Fish, like most complex life forms, require oxygen for their biological processes. While this holds true for most aquatic species, some fish exhibit extraordinary adaptations, allowing them to endure very low oxygen levels, or even complete absence, for extended periods. These abilities enable certain species to thrive in environments lethal to others.
Oxygen’s Essential Role for Fish
Oxygen plays a fundamental role in the survival of nearly all fish species, primarily through its involvement in cellular respiration. This biological process generates adenosine triphosphate (ATP), the energy currency that powers virtually all cellular activities within the fish’s body. Without sufficient ATP, vital physiological functions cannot be maintained. Fish extract dissolved oxygen from water using specialized respiratory organs called gills.
Gills are typically located on either side of the fish’s head and consist of numerous thin, feathery structures known as gill filaments. These filaments are richly supplied with tiny blood vessels, forming an extensive surface area designed for efficient gas exchange. Water enters the fish’s mouth and is pumped over the gill filaments, where oxygen diffuses into the bloodstream while carbon dioxide, a waste product, is released. This countercurrent exchange mechanism, where water flows in the opposite direction to blood, maximizes oxygen uptake from the water.
How Fish Cope with Low Oxygen
Some fish species possess physiological adaptations that allow them to survive in environments with limited or no oxygen. One such adaptation involves metabolic adjustments to produce energy without oxygen, a process known as anaerobic respiration. In most vertebrates, anaerobic respiration produces lactic acid, which can quickly become toxic if it accumulates.
Some fish, particularly those in the genus Carassius, have evolved a unique pathway to convert lactic acid into ethanol (alcohol). This ethanol is then released through the gills into the surrounding water, preventing the toxic buildup of lactic acid in the body. This allows them to maintain ATP supply even when oxygen is absent. Other strategies include metabolic suppression, where fish significantly reduce their energy demands to conserve resources during periods of oxygen deprivation. They can also alter neural signaling to make brain cells less excitable, thereby lowering their energy needs.
Another coping mechanism involves the ability of some fish to breathe atmospheric air directly. These fish have evolved specialized organs that allow them to extract oxygen from the air above the water surface. Such air-breathing capabilities provide an alternative oxygen source when dissolved oxygen levels in the water become too low to support gill respiration alone. This adaptation is particularly beneficial in stagnant or oxygen-poor aquatic habitats.
Fish That Survive Without Much Oxygen
The crucian carp and its domesticated relative, the goldfish, are known for their remarkable tolerance to low oxygen. These species can survive for months in oxygen-free water, especially during winter when ponds are covered by ice. They convert lactic acid into ethanol, which is safely excreted. The crucian carp, for instance, stores large amounts of glycogen in its liver, providing fuel for this ethanol-producing pathway during prolonged anoxia.
Lungfish are another group of fish known for their air-breathing abilities, possessing lungs that are structurally similar to those of amphibians. African and South American lungfish can survive seasonal dry spells by burrowing into mud and aestivating for months, breathing air through vents in their cocoons. They use a buccal pump mechanism to gulp air and inflate their lungs. Similarly, labyrinth fish, such as gouramis and Siamese fighting fish (bettas), have a specialized labyrinth organ located above their gills. This organ, richly supplied with blood vessels, enables them to absorb oxygen directly from gulped air, allowing them to thrive in stagnant or densely vegetated waters with low dissolved oxygen.
Causes of Low Oxygen in Water
Low oxygen levels in aquatic environments, often termed hypoxia or anoxia, can result from various natural and human-induced factors. One significant cause is eutrophication, which occurs when excessive nutrients, primarily nitrogen and phosphorus from sources like agricultural runoff and wastewater discharge, enter a body of water. This nutrient overload stimulates rapid growth of aquatic plants and algae, leading to algal blooms.
While algae produce oxygen during daylight hours, their respiration at night consumes oxygen. When these large algal blooms die, bacteria decompose the organic matter, a process that consumes substantial amounts of dissolved oxygen. This decomposition can deplete oxygen levels, creating zones where aquatic life struggles to survive. High water temperatures also contribute to lower dissolved oxygen, as oxygen is less soluble in warmer water. Stagnant water, lacking circulation and aeration, naturally loses its oxygen content over time, further exacerbating hypoxic conditions.