Fin and flipper locomotion

Fin and flipper locomotion occurs mostly in aquatic locomotion, (and rarely in terrestrial locomotion) of nektonic animals with paired appendages, more specifically aquatic vertebrates (fish and marine tetrapods) and most coleoids. These appendages have a large wetted area that interacts with water like paddles and/or hydrofoils to propel, steer and maintain postural control/balance of the body during locomotion. Some semi-aquatic animals with land-dwelling limbs also have webbed feet that are analogous to fins and flippers.

While most aquatic animals utilize body-caudal, flagellal and/or jet propulsion for locomotion in water, some rely on flapping movements of paired appendages — particularly the pair towards the front of the body — to generate propulsion and lift. Ray-finned fish have thin, folding fan-like pectoral fins that have a high thrust-to-weight ratio and can readily change shape and surface area, while batomorph cartilaginous fish (rays and skates) and aquatic tetrapods such as cetaceans, sirenians, pinnipeds and sea turtles have more bulky and rigid fins, flippers and flukes that they use for propulsion.

Some benthic fish such as handfish, frogfish, flying gurnard, epaulette shark, lungfish and likely the prehistoric tetrapodomorph sarcopterygians (ancestral stem tetrapods) use their paired fins (especially the pelvic fins) for crawling at the seafloor and streambeds, earning them a reputation as "walking fish". Many amphibious fish such as mudskippers, leaping blenny and walking catfish can also ambulate out of water by limb-like movements of their paired fins aided by body-tail propulsion, while flippers are routinely also used for terrestrial locomotion by sea turtles, pinnipeds and penguins, who rely on marine habitats for foraging food but have to return to dry land for sleeping and nesting or for reproduction and nurturing hatchlings/juveniles.