analfin

The anal fin is a median unpaired fin located on the ventral side of many jawed fishes, positioned posterior to the urogenital opening and near the base of the tail. In ray-finned fishes it is supported by fin rays that may be soft or partly stiffened, and some groups also have a spine in the anterior portion. Its exact size and shape vary greatly across species.

Function and use in swimming are its primary roles. The anal fin contributes to stability, helping with braking, balance, and precise steering during locomotion. In some taxa the fin is relatively large and aids maneuvering in complex habitats, while in others it is small or reduced. In certain live-bearing fishes, such as some members of the family Poeciliidae, the anal fin is modified into a gonopodium, a tubular intromittent organ used to transfer sperm during reproduction.

Development and variation: The anal fin develops from an embryonic ventral fin bud. Across species, evolutionary pressures related to swimming mode, habitat, and mating strategies drive substantial variation in its size, shape, and sometimes the presence of a spine or other modifications.

Comparative notes: The anal fin is one of several paired or median fins that contribute to the stability and maneuverability of fish. Its morphology can reflect ecological niche, swimming performance, and reproductive biology. Related fins include the dorsal fin on the back and the pectoral fins near the chest.