Ambulatory fish possess specialized adaptations that allow them to overcome the physical limitations of their aquatic environment and move purposefully on a solid substrate. The ability to navigate outside of water is a biological anomaly, revealing an evolutionary flexibility that pushes the boundaries of life as we generally understand it. This phenomenon is not limited to just emerging onto land, but also includes fish that employ a walking gait along the ocean floor, showcasing a wide range of non-standard locomotion.
Defining the Phenomenon
The term “walking fish” refers to a fish species capable of sustained, controlled locomotion across a solid surface, whether dry land or a submerged seabed. This movement is a distinct form of propulsion, which differentiates it from the uncontrolled flopping or wriggling seen when a typical fish is stranded outside of water. The key characteristic is the use of modified fins or body structures to generate forward thrust and support the body weight against gravity.
This unique capacity is generally associated with fish that inhabit unstable or marginal aquatic environments. These may include tidal zones, shallow temporary pools, or bodies of water that frequently experience low oxygen levels, known as hypoxia. The ability to move efficiently across the substrate, therefore, functions as a highly specialized survival mechanism. The nature of the movement varies widely, ranging from a tripod-like crutching gait to a serpentine slithering motion assisted by the fins.
Diverse Examples of Walking Fish
Among the most well-known examples are the mudskippers, a group of amphibious fish from the Indo-Pacific region that spend more time out of the water than in it. These fish use their muscular, elbow-like pectoral fins to “crutch” or “skip” across the mudflats of mangrove swamps. They are considered the most terrestrially adapted of all living fish, capable of climbing low-lying mangrove roots.
Another distinct example is the walking catfish (Clarias batrachus), a freshwater species native to Southeast Asia. This fish is known for using a combination of serpentine body undulation and stiff, serrated pectoral fin spines to move over moist ground between drying ponds. Their terrestrial excursions are generally short-distance migrations, driven by the need to find a new, more stable aquatic habitat.
Moving away from fully terrestrial examples, the epaulette shark (Hemiscyllium ocellatum) demonstrates a form of aquatic and intertidal walking. Found in shallow coral reefs, this small shark uses its highly muscular, paddle-shaped pectoral and pelvic fins to crawl or “stroll” along the seabed and across rocks in tide pools. This deliberate movement allows it to navigate complex, oxygen-poor environments where swimming is inefficient or impossible.
The Mechanics of Terrestrial Movement
The transition from swimming to walking requires significant anatomical modifications, particularly in the structure of the paired fins and the associated skeletal girdles. In species like the mudskipper, the pectoral fins have evolved thickened fin rays and highly complex muscle arrangements, which allow the fins to function as weight-bearing struts rather than just hydrofoils. This adaptation includes specialized structures, such as aponeuroses, that enable the extension of the pectoral fin to push off the substrate.
The skeletal support for these modified fins is also enhanced, seen in a reinforced shoulder girdle and a stronger articulation between the fin and the body. This provides the necessary stability and leverage to lift and propel the body mass against the force of gravity. Some benthic walkers, like the little skate, use their smaller pelvic fins with an alternating left-right motion, demonstrating a gait pattern similar to the stepping sequences of four-legged land vertebrates. Beyond the skeletal structure, many walking fish possess specialized respiratory organs to sustain them during their terrestrial activity. The walking catfish, for example, utilizes a suprabranchial air-breathing organ located above the gills, which allows it to extract oxygen directly from the air. Mudskippers absorb oxygen through their moist skin and the lining of their mouth and throat, as long as they remain damp.
Evolutionary Drivers and Ecological Role
The evolution of walking behavior in fish is a direct response to selective pressures within their unstable habitats. The most significant driver is often the avoidance of poor water quality, especially low dissolved oxygen levels that can occur in warm, stagnant, or drying pools. Moving onto land or to a new body of water represents an escape from potentially lethal aquatic conditions. Seeking new food sources and escaping aquatic predators are also strong motivating factors for terrestrial excursions. By temporarily leaving the water, a fish can access previously unavailable invertebrate prey or avoid larger fish predators. The ability to move between water bodies ensures survival and allows for the colonization of new, unexploited territories.
The existence of walking fish provides a living model for understanding the movement of vertebrates from water to land. The biomechanical changes seen in modern walking fish, such as reinforced pectoral skeletons and altered fin postures, mirror features observed in the fossil record of early tetrapod ancestors. This convergence suggests that the capacity for terrestrial movement is a highly successful survival strategy for fish living in marginal environments.