All animals require oxygen for cellular functions, but the method of acquisition must be tailored to the medium. Terrestrial animals draw highly concentrated oxygen from the air, which is a gas, using internal, moist sacs (lungs). Fish, however, live in an entirely different physical environment where oxygen is dissolved into a liquid medium, water. This difference necessitates a highly specialized biological system to efficiently extract the far scarcer oxygen molecules from the water flowing around them.
Gills Versus Lungs: Specialized Structures for Gas Exchange
Perch, like the vast majority of fish species, do not possess lungs for respiration. Lungs are internal, balloon-like structures designed to pull oxygen from the air, where the concentration of oxygen is approximately 20% of the volume. This design is highly inefficient for aquatic life because water is a dense medium that holds only about 1% dissolved oxygen by volume, roughly 30 times less than air. A fish attempting to use lungs would expend too much energy moving water in and out of the internal sacs, and the water would quickly strip away the necessary moisture.
Instead of lungs, fish use gills, which are structures adapted for extracting oxygen from a liquid medium. Gills are composed of delicate, feathery filaments supported by bony arches on either side of the head. These filaments are subdivided into thin plates called lamellae, which are richly supplied with blood capillaries. This arrangement maximizes the total surface area exposed to the water, creating a minimal barrier for gas exchange.
The Mechanics of Aquatic Respiration
The process of aquatic respiration begins with the fish actively creating a current to move water over the gills. Most fish draw water in through the mouth, then close the mouth and use muscles to force the water backward over the gill arches and out through the opercula (gill covers). This unidirectional flow is necessary because oxygen must be continuously supplied, and the used water containing carbon dioxide must be expelled. The oxygen molecules are then transferred from the water into the bloodstream across the thin lamellae via diffusion.
The remarkable efficiency of a fish’s respiratory system stems from a mechanism known as countercurrent exchange. In this system, the blood flowing through the capillaries of the gill lamellae moves in the direction opposite to the flow of water passing over them. This opposing movement ensures that as the blood picks up oxygen, it continually encounters water that has a slightly higher concentration of oxygen.
This continuous gradient allows oxygen to diffuse into the blood across the entire surface of the gill. If the water and blood flowed in the same direction, oxygen transfer would stop once concentrations equilibrated halfway. Countercurrent exchange allows fish to extract up to 80% or more of the available dissolved oxygen from the water, which is a necessary adaptation given the low oxygen concentration in their environment.
Air-Breathing Fish: Adaptations for Low-Oxygen Environments
While the gill system is highly efficient, some fish inhabit shallow, stagnant, or warm waters where oxygen levels can drop to near zero, a condition called hypoxia. In these specialized environments, certain fish species have evolved supplementary organs that allow them to breathe air from the surface. These air-breathing adaptations are modified structures that function similarly to lungs, though they are not true mammalian lungs.
Lungfish, for instance, possess a highly vascularized swim bladder that functions as a primitive lung. This allows them to survive periods of drought or severe hypoxia by gulping air at the surface. Other species, such as gourami and the Siamese fighting fish, have evolved a labyrinth organ. This intricate structure is located above the gills and acts as a rudimentary lung, extracting oxygen directly from a bubble of air taken from the surface.
Catfish and electric eels also exhibit air-breathing behavior using highly vascularized tissues in their mouths or intestines. They rely on their gills for most gas exchange, particularly for expelling carbon dioxide. However, they supplement their oxygen intake by surfacing to gulp air when dissolved oxygen in the water becomes too scarce.