Most fish are entirely aquatic, relying on water for survival. However, a minority of fish species has evolved the ability to venture onto land, sometimes for extended periods. This adaptation allows them to exploit terrestrial environments.
Unique Adaptations for Terrestrial Survival
Fish that survive on land possess adaptations to overcome non-aquatic challenges. Breathing air is a primary concern, as gills collapse and dry out in air, preventing efficient gas exchange. Many terrestrial fish employ cutaneous respiration, absorbing oxygen directly through their skin, mouth lining, or throat. This process requires their skin to remain moist.
Some species have evolved specialized respiratory organs beyond their gills. Labyrinth organs, for example, are highly vascularized structures in the head that allow direct air breathing, as seen in climbing perch. Modified swim bladders can also function like primitive lungs, enabling certain fish, such as lungfish, to gulp air at the surface and extract oxygen from it. These internal air-breathing organs allow for sustained periods out of water.
Terrestrial locomotion is another adaptation, as typical fish fins are not suited for movement on solid ground. Amphibious fish often use their pectoral fins, sometimes modified to be more robust and jointed, to “walk” or “skip” across surfaces. Other methods include serpentine movements of their bodies, lateral undulation, or using their caudal peduncle in conjunction with fins to propel themselves.
Preventing desiccation, or drying out, is important for survival on land. Many land-dwelling fish secrete a protective layer of mucus over their bodies, which helps reduce water loss through evaporation. Some species burrow into mud during dry seasons, creating a humid microenvironment where they can aestivate until water returns. These strategies allow fish to temporarily inhabit terrestrial niches.
Notable Land-Dwelling Fish Species
Several fish species demonstrate adaptations for terrestrial survival. Mudskippers, gobies found in mangrove swamps and mudflats, are recognized amphibious fish. They use their strong, muscular pectoral fins to walk, skip, and even climb on land, and they can absorb oxygen through their skin and the lining of their mouths and throats. Their eyes protrude from the top of their heads, allowing them to see effectively in air.
Walking Catfish, prevalent in tropical regions, can “walk” short distances over land, particularly between bodies of water. They possess specialized gill structures that resist collapse and often a modified respiratory tree-like organ, enabling them to breathe atmospheric oxygen. This allows them to survive out of water for extended periods.
Lungfish are an ancient group with a modified swim bladder that functions as a lung, allowing them to breathe air. Australian, African, and South American lungfish can survive droughts by burrowing into mud and forming a cocoon, entering a state of aestivation for months or even years until water returns. Some species are obligate air-breathers.
The Climbing Perch, found in Africa and Southeast Asia, utilizes a labyrinth organ for air breathing, which enables it to survive outside water. These fish move between ponds and other water bodies, using their pectoral fins and gill covers to propel themselves across land. Their ability to traverse terrestrial environments allows them to seek out new, more favorable aquatic habitats.
Ecological Drivers for Leaving Water
Fish venture onto land due to a variety of environmental pressures and opportunities. A primary driver is escaping adverse aquatic conditions, such as dangerously low oxygen levels, which often occur in warm, stagnant water bodies. Overcrowding within aquatic habitats can also prompt fish to seek new territories, reducing competition for resources. Pollution in their water source is another factor that can compel some species to temporarily leave the water.
Seeking new food sources is a significant advantage of terrestrial excursions. Land-dwelling fish can exploit insects, small crustaceans, or other invertebrates found on land that are unavailable in their aquatic environment. This expansion of their diet opens up new ecological niches and reduces competition with purely aquatic species.
Evading aquatic predators is another compelling reason for fish to leave the water. By moving onto land, even for short periods, fish can find temporary refuge from larger fish or other aquatic animals that cannot follow them. This behavioral adaptation provides a survival advantage in predator-rich waters.
The search for new habitats or water bodies is particularly relevant in environments prone to seasonal drying. When ponds or rivers begin to shrink, fish may migrate overland to find more permanent water sources. This overland movement can also be driven by the need to find suitable breeding grounds or to reproduce, with some species even laying eggs on land.
Distribution and Habitats
Land-dwelling fish are predominantly found in tropical and subtropical regions across the globe. These climates provide conditions conducive to such adaptations, including warm temperatures, seasonal droughts, and abundant wetland areas. The dynamic nature of these environments, with fluctuating water levels, favors species capable of surviving both in and out of water.
Specific habitats where these amphibious fish thrive include mangrove swamps, vast mudflats, and floodplains. These environments are characterized by their intertidal nature or by periods of inundation and drying, making them challenging for purely aquatic life. Temporary ponds and stagnant freshwater bodies also frequently host land-dwelling fish, especially during periods of low water.
Intertidal zones, the areas between high and low tide, are particularly common habitats for many amphibious fish. Species here must contend with regular exposure to air as the tide recedes. The ability to breathe air and move on land allows them to exploit food resources and escape predators in these transitional environments, providing a significant survival advantage.