Some fish species possess the unique ability to survive and move across land, blurring the line between aquatic and terrestrial life. This phenomenon, known as amphibious behavior, has evolved independently multiple times across various fish groups, demonstrating convergent evolution. The capacity for terrestrial excursions is a direct result of specialized biological changes that allow these animals to overcome the challenges of air breathing and movement outside of a buoyant medium. These adaptations are driven by environmental pressures, enabling fish to exploit resources or escape unfavorable conditions in their native aquatic habitats. The study of these “walking fish” provides insight into the evolutionary transition from water to land that occurred millions of years ago.
Species That Master Terrestrial Movement
One of the most recognized groups of amphibious fish is the Mudskippers, belonging to the goby subfamily Oxudercinae, which inhabit the intertidal zones of mangrove forests and mudflats across the Indo-Pacific. These fish are highly terrestrial and can spend up to three-quarters of their lives out of the water, where they engage in feeding, social interaction, and burrow maintenance. Mudskippers utilize the exposed mudflats as an extension of their habitat, moving actively during low tide.
The Climbing Perch, or Anabas testudineus, is a freshwater fish native to South and Southeast Asia, known for its ability to travel overland between bodies of water. These fish are facultative air-breathers that typically leave the water when their shallow habitats become overcrowded or experience poor conditions. This overland movement allows them to colonize new aquatic environments.
Certain species of Snakeheads, such as the Northern Snakehead (Channa argus), also exhibit significant terrestrial movement capabilities. These predatory fish are obligate air-breathers that live in slow-moving or stagnant waters, tolerating extreme conditions like low oxygen levels. When their habitats become inhospitable, they can wriggle and propel themselves across moist ground for considerable distances to find better habitat.
Physiological Adaptations for Land Survival
Survival outside of water requires specialized respiratory modifications, as the delicate structure of typical fish gills collapses in air, preventing oxygen absorption. Many amphibious fish have developed accessory breathing organs that function much like a primitive lung, allowing them to extract oxygen directly from the atmosphere. The Climbing Perch possesses a suprabranchial organ, a labyrinth-like structure located above the gills that is heavily vascularized to facilitate air breathing when the fish is out of water or in hypoxic aquatic conditions.
In Mudskippers, the gill chambers are enlarged and can hold a volume of water that keeps the gills moist and functional for a time while on land. Their primary method of air breathing involves cutaneous respiration, absorbing oxygen through their highly vascularized skin and the lining of their mouth and throat. This reliance on the skin requires the fish to keep their bodies damp, limiting them to humid environments.
Another challenge is preventing desiccation, or drying out, which is achieved mainly through behavioral responses and specialized skin. These fish minimize water loss by seeking out moist substrates, burrowing into wet mud, or maintaining a coating of mucus. Managing nitrogenous waste is different in air; while aquatic fish excrete ammonia, some terrestrial fish can store nitrogen temporarily or convert it to less toxic urea, reducing the need for constant water flushing.
Specialized Locomotion Techniques
Moving on land requires overcoming gravity without the buoyancy of water, leading to the evolution of unique physical mechanics for terrestrial locomotion. Mudskippers employ a distinct movement called “crutching,” where their robust, muscular pectoral fins function almost like a pair of forelegs. They synchronously plant these fins into the substrate, vaulting the front half of their body forward before landing on their pelvic fins.
The Climbing Perch utilizes a different strategy that incorporates both fins and other body structures. When moving overland, they use their gill covers, which are reinforced with small spines, as an anchored support pole. They then use the powerful axial muscles of their body to generate propulsive force through lateral undulation, pushing against the ground to move forward in a sustained manner.
Eel-like fish and the younger, smaller Snakeheads often rely on undulating or jumping movements. The Northern Snakehead combines cyclic oscillations of its body with near-simultaneous movements of its pectoral fins for an axial-appendage-based crawl. Smaller individuals frequently use a ballistic “tail-flip” jump, where they bend their body into a C-shape and rapidly straighten, pushing off the ground with the tail to launch themselves forward multiple body lengths.
Ecological Reasons for Leaving Water
The primary driver for fish to leave their aquatic environment is often to escape poor water quality, specifically low oxygen levels (hypoxia). In stagnant ponds, swamps, and tidal pools, decomposition and high temperatures can rapidly deplete dissolved oxygen, forcing air-breathing species onto land where oxygen is plentiful. This behavioral choice acts as a survival mechanism when the water becomes inhospitable.
Terrestrial excursions are also motivated by the search for new resources, as the land surface can offer untapped food sources like insects, worms, and vegetation. By moving out of the water, these fish temporarily avoid aquatic predators. This trade-off, however, exposes them to new threats from aerial or terrestrial hunters.
The ability to move overland is also an important component of dispersal and migration, particularly in freshwater species. During periods of drought or when a habitat is drying up, fish like the Climbing Perch use their terrestrial mobility to migrate to more stable or distant bodies of water. This capacity to navigate between fragmented habitats enhances survival and allows for the colonization of new territories.