Do All Fish Have Gills? A Look at Fish Breathing Methods

Virtually every fish species possesses gills, but gills are far from the whole story. Across roughly 35,000 known fish species, evolution has produced an impressive range of supplementary and even replacement breathing strategies, from true lungs to intestinal gas exchange to oxygen absorption through the skin. Some fish are so committed to air-breathing that they drown if denied access to the surface. The diversity of fish respiration is one of the more surprising corners of animal biology, and it reveals just how flexible vertebrate breathing can be.

How Standard Gills Work

The typical fish gill operates on a principle called countercurrent exchange: water flows across thin, plate-like structures called lamellae in one direction while blood flows through them in the opposite direction. This arrangement is extremely efficient because blood always encounters water that has a slightly higher oxygen concentration than the blood itself, allowing oxygen to continuously diffuse inward along the entire length of the gill surface.

Modeling studies have confirmed that the countercurrent setup is indeed the accepted framework for gas exchange in fish gills, based on the anatomical arrangement of gill elements and their blood vessels.¹ Interestingly, the raw oxygen-uptake advantage of countercurrent over a hypothetical same-direction system is fairly modest, estimated at around 3 to 17 percent for three species examined. The real payoff is energetic: a fish with a same-direction gill design would need to spend more than 46 percent more energy pumping water to compensate for even a 10 percent uptake shortfall.² In other words, countercurrent gills save a lot of metabolic fuel, even if they do not dramatically increase total oxygen capture.

Not All Gills Are Created Equal

Even within the world of gill-breathing fish, the anatomy and mechanics can vary enormously. The hagfish, one of the most ancient lineages of fish-like animals, breathes through a set of pouch-like gills rather than the familiar comb-shaped structures found in most bony and cartilaginous fishes. Water enters through a single nostril, gets pumped through these pouches, and exits through separate openings. The system relies on a two-phase pumping mechanism: a fast suction pump draws water in through the nostril at roughly 22 cycles per minute, while a slower force pump at about 4 to 5 cycles per minute pushes water out through the gill pouches.³ It is a fundamentally different plumbing layout from what you see in a trout or a shark, yet it accomplishes the same job.

At the other extreme are tunas and billfishes, whose gills have become specialized for high-speed, open-ocean life. These fish rely on ram ventilation, meaning they push water over their gills simply by swimming forward with their mouths open. Tunas of the genus Thunnus are obligate ram ventilators, which means they must keep swimming to breathe. Their gills have evolved two distinct types of structural fusions, one binding the gill lamellae and another connecting the gill filaments, that stiffen the gill architecture so it can withstand the continuous rush of water at cruising speed.⁴ Mackerels, their less extreme relatives, only switch to ram ventilation at fast speeds and lack these fusions entirely.

Fish with True Lungs

One of the more counterintuitive facts in vertebrate biology is that lungs did not originate in the animals that crawled onto land. The evidence points strongly in the other direction: lungs evolved in fish first and were later inherited by terrestrial vertebrates. Ancestral state reconstructions based on blood-vessel anatomy support the idea that the pulmonary arteries supplying lungs (and, later, swim bladders) are an ancient feature shared across bony fishes and their descendants.⁵ In most modern bony fish, the lung was repurposed into a swim bladder used for buoyancy rather than breathing. But a handful of lineages kept the original plan.

Lungfish (order Dipnoi) and bichirs (order Polypteriformes) are both obligate air-breathers that rely on lung-like organs for a significant portion of their oxygen intake. Their lungs differ structurally: the lungfish lung sits in a dorsal position and has a highly folded inner surface that maximizes the area available for gas exchange, while the bichir lung sits ventrally and has a comparatively smooth interior.⁶ The Australian lungfish is an especially interesting case because much of its lung’s inner surface is lined with a ciliated epithelium, which limits the respiratory surface area and likely explains why this species, unlike its African and South American relatives, cannot survive prolonged droughts by burrowing into dried mud and aestivating.

The surfactant system that keeps mammalian lungs from collapsing between breaths also appears in fish. Characteristic surfactant lipids and surfactant proteins have been identified in the lungs and swim bladders of several teleost species, providing strong evidence that this system has a single evolutionary origin shared by fish and land animals alike.⁷

The Labyrinth Organ and Other Accessory Air-Breathing Structures

If you have ever kept a betta fish or a gourami, you may have noticed them periodically gulping air at the surface. These fish belong to the suborder Anabantoidei, and they all possess a labyrinth organ: a bony, maze-like structure housed in a chamber above the gills, lined with thin, richly vascularized tissue that absorbs oxygen directly from air.⁸ This organ is not just a convenient backup. Some species are obligate air-breathers and will suffocate in well-oxygenated water if they cannot reach the surface.

The labyrinth organ shows developmental plasticity that responds to the environment. In Siamese fighting fish (Betta splendens), which are facultative air-breathers, mild low-oxygen conditions during development increase the labyrinth’s surface area by about 30 percent. The blue gourami (Trichopodus trichopterus), an obligate air-breather, does not show this same labyrinth response to low oxygen.⁹ The obligate species appears to develop a large labyrinth regardless of conditions, while the facultative breather calibrates its investment based on need.

Walking catfish (Clarias batrachus) take a different approach entirely. They have accessory respiratory organs derived from modified gill tissue, including gill fans, branching dendritic organs on certain gill arches, and a vascularized lining in the chamber above the gills.¹⁰ These structures retain architectural features similar to standard gill filaments but are adapted to extract oxygen from swallowed air rather than from water flowing past.

Breathing Through Skin and Guts

Some fish species have turned body surfaces you would never associate with breathing into functional respiratory organs. Mudskippers, the famously amphibious gobies that spend much of their time on mudflats, absorb oxygen through specialized structures in the skin, the lining of the mouth cavity, and their (reduced) gills. At the microscopic level, mudskipper skin contains respiratory papillae, small projections each supplied by an arteriole from a systemic artery, that are specifically adapted for absorbing oxygen from air.¹¹ The gills, the inner mouth lining, and the skin all show vascular specializations for aerial gas exchange, making the mudskipper a whole-body breathing system when it is on land.

Perhaps the strangest respiratory adaptation in fish is intestinal breathing. Several species of loach swallow air and pass it through their digestive tract, absorbing oxygen through the walls of the hindgut. Histological studies of the weather loach show that its posterior intestine is packed with capillaries at a far higher density than in non-air-breathing fish, and the air-blood barrier (the distance oxygen must diffuse to reach the bloodstream) is extremely thin, around 1.3 micrometers.¹² The mucosal folds in the posterior intestine are flattened and thinned compared to the anterior intestine, effectively turning the back end of the gut into a gas-exchange surface.

Experiments on the large-scale loach (Paramisgurnus dabryanus) confirm that this intestinal breathing is not just a curiosity. When researchers blocked the fish’s access to surface air (forcing reliance on gills alone), the gill filaments enlarged to compensate. When they instead blocked gill function by exposing fish to air, the number of capillaries in the hindgut rapidly increased and the diffusion distance shrank to about 1.24 micrometers.¹³ The fish can dynamically ramp up whichever respiratory system is available, a remarkable degree of physiological flexibility.

The electric eel is another obligate air-breather, but it uses the lining of its mouth rather than its intestines. It surfaces regularly to gulp air and absorbs oxygen across the highly vascularized oral cavity. Despite having gills, the electric eel cannot survive on dissolved oxygen alone.

How Baby Fish Breathe Before Their Gills Grow In

Newborn fish typically hatch without fully functional gills, raising an obvious question about how they get oxygen at all. The answer is cutaneous respiration: the skin handles gas exchange in early life. In zebrafish larvae, the skin is the dominant site of oxygen uptake right after hatching. The larvae use their pectoral fins to fan water across their bodies, and this behavior ramps up under low-oxygen conditions at four days post-fertilization. By 15 days post-fertilization, as the gills mature and take over as the primary respiratory surface, the fin-fanning response to low oxygen weakens, and by 21 days it disappears entirely.¹⁴ There is a clear developmental handoff from skin to gills over the first few weeks of life.

Gills That Reshape Themselves

A handful of fish species can physically remodel their gill tissue in response to changing conditions, which blurs the line between a fixed organ and a plastic one. Crucian carp and goldfish are the most dramatic examples. Under normal oxygen levels or cold temperatures, the spaces between their gill lamellae fill with a mass of cells, effectively burying the respiratory surface. When the water becomes oxygen-poor or warm, these cells undergo programmed death, retracting to expose a much larger gill surface area.¹⁵ The process reverses when conditions improve: cells regrow and fill the spaces again.¹⁶

Why would a fish voluntarily shrink its own gill surface area? Exposed gills are not free. A larger gill surface means more ion and water exchange with the surrounding environment, which is metabolically expensive to regulate. By covering the lamellae during times when oxygen demand is low, these fish reduce the energetic cost of maintaining their internal salt and water balance. They essentially trade respiratory capacity for osmoregulatory savings, then reverse the trade when oxygen becomes scarce.

The Survival Cost of Breathing Air

For fish that must surface to breathe, every trip to the top is a calculated risk. Experiments with dwarf gouramis and predatory snakeheads illustrate this vividly. When dissolved oxygen was near saturation, the gouramis barely surfaced and stayed hidden in vegetation. As oxygen dropped to extremely low levels, the gouramis had to surface more and more frequently and spent less time under cover. They were far more likely to be caught by the snakeheads when out of the vegetation, so capture rates rose sharply at lower oxygen levels.¹⁷ Even in the presence of predators, the gouramis reduced their air-breathing somewhat but could not eliminate it entirely, because the physiological need to breathe overrode the behavioral instinct to hide.

Some species have evolved behavioral shortcuts to deal with low-oxygen water without the full predation cost of breaking the surface. The tambaqui, a large South American fish, performs aquatic surface respiration by skimming the thin layer of oxygen-rich water right at the air-water interface. When oxygen drops, the tambaqui develops a swollen lower lip that channels this surface film of oxygenated water directly over the gills.¹⁸ The lip swelling appears to be triggered directly by local tissue hypoxia rather than by the same sensory pathways that trigger the surface-skimming behavior itself. A similar lip protuberance response has been documented in a piscivorous characid, Salminus brasiliensis, where lip width increased noticeably when dissolved oxygen dropped below about 13 percent saturation.¹⁹

When Gills Fail

Because gills sit at the interface between a fish and its environment, they are vulnerable to parasites, pollutants, and warming water. Amoebic gill disease, caused by the parasite Paramoeba perurans, is a major problem in farmed Atlantic salmon. Infected fish showed a maximum oxygen-uptake rate cut roughly in half compared to healthy fish, with aerobic scope dropping from about 406 to 203 mg O₂ per kilogram per hour. Swimming performance also declined, and infected fish showed signs of chronic stress and disrupted salt balance even at rest.²⁰ Compromised gills affect far more than breathing: they undermine a fish’s ability to eat, grow, and survive.

Climate change adds another dimension. As water warms, it holds less dissolved oxygen, and fish metabolic rates increase, creating a supply-and-demand mismatch at the gills. One prominent idea, known as the gill-oxygen limitation hypothesis, proposes that as fish grow larger, their gill surface area cannot keep pace with their rising oxygen demands, and warming temperatures would force fish to mature at smaller body sizes. Some models forecast fish body-size declines of 14 to 24 percent by 2050 under this scenario. However, recent empirical work measuring how gill surface area and metabolic rate scale across different sizes of the same species found that, for most species tested, gill area appears sufficient to meet oxygen needs throughout life.²¹ The debate is far from settled, but the straightforward version of “warming water means fish can’t breathe and must shrink” may be too simple.

Why So Many Backup Plans

The sheer variety of fish breathing strategies makes more sense when you consider where these fish live. Tropical floodplains, swamps, and stagnant pools routinely see dissolved oxygen plummet to near zero, especially at night when aquatic plants stop photosynthesizing and start consuming oxygen. A fish stuck in a shrinking, warming puddle at the end of the dry season cannot afford to rely on gills alone. Labyrinth organs, lung-like swim bladders, intestinal gas exchange, and cutaneous respiration all evolved independently in lineages that inhabit these extreme environments.

Even in well-oxygenated habitats, auxiliary breathing can confer advantages. Mudskippers exploit terrestrial food sources that gill-only fish cannot reach. Walking catfish can migrate overland between ponds during droughts. The electric eel inhabits murky, low-oxygen rivers in the Amazon basin where obligate gill-breathers would struggle. In each case, the supplementary breathing method opened up an ecological niche that would otherwise be unavailable, a pattern repeated across dozens of unrelated fish families on every continent where freshwater habitats experience seasonal or chronic oxygen stress.