Fish are aquatic creatures, their existence defined by water. While most cannot survive long outside their watery homes, their biology is exquisitely adapted to submerged life. This article explores why fish struggle on land and highlights remarkable exceptions.
The Immediate Challenge: Why Fish Drown in Air
Most fish rely on gills to extract oxygen, an organ uniquely designed for aquatic respiration. Gills are composed of numerous delicate filaments, which are further covered in tiny structures called lamellae. This intricate arrangement creates a vast surface area, rich with capillaries, allowing dissolved oxygen from the water to diffuse into the bloodstream. Water must continuously flow over these gill structures for effective gas exchange to occur.
When a fish is removed from water, these delicate gill filaments collapse, similar to how the pages of a wet book would stick together when dry. This collapse drastically reduces the surface area available for oxygen uptake from the air. Simultaneously, the gills begin to dry out, impairing their function. Even though air contains a higher concentration of oxygen than water, the fish’s respiratory system is unable to utilize it, leading to suffocation.
Factors Influencing Survival Time
The duration a fish can survive out of water varies based on several factors. The fish species itself is primary, as some are more resilient to brief air exposure than others. Larger fish often endure longer than smaller individuals. Ambient temperature is also important; colder conditions slow metabolism and extend survival, while warmer temperatures accelerate distress.
Higher humidity reduces desiccation. Proper handling minimizes stress and improves chances if the fish is quickly returned to water. Oxygen levels in the water before removal also influence resilience, as fish from oxygen-depleted environments may already be stressed. Most common aquarium fish typically last only a few minutes to a few hours out of water.
Remarkable Adaptations: Fish That Defy the Odds
While most fish are bound to water, some species have evolved remarkable adaptations that allow them to survive for extended periods on land. Mudskippers, for example, are amphibious fish capable of spending up to three-quarters of their lives out of water. They breathe through their skin and the lining of their mouth and throat, which are highly vascularized. They can also trap water in their enlarged gill chambers, keeping their gills moist and functional while on land.
Lungfish are another example, known for their ability to breathe air using modified swim bladders that function as lungs. Some lungfish can survive for months or even years buried in mud during dry seasons, drastically slowing their metabolism in a state called estivation until water returns.
Walking catfish possess a specialized accessory respiratory organ, a suprabranchial arborescent organ, which acts much like a lung and allows them to absorb atmospheric oxygen. This adaptation enables them to survive out of water for up to 18 hours and even move across land between bodies of water.