Anal fin

Knifefish use their elongated anal fin for locomotion

The anal fin is found on the ventral side of fish, often, but not always, at the base of the anus.  Along with the dorsal fin, the main purpose of the anal fin is stabilize the fish and keep it from rolling in the water.  For knifefish (order Gymnotiformes), which have neither pelvic or dorsal fins, the anal fin has an additional purpose.  It is almost the entire length of their bodies.  Knifefish swim by rippling their anal fin while keeping the rest of their bodies rigid and straight.  As their primary form of locomotion, knifefish can swim backwards as easily as forward.

Snakeheads (family Channidae) also have an elongated anal fin.  Not native to the US, snakeheads were found in a pond in Maryland in 2002.  They are now permanently established in the Potomac River as an invasive species.  Snakeheads are food fish in their native range, so the introductions to the US were likely intentional.  Snakeheads are an ecological concern because they are top-level predators with no natural predators in US.  They also can survive on land for up to four days (as long as they are still wet) and can “walk” up to a quarter mile on wet land to find other habitable water bodies.

Snakeheads are invasive to the Potomac River