While commonly believed fish cannot survive outside water, their endurance varies significantly. This survival time depends heavily on the specific fish species and the conditions of its surroundings. Understanding their respiratory mechanisms and unique adaptations clarifies why some perish quickly while others persist longer on land.
Why Gills Fail in Air
Fish breathe using gills, intricate respiratory organs on either side of their heads. Gills are composed of delicate gill filaments with numerous smaller folds called lamellae. This branched arrangement creates a large surface area with capillaries, allowing efficient oxygen exchange from water into the bloodstream and carbon dioxide release. Water constantly flows over these structures, and oxygen diffuses into the blood through the lamellae’s thin walls.
When removed from water, gills, designed to be buoyant and spread out, collapse. This collapse causes gill filaments and lamellae to stick together. The reduced surface area hinders oxygen absorption from the air. Gills also quickly dry out, preventing effective gas exchange and leading to suffocation, despite air’s higher oxygen content.
Factors Affecting Out-of-Water Survival
Several factors influence how long fish without specialized air-breathing can survive out of water. Species characteristics play a role; some fish have more robust gills or lower metabolic rates. Environmental temperature is a significant determinant; colder conditions reduce metabolic rate and oxygen demand, extending survival. Warmer temperatures increase metabolic activity, depleting oxygen faster.
Humidity also impacts survival by affecting water loss from gills and skin. Higher humidity keeps these surfaces moist, preventing dehydration and maintaining gas exchange. Fish size is another consideration; smaller fish dry faster and have higher metabolic rates, resulting in shorter survival. High stress levels elevate metabolic rate, consuming oxygen faster and further limiting time out of water.
Fish With Air-Breathing Adaptations
While most fish cannot survive long out of water, many species have evolved adaptations to breathe atmospheric air. Some fish, like lungfish and bichirs, possess true lungs similar to terrestrial vertebrates. These lungs, often modified swim bladders, feature honeycomb-like cavities rich in blood vessels, allowing direct oxygen uptake from gulped air. Australian lungfish primarily use gills but switch to air breathing when oxygen levels drop. African and South American lungfish are obligate air-breathers, needing to surface regularly and capable of surviving years dormant in mud during dry seasons.
Other species, such as gar, bowfin, arapaima, and tarpon, use highly vascularized modified swim bladders as supplementary respiratory organs. These structures act as primitive lungs, enabling fish to gulp air from the surface, useful in stagnant, oxygen-poor waters. The walking catfish has specialized suprabranchial arborescent organs. These tree-like organs extend the gill apparatus, supporting filaments to prevent collapse and allowing atmospheric oxygen absorption, functioning as an accessory lung.
Labyrinth fish, including bettas and gouramis, possess a unique labyrinth organ above their gills. This complex, highly vascularized structure allows them to take in oxygen directly from the air, supplementing gill respiration and enabling survival in oxygen-depleted or out-of-water conditions. Certain fish like mudskippers and eels can engage in cutaneous respiration, absorbing oxygen directly through their skin and mouth lining. This method requires moist skin, limiting them to humid environments or damp surfaces.
What to Do if a Fish is Out of Water
If a fish is found out of water, immediate, gentle action can sometimes improve its chances. First, carefully pick up the fish, minimizing direct contact to protect its delicate slime coat. The primary goal is to return the fish to water as quickly as possible.
Once back in water, provide clean, oxygenated water at an appropriate temperature. If unresponsive, “fish CPR” can be attempted. This involves gently moving the fish forward through water, allowing water to flow over its gills in the correct direction for oxygen uptake. This process should be done carefully, without forcing water backward over the gills, which can cause damage. Even with quick intervention, survival is not guaranteed, but signs of recovery like gill movement or active swimming indicate a positive response.