Fish are broadly defined as aquatic vertebrates that possess gills and, typically, fins, but this definition encompasses a vast range of evolutionary adaptations. While the majority rely on constant movement for survival, a significant number of species have evolved specialized forms of locomotion or a stationary existence. This means the assumption that all fish swim by rapidly propelling themselves through the water is incorrect.
The Mechanics of Standard Aquatic Locomotion
Traditional fish locomotion, or swimming, is primarily achieved through a systematic, wave-like bending of the body and tail, a movement classified as body and caudal fin (BCF) propulsion. The caudal fin, or tail fin, is the main engine, providing the raw force needed to overcome water resistance and move through the fluid medium. The other fins, such as the paired pectoral and pelvic fins, function mainly as stabilizers and steering mechanisms. Pectoral fins, located on the sides of the body, are used for precise maneuvering, braking, and maintaining balance. The dorsal and anal fins act like the keel of a boat, preventing the fish from rolling uncontrollably.
Fish That Walk, Crawl, or Stand Still
Evolutionary pressures have led some fish to adapt their fins from aquatic rudders into terrestrial or benthic appendages. The mudskipper, a semi-aquatic fish from the Indo-Pacific, spends significant time out of water on tidal flats. It uses its muscular, jointed pectoral fins like crutches, performing a distinctive “skip” or “crutching” walk across the mud.
Other species have modified their fins for movement along the seabed. Handfish, found in the waters around Australia, possess pectoral and pelvic fins that resemble small hands, which they use to “walk” slowly across the ocean floor. Similarly, the batfish family, such as the tripodfish, uses elongated, stilt-like fins—specifically its pelvic fins and the lower rays of its caudal fin—to prop its body up off the soft sediment. This standing posture allows the fish to wait for prey or maintain position without actively swimming. Even the epaulette shark uses its muscular pectoral and pelvic fins to “walk” across shallow coral and rock flats when the tide is low.
Sessile and Burrowing Fish Species
A different set of adaptations is seen in fish that have traded continuous movement for a largely stationary or subterranean existence. Ambush predators like the deep-sea anglerfish are masters of standing still, often resting directly on the substrate. While they can swim, their primary feeding strategy involves remaining motionless and using a bioluminescent lure, or esca, to attract prey toward their large mouths.
Burrowing species also minimize swimming, as their habitat is the sediment itself. Garden eels, for instance, live in colonies where each individual anchors the tip of its tail in the sand, emerging only to feed on plankton drifting by. While they possess the ability to swim, they spend the vast majority of their lives anchored, swaying in the current. Hagfish, ancient jawless fish, actively burrow into the mud using a two-phase process: a “thrash” of vigorous sinusoidal movement to penetrate the sediment, followed by a “wriggle” powered by an internal concertina strategy to move through it.