heterocercal

Heterocercal is a term used in ichthyology to describe a caudal fin in which the two lobes are unequal in size. In a heterocercal tail, the vertebral column extends into the larger (usually upper) lobe, producing an asymmetrical profile. This condition is contrasted with homocercal tails, in which the lobes are more or less equal and the vertebral column ends near the base of the tail.

Heterocercal tails are most characteristic of cartilaginous fishes, particularly sharks, where the tail contributes substantially to forward propulsion and maneuverability. The arrangement is also found in some primitive ray-finned fishes, including sturgeons and lungfishes, and in certain extinct groups. The exact shape can vary, with some species showing a pronounced upper lobe, others a more moderate asymmetry.

The asymmetrical tail allows the generation of thrust and, to some extent, lift, aiding rapid swimming and directional control. In evolutionary terms, heterocercal tails represent an early tail design in ray-finned fishes that was later modified into the more symmetrical (homocercal) tail seen in most derived teleosts.