Adipose fin
 Charlie the catfish - CIA video
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AquaPenguin - Festo, YouTube
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AquaRay - Festo, YouTube
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AquaJelly - Festo, YouTube
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AiraCuda - Festo, YouTube
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Fins are usually the most distinctive features of a fish. They are composed of bony spines or rays protruding from the body with skin covering them and joining them together, either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as seen in sharks. Apart from the tail or caudal fin, fish fins have no direct connection with the spine and are supported only by muscles. Their principal function is to help the fish swim. Fins located in different places on the fish serve different purposes such as moving forward, turning, keeping an upright position or stopping. Most fish use fins when swimming, flying fish use pectoral fins for gliding, and frogfish use them for crawling. Fins can also be used for other purposes; male sharks and mosquitofish use a modified fin to deliver sperm, thresher sharks use their caudal fin to stun prey, reef stonefish have spines in their dorsal fins that inject venom, anglerfish use the first spine of their dorsal fin like a fishing rod to lure prey, and triggerfish avoid predators by squeezing into coral crevices and using spines in their fins to lock themselves in place.
For every type of fin, there are a number of fish species in which this particular fin has been lost during evolution.
Dorsal fins are located on the back. A fish can have up to three dorsal fins. The dorsal fins serve to protect the fish against rolling, and assist it in sudden turns and stops.
Research published in 2014 indicates that the adipose fin has evolved repeatedly in separate lineages.
(A) - Heterocercal means the vertebrae extend into the upper lobe of the tail, making it longer (as in sharks).
(B) - Protocercal means the vertebrae extend to the tip of the tail and the tail is symmetrical but not expanded (as in amphioxus)
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Wikipedia