The idea of a fish leaving the water to move on land seems contradictory, yet several species of amphibious fish routinely exhibit this capability. These creatures represent a remarkable evolutionary transition, possessing specialized features that allow them to survive and move outside of their aquatic environments. This terrestrial activity is a deliberate behavior driven by environmental pressures. Studying these “walking fish” offers insight into the biological adaptations required for the move from water to land.
Specialized Species That Walk
The most highly terrestrial species are the Mudskippers, a group of gobies found in the intertidal zones of the Indo-Pacific. These fish spend up to three-quarters of their lives out of the water, primarily using a unique mode of locomotion called “crutching.” They employ their muscular, jointed pectoral fins like forelimbs, moving them synchronously to propel their bodies across the mudflats.
Another notable species is the Climbing Perch (\(Anabas\ testudineus\)), native to fresh and brackish waters across Asia. This fish moves by a jerky, wriggling motion, using its extended gill plates and sharp opercular spines to anchor itself on the substrate. It pushes off with its fins and tail, allowing it to traverse ground for several hundred meters to find new aquatic habitats.
The Northern Snakehead (\(Channa\ argus\)) is an elongate, serpentine fish infamous for its ability to travel over land. When out of water, the snakehead uses a combination of body undulation and synchronized movements of its pectoral fins, known as axial-appendage locomotion. This method allows it to slither across moist terrain for several days, provided its skin remains damp.
Essential Physical Adaptations for Terrestrial Life
Survival outside of water requires profound modifications, particularly in the respiratory system. Many amphibious species, including the Climbing Perch, possess a suprabranchial organ, often called a labyrinth organ, located above the gills. This maze-like structure is highly vascularized, enabling the fish to extract oxygen directly from the air when it gulps at the surface.
Mudskippers rely on a dual breathing mechanism, utilizing cutaneous respiration—absorbing oxygen through their moist skin and the lining of their mouth and throat. They also retain a bubble of water within their enlarged gill chambers, which acts as a portable supply to keep the gills functional while exposed to air.
Locomotion on land is achieved by repurposing existing fins and body musculature for weight support and propulsion. Mudskippers have robust pectoral fins with modified internal skeletons that function much like limbs, supporting their body weight against gravity. Snakehead, in contrast, uses its entire body in a powerful, sinuous motion, with pectoral fins providing stability and additional thrust.
Survival Reasons for Leaving the Water
The primary motivation for these fish to leave the water is the avoidance of poor environmental conditions, particularly low oxygen levels. When water becomes stagnant or warm, the dissolved oxygen concentration drops, creating a hypoxic environment intolerable for most aquatic life. Air-breathing fish can simply emerge onto land to access the oxygen-rich atmosphere.
Movement over land also serves as a strategy for dispersal and resource acquisition. Fish may traverse short distances overland to escape drying pools or to reach a new body of water that offers abundant food sources or better foraging opportunities. Some species, such as Mudskippers, engage in terrestrial activity as part of their reproductive cycle, constructing burrows in the mudflats for nesting and mating.
The ability to move onto land can also offer a temporary refuge from aquatic predators. By escaping the water, fish can evade larger, fully aquatic hunters that cannot follow them onto the shore. This behavior is a short-term survival mechanism, though they must still contend with terrestrial predators like birds or snakes.
Environmental Impact and Invasive Potential
The same adaptations that allow these fish to survive in harsh native conditions make them highly effective invasive species when introduced elsewhere. Their ability to survive for extended periods without continuous water and to cross land barriers lets them colonize isolated ponds and lakes. This capability bypasses the natural geographic barriers that limit the spread of aquatic organisms.
The Northern Snakehead serves as a clear example of this invasive threat in North America. Its voracious appetite and predatory nature mean it competes directly with, and preys upon, native fish and amphibian populations, disrupting local aquatic food webs. Because of the ecological risk posed by these mobile invaders, the Northern Snakehead is federally listed as injurious wildlife in the United States. This designation restricts its importation and interstate transport, recognizing the severe environmental damage it can cause once established.