Fins

Fins are paddle-like appendages used by aquatic vertebrates, especially fishes, to propel, steer, and stabilize in water. In fishes, fins are supported by bone or cartilage and reinforced by flexible fin rays covered by skin. The main fins are the caudal (tail) fin, the dorsal fins along the back, the anal fin on the underside near the rear, and the paired pectoral fins near the gills and pelvic fins on the underside. Many species also possess a small adipose fin between the dorsal and caudal fins.

Function and variation: The caudal fin provides most thrust during swimming, while dorsal and anal fins help maintain stability. Pectoral and pelvic fins enable precise steering, braking, and hovering. Fin shapes and placements vary widely, supporting diverse swimming modes from rapid bursts to slow cruising. Structural differences occur where some fishes have spines in the dorsal fin and others have soft-rayed fins; in many bony fishes the fin framework is formed by segmented rays, whereas cartilaginous fishes rely more on cartilage for support. Caudal fin morphology can be homocercal (symmetrical) or heterocercal (lobe asymmetry), influencing propulsion and steering.

Evolution and human context: Fins are evolutionarily related to limbs in terrestrial vertebrates, reflecting the transition from aquatic to terrestrial locomotion in vertebrate history. Humans interact with fins through fishing, aquaculture, and, in some species, commercial shark fin trade, which has prompted conservation and regulatory measures in many regions.

This overview covers the basic biology, variation, and relevance of fins in aquatic vertebrates.