The fish that walk on land - Noah R. Bressman
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While many different fishes emerge onto land, they do so for a variety of reasons. The mangrove rivulus sometimes goes onto land to cool off through evaporative cooling. As water evaporates off their body, heat energy is lost through their skin, just like how sweat cools people off in hot weather. Northern snakeheads do not cool off on land, but they will go on land to get away from acidic water. While amphibious fishes may share similarities, each species is uniqule different.
While lungfish breathe air through their lungs and armored catfish breathe air through their gut, other amphibious fishes have different methods of extracting oxygen from the air. Some fish, like the arapaima, have gas bladders that are surrounded by blood vessels to enable air-breathing. Others, like walking catfish and snakeheads, have specialized air-breathing organs above their gills that are lined with blood vessels. Many small amphibious fishes, like killifish and intertidal sculpins, are able to get enough oxygen from the air via diffusion through their thin, permeable skin.
Because of differences in air breathing abilities and other characteristics, different species of amphibious fishes can survive out of water for different amounts of time. While the mangrove rivulus can survive emerged in moist logs for up to two months at a time, species like snakeheads and walking catfish may only be able to survive about one day at a time. Fish without significant air breathing capabilities like mosquitofish may be lucky to survive an hour out of the water, but they likely are not traveling across land, staying emerged for just a few seconds at a time. At the most extreme end are lungfish, which can survive for a few years out of water. They achieve this by hibernating in a mucus cocoon that keeps them moist in a phenomenon known as aestivation.
Terrestrial locomotion using suction cups is not limited to just gobies; lampreys and suckermouth catfishes also use oral suction cups to move around on land. However, most fish use other terrestrial locomotor behaviors. Some fish use axial locomotion which involves only movements of the body. This can include snake-like slithering in eels or jumping by killifish. Other fish use just their appendages to move overland, but this is restricted to the crutching behavior of mudskippers, which “crutch” along the mud using their modified pectoral fins. Axial-appendage-based terrestrial locomotion is more common, like the “walking” behavior of walking catfish or the crawling behavior of snakeheads, and involves coordinated movements of the body and fins.
Vision is the likely the most widely used sense by fish out of water because it requires few adaptations to function in air, albeit not as well. The refractive index of air is different from water. Human eyes are adapted for vision in air, so when we open our eyes underwater without goggles, everything appears blurry. We can still see colors and shapes, but it is hard to make out specific objects. For fish, it is usually the reverse. They likely can see basic visual cues out of water, such as reflections, colors, and shapes, which may help them find water or safety. However, they may have trouble discerning unfamiliar objects or animals, which may make them vulnerable to predators.
The inner ears of fish are another widespread sense (all fish have them) that may help all amphibious fishes orient on land and find their way downhill, but can only give fish a limited amount of information. If a fish is on flat ground, then this sense is not helpful for orientation. Some fish may use chemoreception (taste and smell) to find prey by detecting tasty amino acids or avoid noxious, bad-quality water. However, how fish use these senses out of water are not well understood. Scientists are in the midst of uncovering how specialized amphibious fishes may use chemoreception for terrestrial orientation.
Invasive Amphibious Fish
Some amphibious fishes, like walking catfish, northern snakeheads, and Asian swamp eels, are also invasive species, meaning they have established populations outside of their native range, where cause ecological and/or economical harm. They consume native species, compete with them for resources, alter water quality, and/or cause erosion. Unlike your typical invasive fish species like carp, which can only spread through connected bodies of water, such as a river system, invasive amphibious fishes may be able to expand their range through overland movements. This can make containing their “invasion” much more difficult. Their abilities to leave the water, move across land, and find their way around are important details to consider when managing these species.
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