Risso's smooth head
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Risso's smooth head ( Alepocephalus rostratus ) |
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| Alepocephalus rostratus | ||||||||||||
| Risso , 1820 |
Risso's bald head , Alepocephalus rostratus , was one of the first deep-sea fish to be discovered almost 200 years ago (through Antoine Risso in Nice ). However, its appearance is not very spectacular for a deep-sea fish; the most striking is the black coloring. The fact that the head is scaly (what the scientific name says: (Greek :) a- "not", lepos "scale", kephalé "head") is also found in many other fish of different lifestyles. Alepocephalus has no photophores (light organs) and no swim bladder.
description
Risso's smooth head is hardly longer than 50 cm, but grows very slowly due to the low food supply in the deep zone - it is then already over 20 years old. The large eyes are striking, but also not characteristic of its biotope without daylight. The rear nostrils are quite large. The color is a deep dark brown, the skin mucus shimmers blue. The smooth, thin scales fall out easily; approx. 50 keeled sideline scales.
The upper jaw is usually somewhat saddled (hence the species name rostratus , "with a beak"); the very small mouth is finely toothed (in the upper jaw only the premaxillary and palatine, the maxillary and vomer are toothless, the front teeth of the dental are somewhat larger). Two small supramaxillaries attach to the maxillary. There is a small button on the lower jaw (sensory organ?). Usually 6 (rarely 7) delicate Branchiostegal rays (radii; see Branchiostegal apparatus ). The gill cavity is widened dorsally at the back to accommodate the paired epibranchial or cruminal organ (from Latin crumina "pouch". The curved cavity has a muscular shell in which elongated branchiospins are stuck: food can be ground a little here to facilitate digestion) . The five crevices of the gill trap are also secured by long branch spins.
Carl Gegenbaur described the skull in detail in 1878. All skin bones are tender, the neurocranium (brain capsule) is only slightly ossified, so it mainly remains cartilaginous. The mouth, throat and peritoneum are blackish. The stomach is U-shaped, the intestine about the length of the body. There are 20 to 28 pyloric tubes (other numbers come from animals that now represent other species in the genus). According to Valenciennes (1846), the smooth head has a spiral valve (see spiral intestine) in the rectum - but this is very unlikely. The testicle is unpaired, but bilobed - a rare construction variant in vertebrates.
Fin formula : D 16-20, A 17-21, P 10-11, V 9, C 8-9 / 21 / 8-9 (i.e. dorsally and ventrally the C has anterior rays for stiffening). P and V are very small; A small area of skin above the P is unscaled. The position of D and A is reminiscent of that of the pike - but this does not necessarily speak for "shock robbers".
Ecology and diffusion
The bald head (bald head, slick head, smooth head) lives on continental and island slopes at a depth of 950–2200 m (over soft ground; the oldest individuals at the deepest) of various creatures (hydroid polyps, rib jellyfish , snails to decapods , echinoderms, Tunicates (esp. Salps ), small fish (rare), from plankton especially Euphausiacea , Amphipoda and Mysidacea - whatever it finds and can cope with, but plankton and necton predominate over benthos; with benthos sometimes a lot of sediment gets into the intestine). It occurs in the eastern Atlantic between Iceland, Ireland, Madeira and Angola and Namibia, as well as in the western part of the Mediterranean (up to the Ionian Sea). At night it sometimes rises to a depth of approx. 300 m (warmer water is avoided). Locally, it can dominate fish biocenoses (e.g. the Catalan Sea below 1000 m). It has an extended (or none at all!) Breeding season and large bathypelagic eggs, from which no “larvae”, but rather young fish hatch. It becomes sexually mature with a length of approx. 25 cm (4–6 years old: females larger than males).
relationship
Right from the start, you puzzled over the closer integration into the system. The difficulty lies in the fact that the fish shows almost only ancient (sym plesiomorphic ) features, e.g. B. the epibranchial organ, which was widespread in "primitive" Teleostei who were not pronounced fish-eaters (today still in Heterotis , Chanos , Clupea , Argentina , Gonorynchus ). William Gosline (1969), after carefully weighing the arguments, classifies him as one of the Osmeriformes ; Since then, however, the Argentiniformes have been separated from them and the Alepocephaloidea added to them (epibranchial organ, no adipose fin) - cf. see R. Diogo et al. 2008. Lavoué, S. et al. (2007), on the other hand, count the Alepocephaliformes among the Otocephala , thus returning to the old "herring relationship" (the connection between the swim bladder and the middle ear would therefore have disappeared through the reduction of the former).
fishing
After the end of the cod fishery (around 1990) in the North Atlantic, a replacement was sought in the exploitation of species living deeper in the sea (so-called deep-water fish at a depth of 400–1000 m, which can be obtained with trawls). B. Coryphaenoides sp., Hoplostethus atlanticus . (Their yield cannot, of course, come close to that of Gadids, the destruction of biotopes on coral reefs, among other things, is often very large.) Alepocephalus ( bairdi and rostratus ) are also abundant, but the meat is not marketable because of its tasteless wateriness (Gordon 2001) - the fish have to be thrown (dead) back into the sea ( EU ban on bycatch ). But even if they were kept as a source of protein, they could not be managed in a “sustainable” manner in this way (neither for fishermen nor for nature).
literature
- M. Carrasson and J. Matallanas (1998): Feeding habits of Alepocephalus rostratus (Pisces: Alepocephalidae) in the Western Mediterranean Sea.- J. Mar. Biol. Assoc. UK 78: 1295-1306.
- John DM Gordon (2001): Deep water demersal fisheries - Fisheries reports, Joint Nature Conservation Committee, UK. http://www.jncc.gov.uk/default.aspx?page=2525
- William Alonzo Gosline (1969): The morphology and systematic position of the alepocephalid fishes.- Bull. Br. Mus. Nat. Hist., Zool. 18: 185-218. http://www.biodiversitylibrary.org/name/Alepocephalus_rostratus#247
- B. Morales-Nin, E. Massutí, and C. Stefanescu (1996): Distribution and biology of Alepocephalus rostratus from the Mediterranean Sea.- J. fish biol. 48: 1097-1112.
Individual evidence
- ↑ Carl Gegenbaur (1878): About the head skeleton of Alepocephalus rostratus Risso.- Morph. Jb., Suppl. 4 (Festschr. For the 50th anniversary of CTE von Siebold): 1-42.
- ↑ Rui Diogo, Ignacio Doadrio and Pierre Vandewalle (2008): Teleostean phylogeny based on osteological and myological characters.- Internatl. J. Morphol. 26: 463-522.
Web links
- Risso's bald head on Fishbase.org (English)