Quick Definition
A lateral or ventral ridge on the caudal peduncle of fast-swimming fishes that stabilises the tail beat and reduces drag.
📖 Full Definition
In fish morphology, a keel refers to a sharp or rounded ridge. A caudal keel is a lateral compression of the caudal peduncle in high-speed swimmers such as tunas, swordfish, and some sharks. It stiffens the tail base, decouples body flexion from tail movement, and reduces hydrodynamic drag. Some herring-like fishes (clupeoids) have ventral keeled scutes along the belly which may aid in counterillumination or mechanical protection.
💡 Examples & Context
All five species of tuna in the genus Thunnus have two small horizontal keels at the base of the tail and a larger median keel.