Like a fish out of water – Amphibious fish
Fish are found in bodies of water all over the world, from the tropics to the poles and from mountain lakes to deep-sea canyons. These habitats vary widely, but all have in common the fact that they have enough water for the fish to swim around and find food and mates in. It seems fundamental that fish must be in water to survive, but some of them must not have received the memo, as they have evolved to spend time on land, breathing the same air as people do.
Fish that have evolved to leave water and travel terrestrially must have adaptations that allow them to breathe, move on land, and see in air, all without drying out. They may breathe using their gills or other specialised breathing organs, such as labyrinth organs. Their movement on land may appear clumsy and uncalculated, but their pectoral fins, pelvic fins, and tails have adapted for more efficient movement and propulsion on land. They often have eyes that are set high on their head, structured in order to enhance vision both underwater and in air. Amphibious fish generally stay close to water in order to keep their skin moist and prevent desiccation, which is particularly important in the case of species who rely on wet skin to obtain oxygen. While these adaptations may be challenging, multiple groups of fish have independently evolved to have the necessary characteristics to thrive and live an amphibious life.
Fish on Land
In the case of mudskippers (subfamily Oxudercinae), their pectoral fins function differently on land than in the water. When submerged, the fins keep them stable as they propel themselves with their tail, but on land, their articulated pectoral fins function to thrust them across the ground, supported by radials and rays, with hinged joints where the fin meets the body, and part way down the fin where the radials meet the rays. Their jointed fins allow for extra manoeuvrability on the shores they climb out onto. When exposed to air, their gills begin to dry out, a stress they are able to overcome by utilising a cavity behind their ears which stores seawater that is reoxygenated by the movement of the eye and allows the gill flaps to stay moist enough to continue to function. Mudskippers are so well adapted to life on land that they may spend up to three quarters of their lives out of water.
The Pacific jumping blenny (Alticus arnoldorum), found in the south Pacific, and spends its entire adult life just above the water line, relying on the spray from splashing waves to keep in moist, hopping around between holes when provoked. As well as having enlarged pectoral fins to support movement and grip on land, they move and twist their tails to help them propel and manoeuvre themselves. These convulsions look unpredictable, but blennies have tails strong enough to leap a distance many times longer than their body length. Unlike most blennies, the Pacific jumping blenny respires through both the gills and the skin, allowing them to attain the necessary oxygen for as long as their skin is moist.
Video: The Pacific leaping blenny hops with a whip of its tail (Source: Hsieh, 2010)
Mudskippers, blennies, and other amphibious fish are rather unusual when compared with typical marine fish. The adaptations that make them so suited to life on land allow them to explore a different, extended range, allowing them to escape predation, exploit a different food source, and avoid stressful marine conditions such as rising temperatures, higher salinity, and pollutants. While these adaptations may seem confusing, or even useless, they have evolved separately multiple times, so they’re getting something right! Who knows, maybe someday there’ll be other fish that catch on?