Homocercal

Homocercal refers to a type of caudal fin in fishes in which the two lobes of the tail are approximately equal in size, producing a symmetrical outline about the vertical axis. It is characteristic of most living teleost fishes. This condition contrasts with heterocercal tails, found in sharks, rays, and many primitive ray-finned fishes, in which the upper and lower lobes are unequal and the vertebral column extends into the more prominent lobe.

In homocercal tails the caudal fin is supported by a central axis of skeletal elements and a fused hypural plate that anchors the fin rays. The rays are arranged so that thrust is distributed evenly, enabling efficient forward propulsion and agile steering with minimal yaw. The tail typically forms a bilobed, symmetric fin with no pronounced offset between dorsal and ventral margins.

Homocercal tails are widespread among teleosts and occur in both freshwater and marine species. The feature is viewed as a derived condition relative to heterocercal configurations and is associated with the diversification and high maneuverability that typifies many modern teleosts.

Examples include goldfish, salmon, perch, and tuna.

Related terms include heterocercal tail and caudal fin.