finless

Finless is an adjective used in biology to describe organisms that lack fins or have fins that are greatly reduced in size or function. The term is most often applied to aquatic vertebrates, where fins play a key role in propulsion, steering, and stabilization. The word derives from the ordinary English word “fin” combined with the suffix “-less,” indicating absence.

A notable example is the finless porpoise, a common name for certain species within the genus Neophocaena. Finless porpoises are characterized by the absence or marked reduction of the dorsal fin, and in some individuals the dorsal fin may be entirely missing. They inhabit coastal and estuarine waters in parts of Asia and are adapted to life in shallow, cluttered environments where reduced or absent dorsal fins can be advantageous for maneuverability.

In broader terms, finlessness can occur through evolutionary processes, developmental anomalies, or pathology. Some lineages may evolve reduced fins as an adaptation to specific ecological niches, while others may exhibit fin loss as a result of genetic mutations or environmental stress. The concept is also discussed in paleontology and comparative anatomy when describing fossil forms or living species in which fins are reduced or modified relative to close relatives.

Finlessness is a descriptive term rather than a taxonomic group. It helps scientists discuss morphological variation among aquatic organisms and to compare evolutionary strategies related to locomotion and habitat use. See also dorsal fin, fin reduction, and limb evolution for related topics.