Fish require oxygen to survive. While humans and other land animals obtain oxygen from the air through lungs, most fish extract the oxygen dissolved in water. This process relies on specialized organs called gills, which are efficient at absorbing oxygen from their aquatic environment.
Dissolved Oxygen in Water
The oxygen fish utilize is not the oxygen atom within the water molecule (H₂O), but rather free oxygen molecules (O₂) dissolved in the water. This “dissolved oxygen” (DO) enters water bodies through two main mechanisms. First, oxygen from the atmosphere can dissolve into surface water through diffusion and the aerating action of winds. Second, aquatic plants and algae contribute significantly to DO levels through photosynthesis, releasing oxygen as a byproduct.
Maintaining adequate dissolved oxygen is important for fish and other aquatic organisms. Fish need DO levels between 5 and 6 parts per million (ppm) for optimal growth and survival. Levels below 3 ppm can stress fish, and concentrations dropping below 2 ppm often lead to mortality. Several factors influence DO levels, including water temperature (colder water holds more oxygen) and salinity (freshwater absorbs more oxygen than saltwater).
How Gills Extract Oxygen
The primary respiratory organs in most fish are their gills, located behind and to the side of the mouth cavity. These structures are supported by gill arches, from which numerous comb-like filaments extend. Each gill filament is covered in tiny folds called lamellae, supplied with small blood vessels called capillaries. This arrangement provides an extensive surface area for gas exchange, allowing fish to efficiently absorb oxygen from the surrounding water.
Fish breathe by drawing water into their mouths, pumping it over their gills, where oxygen diffuses from the water into their bloodstream and carbon dioxide moves from the blood into the water. This process is efficient due to countercurrent exchange. In countercurrent exchange, blood flows through the gill capillaries in the opposite direction to the water flowing over the gills. This opposing flow ensures the blood always encounters water with a higher oxygen concentration, maximizing oxygen absorption through a continuous concentration gradient. This allows fish gills to extract over 80% of the available oxygen from the water.
Beyond the Gills: Air-Breathing Fish
While most fish rely solely on gills for underwater respiration, some species have evolved adaptations to breathe atmospheric oxygen. These adaptations often allow them to survive in environments with low dissolved oxygen levels, such as stagnant or drying waters. For instance, lungfish possess true lungs that are highly vascularized, enabling them to gulp air from the surface. African and South American lungfish are obligate air-breathers, meaning they must regularly surface for air to survive, even in well-oxygenated water. They use a buccal force pump mechanism, drawing air into their mouth and then forcing it into their lungs.
Another group, labyrinth fish, includes species like bettas and gouramis. These fish have a specialized labyrinth organ, a complex bony structure above their gills, lined with highly vascularized tissue. This organ allows them to absorb oxygen directly from inhaled air. Electric eels are also air-breathers, using vascular folds in their mouth lining to absorb atmospheric oxygen and must surface every few minutes. These diverse adaptations highlight the varied strategies fish employ to thrive in challenging aquatic conditions.