The adipose fin, adipose, is a fin-like appendage ( fin ) of fleshy consistency between the dorsal and caudal fin in some orders of bony fish . It is mostly built up without real, bony fin rays , but is supported by actinotricha (fin rays in the embryonic phase).
The name adipose fin can be described as misleading. Histological investigations have shown that fat cells are present, but their proportion is so small that the fleshy shape rather than the presence of fat cells gave it its name. In tetras there may also be articulated fin rays, in the catfish genus Synodontis hard rays or sting rays. Its German as well as its scientific name (adipose) are derived from the mistaken assumption that it is made of fat. Adipose fins occur in nine orders of bony fish ( Percopsiformes , Myctophiformes , Aulopiformes , Stomiiformes , Salmoniformes , Osmeriformes , Characiformes , Siluriformes , Argentiniformes ), whereby their size is very variable and can sometimes exceed the area of the actual dorsal fin . A sexual dimorphism was found in the Pacific salmon ( Oncorynchus spp.), Here the adipose fin of the males is larger than that of the females in relation to the body length and can thus serve as a distinguishing feature between the sexes. Despite its appearance in different fish taxa, it has not developed multiple times, but is considered a plesiomorphism .
A comparative study from 2013 dealt with the ontogenetic (individual) development of the adipose fin. It turned out that this develops in two different ways in Natterer's Sägesalmler ( Pygocentrus nattereri ; Characiformes) and the Baltic Sea bill ( Coregonus maraena ; Salmoniformes). Named after the order from which the two examined species originate, a distinction is made here between the characiform development and the salmoniform development. In the salmoniform type, the adipose fin develops parallel to the other unpaired fins from the larval fin edge. In comparison, the development of the characiform type takes place later, here the adipose fin only forms after the other unpaired fins have formed from the apparently almost completely reduced larval fin fringe. This form of development thus also opposes the often made statement of a functionless remnant.
There is disagreement about the function of the adipose fin, recently (for salmonids ) hydrodynamic and flow sensory functions have been increasingly discussed.
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- Anke Bender, Timo Moritz: Developmental residue and developmental novelty - different modes of adipose-fin formation during ontogeny . In: Zoosystematics and Evolution . tape 89 , no. 2 , September 1, 2013, ISSN 1860-0743 , p. 209–214 , doi : 10.1002 / zoos.201300007 ( wiley.com [accessed April 20, 2017]).