The structure and function of the different fins of the rainbow trout, Oncorhynchus mykissThe rainbow trout, belonging to the Salmonidae family, class Actinopterygii (ray-finned fish), inhabits cold streams , rivers and lakes of North America. Those that live in lakes have silver scales apart from those on their backs which are dark green. They are covered with dark spots that usually do not go beyond 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 around it. A pinkish-red band often runs along the sideline. The shape of the rainbow trout is a characteristic shape not uncommon from that of the salmon. (Guidekenairriver.com) Fish fins developed about 500 million years ago. A fin is a thin appendage attached to an organism's body that causes propulsion, stabilization, or the ability to steer and brake. Median caudal, dorsal and anal fins are possessed by most fish species. Rainbow trout have 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 located between the anus and the tail fin, on the ventral surface of the fish. In rainbow trout the anal fin is delimited by a black margin. This is one of the characteristics of the fish that helps identify it. Its function is to stabilize the fish while it swims. Since the dorsal fins are located above the roll axis of the rainbow trout, the movement of the dorsal fins would cause the fish to roll as it swims, were it not for the anal fin which counteracts this movement and stabilizes the fish. (Journal of Experimental Biology). The spine... in the center of the card... is in the water, as is a rapid change of pace and direction. This also serves to evade predation. The pectoral fins are located just behind the operculum: this is the flap of skin that covers the gill slit. As it swims at a constant speed, the rainbow trout's pectoral fins are flattened against its body. However they are then held at right angles to the body as the fish hovers in the water, turning at low speed, and used to suddenly stop the fish when traveling at high speeds. The rainbow trout is able to rotate these pectoral fins over a range of 30 degrees, allowing both hovering and braking. (Drucker EG, Lauder GV, 2003). It took evolution millions of years (500 million) for the rainbow trout to develop the fins it has today, each with a specific purpose involved in the fish's locomotion and stability..