Fish obtain oxygen necessary for life from their watery surroundings. They extract oxygen dissolved within the water, not from the water molecules themselves. These mechanisms are finely tuned to their aquatic habitats, allowing them to thrive globally.
The Vital Role of Dissolved Oxygen
Fish depend on dissolved oxygen in water for respiration. This “dissolved oxygen” (DO) refers to free oxygen molecules (O₂) present in the water, not the oxygen atoms within water (H₂O). Aquatic environments gain DO through natural processes like atmospheric diffusion into surface waters, aided by wind and waves. Aquatic plants and phytoplankton also release oxygen as a byproduct of photosynthesis, contributing significantly to DO levels.
The amount of dissolved oxygen available in water is influenced by factors such as temperature, with colder water holding more oxygen than warmer water. Fish require specific levels of dissolved oxygen to survive and grow, typically needing between 5-6 parts per million (ppm). Levels falling below 3 ppm can stress most aquatic organisms, and concentrations under 2 ppm are often lethal, leading to fish mortality.
The Marvel of Gill Respiration
Most fish breathe using gills, specialized organs located on either side of their head. Each gill is supported by a bony or cartilaginous arch from which numerous delicate structures, called gill filaments, project. Tiny, plate-like lamellae extend from these filaments, further expanding the surface area for gas exchange.
As a fish takes in water through its mouth, it pumps this water over the gill filaments. Within the lamellae, a dense network of capillaries provides a large surface where oxygen and carbon dioxide can be exchanged. The efficiency of this process is greatly enhanced by a mechanism called countercurrent exchange. Blood flows through the capillaries in the opposite direction to the water flowing over the gills. This opposing flow ensures that the blood always encounters water with a higher oxygen concentration, maintaining a continuous diffusion gradient that maximizes oxygen uptake. This allows fish to extract a high percentage, often over 80%, of the available oxygen from the water.
Fish That Breathe Beyond Water
While gills are the primary respiratory organs for most fish, some species have evolved adaptations allowing them to breathe air directly. These air-breathing fish often inhabit environments where water oxygen levels can become very low, such as stagnant ponds, swamps, or areas prone to drying. Using atmospheric oxygen provides a survival advantage in these challenging conditions.
These specialized adaptations include modified swim bladders that function like lungs, vascularized mouth linings, or even specialized sections of their intestines. For instance, lungfish possess one or two primitive lungs, enabling them to gulp air and survive periods of drought by burrowing into mud. Labyrinth fish, such as bettas and gouramis, have a unique labyrinth organ, a complex structure above their gills, that allows them to extract oxygen from air. Other examples include mudskippers, which can spend time on land and absorb oxygen through their skin and modified gills, and certain catfishes and electric eels that use vascularized oral or intestinal surfaces for air breathing.