Monday June 23, 2025
Respiration is a basic process that allows living organisms to function by delivering oxygen to their cells and tissues. The air we breathe is full of oxygen – containing about 21% – compared to water which is less than 1% oxygen. Fish have evolved many approaches to efficiently extract the low amount of available oxygen from water. For most fish, gills are the focal point of respiration. Gill filaments are blood vessel-rich tissues that resemble feathers or palm fronds and are covered by lamellae, small structures that maximize available surface area for gas exchange. When water flows over a fish’s gills, diffusion – whereby molecules in areas of high concentration move to areas of low concentration – brings oxygen from the water into oxygen-depleted blood. But diffusion only works when there is a difference in oxygen concentration between two solutions – in this case, blood and water.
Fish blood in the gills flows in the opposite direction of water, meaning that there is a constant supply of lower oxygen blood meeting a supply of high oxygen water, allowing diffusion to occur continuously to oxygenate the blood. This flow pattern is called counter-current exchange. Oxygenated blood is then carried throughout the body, where it facilitates the necessary processes that provide fish with energy to carry out daily activities and meet metabolic demands.
Fish can increase the rate of this process when faced with elevated energy demands by taking larger and faster ‘breaths’ to pump more water over their gills. However, some fish like certain shark and tuna species lack the ability to pump water over their gills and must swim constantly with their mouths open to maintain a stream of oxygenated water over their gills.
The structure and function of a fish’s gills differs among species and can change in response to the oxygen concentration in the water. When exposed to low oxygen conditions, fish can meet oxygen demands by increasing the number of lamellae they use in respiration. More active species tend to have thinner and more abundant, closely spaced lamellae, though thinner lamellae are more susceptible to damage by high water velocity. Some tuna species have overcome this challenge by evolving fused tips between gill filaments to increase their rigidity. Walking catfish developed specialized structures to prevent gill collapse when out of the water, allowing the species to breathe as they move across land.
Many fishes use organs other than their gills for respiration, including their skin, mouths, guts, or even lungs. In fact, most fish have some ability to carry out gas exchange using their skin. In the larval stage, most respiration happens exclusively through the skin until the gills develop. Mudskippers and European eels utilize their highly vascularized skin to exchange oxygen with the atmosphere during on-land adventures.
Some fishes are obligate air breathers and would suffocate if they did not have access to air! Electric eels, for example, must come to the surface frequently to breathe. They use their mouths to breathe air, and their gills expel carbon dioxide rather than absorb oxygen. Arapaima follow a similar approach, but they gulp air into their gas bladder for respiration, instead. Lungfishes, as their name suggests, have true lungs. Like electric eels, African and South American lungfishes use their gills mainly to express carbon dioxide. In times of drought, these fish rely wholly on lung breathing. When their habitat dries, African lungfish create a hard mucous cocoon that they aestivate (‘sleep’) in for up to five years, slowing down their metabolic rate, only to be woken up when water permeates their cocoon.
Unlike their African and South American cousins, the Australian lungfish is not an obligate air breather, but a facultative air breather, relying mainly on their gills to respire and using their lungs to supplement oxygen in times of stress. Other notable facultative air breathers include tarpon, gar, loaches, and armored catfish.
Tarpon and gar gulp air into their gas bladders for additional oxygenation, while armored catfish and some loaches gulp air into highly vascularized portions of their gastrointestinal tract. Each of these species lives in habitats that experience large swings in environmental conditions, necessitating their use of alternative means to respire when times get tough.
The diverse and physiologically impressive breathing strategies of fishes highlight the challenges of thriving in low-oxygen environments – whether by supercharging oxygen absorption in the highly specialized gills of oceanic predators or by using lungs and ramping down metabolism to survive for years as the proverbial “fish out of water”.
Header Image Caption: FISHBIO science explained graphic of ways fish “breathe”.
This Fish Report is part of the ongoing Fish Physiology Series, where we highlight some of the important physiological characteristics that enable fish to survive, persist, and thrive in aquatic environments on an ever-changing planet. Subscribe to the Fish Report and follow these posts to learn more about fish physiology!