Rinnalülid

Rinnalülid, also known as pectoral fins, are paired fins located on either side of a fish's body, typically just behind the gill covers. They are homologous to the forelimbs of tetrapods. In most fish, the pectoral fins are used for steering, braking, and maneuvering in the water. They can also be used for propulsion, especially in slower-moving fish or when making sudden turns.

The size, shape, and structure of pectoral fins vary greatly among different fish species, reflecting adaptations to their specific environments and lifestyles. For instance, fish that live near the seabed often have broad, flattened pectoral fins that help them rest on the bottom or "walk" along the substrate. Many fast-swimming fish, such as tuna and sharks, have long, stiff pectoral fins that provide stability and lift during high-speed movement. Some fish, like flying fish, have greatly enlarged pectoral fins that allow them to glide through the air after leaping from the water.

In some species, pectoral fins have been modified for purposes other than locomotion. For example, the pectoral fins of male seahorses are reduced and used for gripping the female during mating. In certain deep-sea anglerfish, the pectoral fins are small and serve little purpose. The skeletal structure supporting the pectoral fins consists of a pectoral girdle, which attaches to the skull or vertebral column, and fin rays that provide support and flexibility to the fin itself. The musculature controlling the pectoral fins is also highly developed, allowing for a wide range of movements.