Threefin

Threefin is a common name used for several small fishes that share a characteristic: three distinct dorsal fins. It is not a formal taxonomic group; rather, it is a vernacular label used in field guides and the aquarium trade to describe multiple species that have three dorsal fins. The best-known use is for the threefin gobies, gobies in the genus Tridentiger, which are widely referred to as threefins. These species are found in shallow coastal, estuarine, and sometimes brackish habitats in the Pacific region and adjacent waters. They are small, typically just a few centimeters in length, and display a variety of color patterns that aid camouflage among rocks, algae, and sandy substrates.

Morphology typically includes a dorsal region divided into three separate fins, a characteristic feature that distinguishes them from many other gobies that have two dorsal fins. The body is elongated, and the head is often pointed. Behaviorally, threefins are usually active, benthic foragers that feed on small invertebrates.

In the aquarium hobby, threefins are valued for their compact size, hardiness, and lively behavior, but they require appropriate salinity and habitat conditions to thrive. The term may also be applied to other small fishes in different regions that have three dorsal fins, but such usage is less standardized.

Conservation status varies by species and region; no broad threat status applies to the term as a whole.

See also: goby; threefin goby; Tridentiger.