Structure and Function of Rainbow Trout Fins

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The structure and function of the different fins of the Rainbow Trout, Oncorhynchus mykiss

The Rainbow trout, belonging to the family Salmonidae, Class Actinopterygii (ray finned fish), inhabitscold streams, rivers and lakes in North America. Ones living in lakes have silver scales apart from those on their back which are dark green. They are covered in dark spots that usually go no further than the lateral line. The lateral line is a line of sense organs found on the side of the Rainbow trout. It is used to detect vibrations (and therefore movement) in the water surrounding it. A pinkish red band often runs the length of the lateral line. The shape of the Rainbow trout is a characteristic shape not uncommon from that of salmon. (Guidekenairriver.com)
Fish fins developed around 500 million years ago. A fin is a thin appendage attached to the body of an organism that causes either propulsion, stabilization, or the ability to steer and brake. The median caudal, dorsal, and anal fins are possessed by most fish species. The rainbow trout has six different types of fins- an anal fin, an adipose fin, a dorsal fin, a caudal fin, a pair of pelvic fins, and a pair of pectoral fins.
The anal fin is situated between the anus and the tail fin, on the ventral surface of the fish. In Rainbow trout, the anal fin is bordered by a black margin. This is one of the fish’s features which help to identify it. Its function is to stabilise the fish whilst it’s swimming. Due to the dorsal fins being located above the rolling axis of the Rainbow trout, the movement of the dorsal fins would cause the fish to roll whilst swimming if not for the anal fin which counteracts this movement and stabilises the fish. (Journal of experimental Biology).
The dorsa...

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...s in the water, as well as quick change in pace and direction. This again is to evade predation.
The pectoral fins are located just behind the operculum- this is the flap of skin which covers the gill slit. Whilst swimming at a constant steady speed, the pectoral fins of the Rainbow trout are flattened against its body. However they are then held out at right angles to the body when the fish is hovering in the water, turning at slow speeds, and used to suddenly stop the fish when travelling at fast speeds. The rainbow trout is able to rotate these pectoral fins over a 30 degree range, allowing for both sculling when hovering, and braking. (Drucker EG, Lauder GV, 2003).
Evolution has taken millions of years (500 million) for the Rainbow trout to have developed the fins it has today, each with a specific purpose involved in the locomotion and stability of the fish.

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