without a thorn. The beginning of the first dorsal fin is above the free tip of the pectoral fins. An anal fin is missing. The tail stalk has clear keels, the tail consists only of a large upper lobe while the lower lobe is absent in all sawsharks. The hull is covered with flat, three-pointed placoid scales that overlap like roof tiles.
On the back the fish are monochrome, light or dark brown, sometimes with more or less pronounced yellow longitudinal stripes, on the belly they are whitish, sometimes with some dark markings. The fins, especially the caudal fin and the pectoral fins, sometimes have a whitish edge. The teeth of the rostrum are dark.
Way of life
Six-gill saw sharks are bottom sharks that are found mainly in shallow water. They predatorily feed on various fish , and they also hunt crustaceans , snails and octopuses . The long rostrum is used to search for food, which is equipped with barbels and other sensory organs and can therefore perceive movements and probably also electrical fields and chemical impressions in the sea floor. The prey is tracked down with a saw, dug up and probably also partly killed. In addition, she also uses the saw sharks against attackers (especially large sharks such as the tiger shark ) as well as against conspecifics.
The sharks are viviparous and do not form a placenta (aplacentally viviparous ). The females get between five and seven young animals with a length of about 35 cm in one litter. However, they produce seven to 17 eggs per litter, which develop into embryos with a yolk sac , but cannot fully hold until birth. The birth probably takes place in shallow coastal areas in the tidal range.
distribution
Six-gill saw sharks are found in the southeast Atlantic and southwest Indian Ocean in very demarcated areas off the coasts of South Africa , Mozambique , near Zanzibar , Madagascar and above the Mascarene Plateau north of Mauritius . Their habitat is in the area of the continental shelf near the sea floor in water depths of around 25 to 430 meters, whereby the adult animals often live in deeper regions than the young animals.
Systematics
The genus of Sechskiemer-sawshark ( Pliotrema ) was in 1906 with Warren Sixgill Sawshark ( Pliotrema warenni ) by British ichthyologist Charles Tate Regan first described scientifically and was long considered monotypic . In 2020 the genus was revised and two more species were described, so that there are three valid species today.
- Anna's six- gill saw shark ( Pliotrema annae Weigmann, Gon, Leeney & Temple , 2020)
- Kaja's six-gill saw shark ( Pliotrema kajae Weigmann, Gon, Leeney & Temple , 2020)
- Warren's six-gill sawshark ( Pliotrema warreni Regan , 1906)
Individual evidence
- ↑ a b c d e Simon Weigmann, Ofer Gon, Ruth H. Leeney, Ellen Barrowclift, Per Berggren, Narriman Jiddawi, Andrew J. Temple: Revision of the sixgill sawsharks, genus Pliotrema (Chondrichthyes, Pristiophoriformes), with descriptions of two new species and a redescription of P. warreni Regan. PLOS ONE , March 2020, doi: 10.1371 / journal.pone.0228791
- ^ A b Leonard Compagno , Marc Dando, Sarah Fowler: Sharks of the World. Princeton Field Guides, Princeton University Press , Princeton and Oxford 2005; Pp. 131-132, ISBN 978-0-691-12072-0 .
- ↑ Chales Tate Regan: Descriptions of new or little known fishes from the coast of Natal. Annals of the Natal Government Museum. Vol. 1, No. 1, pp. 1-6 ( BHL ).