Why Lungs can't work in water and gills can't work in air?

Animals with lungs (such as most terrestrial mammals) cannot breathe if submerged in water, and soon suffocate.

Whereas, fish can variously breathe air (at least for few-hours, as some fish frequently need to get air periodically), including those that are bigger than a human (say sharks).

But why? Do fish require less oxygen than lunged terrestrial-animals, despite being active and variously engaging in activities like predation? (so that uptaking only dissolved oxygen, and probably less surface area of gill than lung-alveoli is enough for them?)

Misleading sentences in your question

fishes can breathe in water (at least for few-hours)

Fishes can breathe for much longer than a few hours under water as they spend their whole life underwater.

Animals-with-lungs (such as most terrestrial mammals)

All mammals, whether terrestrial (like a cow) or not (like a dolphin) have lungs. While all mammals have lungs, most fish (but not all) have gills.

Lungs vs Gills

To understand the answer, you need to understand two key organs involved in gathering oxygen from the environment: gills and lungs.

Lungs are organs that allow the transfer of oxygen into the blood when inhaling air, while gills are organs that allow the transfer of oxygen into blood when inhaling water. Lungs don't do their job properly in water and gills don't do their job properly in the air. You should have a look at the following wikipedia entries to further your understanding of these organs:

Energy consumption

You say

does fishes require much-much less-amount of oxygen than lunged terrestrial-animals, though they stay in physical activities including predation?

I am not sure I understand the meaning of this sentence but anyway.

Making generalization about fish metabolic rate can only be misleading as there are an immense diversity of fishes. For example some fishes are endotherm (see here to understand the definition of this term) while some are not and this will vastly affect the metabolic rate.

Although eventually too advanced for the OP, Chabot et al. (2016) is an interesting reading to get an general overview of the Metabolic rate in fishes.

Why Lungs can't work in water and gills can't work in air? - Biology

I tried to find the difference between lungs and gills. I know some of you already gave me the answer, but I'm not satisfied with "lungs work in air and gills work in water".

I couldn't find much detailed information in the vast web of internet stupidity, but their chemical workings seem similar. The surface area and the way to create this surface area appear to be the only difference between lungs and gills. It's a structural difference and not a chemical one. Water contains about 20 times less oxygen than air, which explains why we can't breathe water. However, this makes you think fish should be able to breathe air.

The problem is that the structure that creates the enormous surface area of their gills has to be supported by water. "Out of water, the gills collapse like wet tissue paper, and very little surface area is left exposed for gas exchange. Most fish, therefore, can only survive a short time out of water before oxygen deficiency catches up with them and they asphyxiate." What about zero-g? "even in a humid air-filled chamber at zero gravity, the gill filaments will simply adhere to one another.". Too bad.

Can we breathe a liquid if it contains enough oxygen? "If water could hold about 20 times more oxygen than it does, things would be different - there are apparently a few liquids (though not water) that can hold that much dissolved oxygen, and one can breathe a liquid of this sort". Despite their similarities, it is currently believed that lungs evolved from gas bladder that fish use to float, and not from their gills. All vertebrates use Hemoglobin for oxygen transport.


i thought it was lungs work in water and gills work in air? i heart the internets.

i thought humans were just desinged to not breathe water, i didnt know about the O2 difference!

Too true man. rock on respiration! :P )))))
i<3 biology. My elbows are warm for this article. oxooxoxoxoxo bby.

A Student Claims to Have Designed Working Artificial Gills

Out of all the attributes that separate humans from fish, the ability to breathe underwater is one that makes us land dwellers most envious. So it's hard not to get worked up over word this past week that a Korean design student may have come up with a blueprint for a wearable device that can extract sufficient air from seawater, enabling just about anyone to breathe like a fish.

It's a remarkable claim considering that no one has yet come up with anything resembling actual “artificial gills.”  Codenamed "Triton," the mysterious concept comes in the form of a small mouthpiece, reminiscent of the "rebreather" James Bond uses in Thunderball (1965) and Die Another Day (2002). It is designed to mechanically capture the oxygen gas present in water and store it in a compressed air tank. As creator Jeabyun Yeon describes on his website, water is filtered using a pair of cylindrical shaped gills that house fine threads with "holes smaller than water molecules." A built-in micro compressor, powered by a quick-charging miniaturized battery, then condenses the oxygen, making it readily available as the wearer inhales.

Several skeptics have since chimed in, pointing to certain technological challenges that would ultimately render Yeon's idea, as it's detailed, anywhere from implausible to ridiculously far-fetched. To grasp why artificial gills have been nothing more than a pipe dream thus far, one must understand some of the intrinsic biological differences between man and finned sea creature. First, and most obvious, is that fish possess gills that have evolved to absorb oxygen while keeping out waste gases human respiratory systems are equipped to tap into the oxygen in the air. Fish are also cold-blooded, meaning they require a lot less energy. This adaptation is essential as the concentration of dissolved oxygen in water is scarce, about 20 times less than what's found in the same volume of air.

The Blog ZidBits explains that artificial gills would need to be huge to provide an adequate amount of oxygen for humans:

This problem is amplified thanks to sea water only containing 7 ppm of oxygen. As a result of this low concentration, 1,000 tonnes of sea water holds only 14 lbs. of O2. Since an average diver needs 1 quart of oxygen per minute, you would need 51 gallons of sea water per minute to pass through the ‘gills’.

The blog DeepSeaNews critiqued Yeon's technology, estimating that, even at the low-end, such a system would need to pump and extract oxygen from around 24 gallons of water for every minute spent submerged. Moreover, inhaling pure oxygen filtered from water can be highly toxic. While 20 percent of air is made up of oxygen, scientists have discovered that breathing air comprised of 100 percent oxygen can cause symptoms such as blurred vision, seizures and convulsions due to fluid accumulating in the lungs.

That said, these challenges haven't thwarted others' attempts to ditch pressurized scuba tanks. Israeli inventor Alon Bodner has been developing a battery-powered prototype that uses a high-speed centrifuge to reduce the pressure of captured seawater, which causes oxygen to bubble up and escape into a separate chamber, much the same way carbon dioxide gases are released when opening a can of soda. The drawback is that the contraption, dubbed "LikeAFish," requires a high-capacity (and likely heavy) power source to function. 

Another more exotic approach by scientists at Nottingham Trent University in England was inspired by the great diving beetle, an insect with anatomical features that allow it to survive underwater. Tiny hairs located on its abdomen work to trap a pocket of air between its respiratory opening and the surrounding water. This protective layer of air also acts as a filter, allowing oxygen gases locked up in the water to pass in and carbon dioxide to diffuse out. In one experiment, researchers were able to mimic this effect, to some degree, using a "super-water-repellant porous foam" material wrapped around an oxygen inhaling device. 

But, any way you frame it, it looks like it will be a while before a human can be one with the fishes.

Why can’t fish breathe out of water?

Dear Straight Dope:

I went fishing yesterday with my family and when we caught a fish I thought, "Why can't fish live outside of water?" I mean, they breathe oxygen in the water. Outside of the ocean, they should have more oxygen to breathe - no harm done! So why will taking a fish out of water kill it?

Dear Cecil:

I do a lot of fishing, or maybe it is standing on the shore with a pole looking like an idiot. The other day, I did actually catch a fish, a largemouth bass to be exact. I am a catch-and-release kind of guy - but of course I had to take a few pictures, work the hook out of his mouth, carry him back to the water, etc. All told, the poor little guy was probably removed from his preferred environment for approximately five minutes. This makes me wonder, why can a fish survive for five minutes out of water while a human surely would not survive (or at least remain conscious) for five minutes submerged in water? Could it be that a fish is able to breath somewhat when it is out of water? Please help solve this fish tale, as I hate to think while I fish.

Daniel L., NY, NY Mike T., Philadelphia, PA

Fish gills are remarkable things, but the conditions under which they function are pretty specific. For one thing, they are rather delicate, and their tremendous surface area (the main thing that makes them work so well) is dependent on being immersed in water to support their weight. Out of water, the gills collapse like wet tissue paper, and very little surface area is left exposed for gas exchange. Most fish, therefore, can only survive a short time out of water before oxygen deficiency catches up with them and they asphyxiate.

If it were possible to keep the gills supported and moist without being submerged, a fish could survive quite a bit longer, but that isn’t physically possible – even in a humid air-filled chamber at zero gravity, the gill filaments will simply adhere to one another. Water needs to completely fill the gill chamber to keep all of the filaments in operation. For that matter, the water has to be flowing in the mouth and out the gills in order for oxygen extraction to work properly. If you force water to go in the opposite direction, in the gills and out the mouth, the system only works at about 50% efficiency, since the water flow needs to go counter to the flow of blood for maximum oxygen uptake.

Many fish species have evolved mechanisms to work around this limitation (usually involving the development of lung-like structures in addition to the gills), and some can go for long periods out of water. But land-based critters haven’t developed a comparable ability to breathe while submerged. The lungs of other vertebrates are simply not designed to extract enough oxygen for them to function underwater, where the oxygen concentrations are more than an order of magnitude lower. If water could hold about 20 times more oxygen than it does, things would be different – there are apparently a few liquids (though not water) that can hold that much dissolved oxygen, and one can breathe a liquid of this sort, as in the movie The Abyss. But maintaining those high oxygen levels for long in a closed system might be a major practical stumbling block, so I don’t think liquid breathing systems are going to be easy to design or use.

Send questions to Cecil via [email protected]


Here are a few of our favourite “fish out of water”

Scientists found that fish and aquatic animals that can leave the water without much trouble are typically those that live in intertidal zones. An intertidal zone is a place that is under water when the tide is high, and out of water when the tide is low. Similarly some fish that live in floodplains, areas that are subject both dry and wet times, can also survive comfortably on land.

Cephalopods (Octopus) : Cephalopods, like octopus, are part of the mulluscs phylum. This is the same animal phylum as clams and snails. Octopus are very clever. Some species of octopus that reside in tidal areas or near-shore leave the water to go for crawl. Scientists have found that they like to hunt for food in tidal pools. However, just like people need air, octopi need water to breathe. When they leave the ocean to crawl around tidal pools it’s almost like holding their breath. They can only stay out of the water for several minutes at a time. As long as they stay moist they’re okay, as their bodies can still get oxygen this way. As octopus are typically nocturnal their out of water trips usually happen under the cover of darkness. Inky the Octopus’ great escape from the National Aquarium of New Zealand in April 2016 happened at night. Inky, a common New Zealand octopus, escaped her tank, crossed the floor, and made it home to the ocean by crawling through a 15 cm drain pipe.
Photo by Andrew Reding, Pacific giant octopus slithering between tidepools at low tide

African Lungfish: Lungfish are very special because, instead of breathing water, they breathe air. Like other animals with lungs, lungfish have to come to the surface of the water to take a breath. If they don’t, they can drown. Depending on the species, African lungfish have one or 2 lungs. In West and South Africa, where lungfish live, there is a dry season. The swamps, rivers, and streams they call home can completely dry up and these special fish may have to live outside of the water for many months. In order to survive, they burrow into the mud in a cocoon. They continue to rely on their lungs to breathe air while they wait for the rain to come back and the water to return. Lungfish can survive for years out of water. When the rains return, lungfish go back to swimming.

Clingfish: Clingfish are a type of fish with a very strong sucker on their stomachs, which they use to attach themselves to rocks in very heavy surf. Clingfish can leave the water and breathe air stored in their gills. They can survive up to 3 and a half days out of the water.

Gobies: Gobies, suck as mudskippers, are a truly amphibious fish. They are also the most famous fish-out-of-water. Mudskippers breathe air. And, like other amphibians, when they are out of the water they can get oxygen through their skin and mouth-lining. They can breathe with their gills to absorb oxygen. They also carry water around in their gill chambers to help them from drying out. As long as the mudskipper can keep itself moist, it can stay out of the water as long as it likes.
Photo by Afon

Crabs: There are three different types of crabs: aquatic, meaning they spend their whole life in the water intertidal, meaning they live both in and out of the water and terrestrial, meaning they spend their life on land. Each category of crab breathes in a different way. Aquatic crabs, like fish, have gills to get the oxygen from water. Land crabs have specially adapted gills that work on land. Some types of land crabs even have lungs. Intertidal crabs live both in and out of water. They have cavities throughout their bodies that they can use to store water for when they are on land. Their gills work great out of the water, so long as they are kept moist. Intertidal crabs have a special moveable plates that help seal in moisture around their gills when they are out of the water.
Photo by Rushen/Thai National Parks

Why Whales Don’t Have Gills… or why we don’t colonise the Sea

…basically it’s too much hard work for not enough oxygen. Whales, as mammals, need a good oxygen supply and there just isn’t enough in sea water to get at without the whale getting exhausted. Moving all that water in and out of one’s lungs/gills is very hard work – a mammal would need to breathe kilograms of sea water at a time rather than the mere grams of air it does breathe.

Often people wonder: why don’t we colonize the sea instead of space since it is so much closer?

But how close is it really? Beyond a few metres the pressure rises to levels that challenge our freedom to move – you can’t ascend too rapidly without risking the “bends”. Plus there’s very little oxygen, little light and very little else. In space pressure is never a problem and light is everywhere. In the sea any kind of heat processing suffers from the heat-sapping presence of water, but in space one can vapourise metals and silicates via concentrated sunlight.

So I have a few reservations about the whole “colonise the sea instead of space” idea. As NASA has long realised the two require similar and yet dissimilar technologies – it uses underwater laboratories as training environments for its astronauts, but isn’t planning on colonising the great oceanic deserts anytime soon.

Kurt9’s comment makes another good point…

The ocean is a remarkably hostile environment from a structural engineering standpoint. Seawater is corrosive and you have 50 meter rogue waves to deal with. You also have to deal with typhoons if you are not within 5 degrees latitude of the equator. Space is actually a more benign environment for structures, but is very expensive to access as of yet.

The length of time a crab can stay out of water depends on the type of crab. Some crabs, like coconut crabs and land hermit crabs, are terrestrial and breathe well without water, although they still need to keep their gills moist. As long as their gills stay moist, these crabs can spend their lives out of the water. But if they were submerged in water, they would die.

Aquatic Crabs

Other crabs, like blue crabs, are primarily aquatic and are adapted to receiving their oxygen from the surrounding water. Yet, they can still survive for 1-2 days out of the water.

The European green crab is a species infamous for surviving out of water for a long time—at least a week. These species seem indestructible, which is a problem since they have invaded many areas of the U.S. and are out-competing native species for food and space.

ELI5: When drowning, why can't the lungs produce oxygen

When a human is drowning, why can the lungs not create oxygen from the H2O?

Judging by your wording, you're misunderstanding what lungs do.

Your lungs don't produce oxygen. Your lungs facilitate gas exchange between your blood and the outer world.

Every time you breathe, oxygen is absorbed into your blood through your lung tissue. And carbon dioxide is deposited from your blood stream into the air you exhale. With every cycle you inhale fresh air and take the oxygen and you exhale air loaded with carbon dioxide.

Water doesn't contain nearly enough dissolved oxygen to meet your needs. And trying to breathe a fluid, especially under pressure if you're deeper down underwater, is really hard work.

The way the gills of fish and other underwater creatures works is pretty similar to our lungs really. They have tissues that facilitate the gas exchange between their blood and the surrounding water. They take oxygen from the water and dump carbon dioxide into the water.

But gills do have a few difference. Because there's so little oxygen in the water compared to air, gills have far more surface area than lungs do. The reason fish can't breath on land is that they rely on the water to feather out their gill tissue, much like your hair feathers out when you dive. Outside the water their gills just stick together, much like your hair when you exit the water.

Fish also don't have to work as hard to have water pass their gills. When you try to breathe underwater while you're drowning, you have to suck all that heavy water in and then pump it back out. You're not build for that.

Fish gills are right behind the mouth and open right back up to the water. All they have to do is open their mouths, let the water flow in and flow out past their gills.

So to sum it up. There's not enough oxygen in the water for you to meet your needs. Your lungs aren't built to take water from oxygen. And it's really hard to breathe fluids in and out, especially under pressure when you're deep down.

Eli5 Gills can absorb oxygen from an aqueous environment. Why can’t gills absorb oxygen from a non-aqueous environment?

Additionally, in the instances like salamanders that absorb oxygen through their skin is this the same function?

They can, but they collapse and stick together when out of the water and as a result there's far far far less surface area for oxygen exchange exposed to the air than exposed to the water. If you could somehow keep gills fully open and exposed in air, they would likely work. Gills work in water because there is a huge amount of surface area presented to the water. Lungs don't need to present nearly as much surface area for the same amount of oxygen absorption because there's nearly 20 times the amount of oxygen in air as their is in water. Lungs avoid collapse by having lots and lots of tiny pockets that stay open in air, so there's plenty of surface area for gas exchange.

It's like long hair underwater. It spreads out a lot and each strand has lots of contact with the water. Then you get out of the pool, and all the hair lays really flat, and most of it isn't touching the air.

I would amend the last statement to say "lungs stay open due to a combination of negative pressure driven by the intercostal muscles and the diaphragm in a closed thoracic space and aided by surfactants produced by the alveoli".

Edit: your muscles that make you suck in air, being in a chest without more holes than normal (just your mouth and nose), along with lung goop, allow you to breathe air, unlike these fish.

In addition, lungs are elaborately constructed to preserve the moisture inside them. Gills have not had to adapt to this, and would dry out very quickly.

This is a good answer which covers a big, important part of it, but it's not quite the whole story. The other part is avoiding drying out. Gill cells (And vertebrate cells in general) will die if they dry out too much, which is a part of why fish like mudskippers have to stay near the water even if they can breathe air. If the gills get too dry the cells die and dead cells don't work well. Lungs get around this problem by being inside the body at the end of a tube, where their humidity can be more easily maintained, and there's also a surfactant down there with a lot of important functions.

so im relatively safe from land sharks?

Gills are like tiny little hairs made out of special meat that can pick up oxygen in the water. These meat hairs are better at picking up oxygen when they are in water. When they are in the air, they can still pick up oxygen, but they dry out really fast because they are so used to the water. Remember Spongebob when he went to visit sandy for the first time? He got really dry and started to crack. The same thing happens to the gills and they can't pick up oxygen as well anymore.

They still can as long as they are wet. Gills don't work that much different from lungs really, the big difference being that gill's dry out much more quickly.

When you first pull a fish out of water, at first it is not suffocating. It actually has the opposite problem, and is dealing with too much oxygen, poisonous levels of oxygen. If a fish could keep its gills wet long enough, this would end up killing it. But instead of eventually the gills dry out and then it dies of suffocation.

But instead of eventually the gills dry out and then it dies of suffocation.

That's sad. When I was taught to fish I was taught to whack them on the head/neck. We had basically a small baseball bat for doing the deed.

But thinking of commercial fishing, I could imagine they dump them in a pile and wait for the inevitable.

The thing is though, if you can keep your gills wet somehow, you can go in air just fine. Crayfish are a good example the gills are located in the thorax - inside the body - so they have some control of humidity (plus the enclosed space helps), and also temperature to some extent by their location in sun/shade. This is how they colonize new areas: they go for a walk over land and look for new water bodies to live in. How far they can walk depends on species (some are better at it than others), but still, they have more distance before drying out than you may think.

The main issue is interface, which water provides in a much better way than anything else. Gills have the blood vessels all disposed in an array that allows the water to pass through, which in turn allows the oxygen to be captured by the oxygen-carrying molecules of the organism. Land crustaceans, like some crabs and woodlouses, also breathe through gills, secreting a liquid over the gills to capture the oxygen in the air
Salamanders start their life with gills, some never lose them, and some develop lungs but still keep the skin as a breathing interface. but it needs to be always wet.

More concisely, just about anything living needs water in some way to work (specially if involves proteins) , and breathing is such a case.


Galen observed that fish had multitudes of openings (foramina), big enough to admit gases, but too fine to give passage to water. Pliny the Elder held that fish respired by their gills, but observed that Aristotle was of another opinion. [1] The word branchia comes from the Greek βράγχια , "gills", plural of βράγχιον (in singular, meaning a fin). [2]

Many microscopic aquatic animals, and some larger but inactive ones, can absorb sufficient oxygen through the entire surface of their bodies, and so can respire adequately without gills. However, more complex or more active aquatic organisms usually require a gill or gills. Many invertebrates, and even amphibians, use both the body surface and gills for gaseous exchange. [3]

Gills usually consist of thin filaments of tissue, lamellae (plates), branches, or slender, tufted processes that have a highly folded surface to increase surface area. The delicate nature of the gills is possible because the surrounding water provides support. The blood or other body fluid must be in intimate contact with the respiratory surface for ease of diffusion. [3]

A high surface area is crucial to the gas exchange of aquatic organisms, as water contains only a small fraction of the dissolved oxygen that air does. A cubic meter of air contains about 250 grams of oxygen at STP. The concentration of oxygen in water is lower than in air and it diffuses more slowly. In fresh water, the dissolved oxygen content is approximately 8 cm 3 /L compared to that of air which is 210 cm 3 /L. [4] Water is 777 times more dense than air and is 100 times more viscous. [4] Oxygen has a diffusion rate in air 10,000 times greater than in water. [4] The use of sac-like lungs to remove oxygen from water would not be efficient enough to sustain life. [4] Rather than using lungs, "[g]aseous exchange takes place across the surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism. The density of the water prevents the gills from collapsing and lying on top of each other, which is what happens when a fish is taken out of water." [4]

Usually water is moved across the gills in one direction by the current, by the motion of the animal through the water, by the beating of cilia or other appendages, or by means of a pumping mechanism. In fish and some molluscs, the efficiency of the gills is greatly enhanced by a countercurrent exchange mechanism in which the water passes over the gills in the opposite direction to the flow of blood through them. This mechanism is very efficient and as much as 90% of the dissolved oxygen in the water may be recovered. [3]

The gills of vertebrates typically develop in the walls of the pharynx, along a series of gill slits opening to the exterior. Most species employ a countercurrent exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other. The gills are composed of comb-like filaments, the gill lamellae, which help increase their surface area for oxygen exchange. [5]

When a fish breathes, it draws in a mouthful of water at regular intervals. Then it draws the sides of its throat together, forcing the water through the gill openings, so it passes over the gills to the outside. Fish gill slits may be the evolutionary ancestors of the tonsils, thymus glands, and Eustachian tubes, as well as many other structures derived from the embryonic branchial pouches. [ citation needed ]

Fish Edit

The gills of fish form a number of slits connecting the pharynx to the outside of the animal on either side of the fish behind the head. Originally there were many slits, but during evolution, the number reduced, and modern fish mostly have five pairs, and never more than eight. [6]

Cartilaginous fish Edit

Sharks and rays typically have five pairs of gill slits that open directly to the outside of the body, though some more primitive sharks have six pairs and the Broadnose sevengill shark being the only cartilaginous fish exceeding this number. Adjacent slits are separated by a cartilaginous gill arch from which projects a cartilaginous gill ray. This gill ray is the support for the sheet-like interbranchial septum, which the individual lamellae of the gills lie on either side of. The base of the arch may also support gill rakers, projections into the pharyngeal cavity that help to prevent large pieces of debris from damaging the delicate gills. [7]

A smaller opening, the spiracle, lies in the back of the first gill slit. This bears a small pseudobranch that resembles a gill in structure, but only receives blood already oxygenated by the true gills. [7] The spiracle is thought to be homologous to the ear opening in higher vertebrates. [8]

Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by rapidly swimming forward. In slow-moving or bottom-dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth. [7]

Chimaeras differ from other cartilagenous fish, having lost both the spiracle and the fifth gill slit. The remaining slits are covered by an operculum, developed from the septum of the gill arch in front of the first gill. [7]

Bony fish Edit

In bony fish, the gills lie in a branchial chamber covered by a bony operculum. The great majority of bony fish species have five pairs of gills, although a few have lost some over the course of evolution. The operculum can be important in adjusting the pressure of water inside of the pharynx to allow proper ventilation of the gills, so bony fish do not have to rely on ram ventilation (and hence near constant motion) to breathe. Valves inside the mouth keep the water from escaping. [7]

The gill arches of bony fish typically have no septum, so the gills alone project from the arch, supported by individual gill rays. Some species retain gill rakers. Though all but the most primitive bony fish lack spiracles, the pseudobranch associated with them often remains, being located at the base of the operculum. This is, however, often greatly reduced, consisting of a small mass of cells without any remaining gill-like structure. [7]

Marine teleosts also use their gills to excrete osmolytes (e.g. Na⁺, Cl − ). The gills' large surface area tends to create a problem for fish that seek to regulate the osmolarity of their internal fluids. Seawater contains more osmolytes than the fish's internal fluids, so marine fishes naturally lose water through their gills via osmosis. To regain the water, marine fishes drink large amounts of sea water while simultaneously expending energy to excrete salt through the Na + /K + -ATPase ionocytes (formerly known as mitochondrion-rich cells and chloride cells). [9] Conversely, fresh water contains less osmolytes than the fish's internal fluids. Therefore, freshwater fishes must utilize their gill ionocytes to attain ions from their environment to maintain optimal blood osmolarity. [7] [9]

Lampreys and hagfish do not have gill slits as such. Instead, the gills are contained in spherical pouches, with a circular opening to the outside. Like the gill slits of higher fish, each pouch contains two gills. In some cases, the openings may be fused together, effectively forming an operculum. Lampreys have seven pairs of pouches, while hagfishes may have six to fourteen, depending on the species. In the hagfish, the pouches connect with the pharynx internally and a separate tube which has no respiratory tissue (the pharyngocutaneous duct) develops beneath the pharynx proper, expelling ingested debris by closing a valve at its anterior end. [7] Lungfish larvae also have external gills, as does the primitive ray-finned fish Polypterus, though the latter has a structure different from amphibians. [7]

Amphibians Edit

Tadpoles of amphibians have from three to five gill slits that do not contain actual gills. Usually no spiracle or true operculum is present, though many species have operculum-like structures. Instead of internal gills, they develop three feathery external gills that grow from the outer surface of the gill arches. Sometimes, adults retain these, but they usually disappear at metamorphosis. Examples of salamanders that retain their external gills upon reaching adulthood are the olm and the mudpuppy.

Still, some extinct tetrapod groups did retain true gills. A study on Archegosaurus demonstrates that it had internal gills like true fish. [10]

Crustaceans, molluscs, and some aquatic insects have tufted gills or plate-like structures on the surfaces of their bodies. Gills of various types and designs, simple or more elaborate, have evolved independently in the past, even among the same class of animals. The segments of polychaete worms bear parapodia many of which carry gills. [3] Sponges lack specialised respiratory structures, and the whole of the animal acts as a gill as water is drawn through its spongy structure. [11]

Aquatic arthropods usually have gills which are in most cases modified appendages. In some crustaceans these are exposed directly to the water, while in others, they are protected inside a gill chamber. [12] Horseshoe crabs have book gills which are external flaps, each with many thin leaf-like membranes. [13]

Many marine invertebrates such as bivalve molluscs are filter feeders. A current of water is maintained through the gills for gas exchange, and food particles are filtered out at the same time. These may be trapped in mucus and moved to the mouth by the beating of cilia. [14]

Respiration in the echinoderms (such as starfish and sea urchins) is carried out using a very primitive version of gills called papulae. These thin protuberances on the surface of the body contain diverticula of the water vascular system.

The gills of aquatic insects are tracheal, but the air tubes are sealed, commonly connected to thin external plates or tufted structures that allow diffusion. The oxygen in these tubes is renewed through the gills. In the larval dragonfly, the wall of the caudal end of the alimentary tract (rectum) is richly supplied with tracheae as a rectal gill, and water pumped into and out of the rectum provides oxygen to the closed tracheae.

Plastrons Edit

A plastron is a type of structural adaptation occurring among some aquatic arthropods (primarily insects), a form of inorganic gill which holds a thin film of atmospheric oxygen in an area with small openings called spiracles that connect to the tracheal system. The plastron typically consists of dense patches of hydrophobic setae on the body, which prevent water entry into the spiracles, but may also involve scales or microscopic ridges projecting from the cuticle. The physical properties of the interface between the trapped air film and surrounding water allow gas exchange through the spiracles, almost as if the insect were in atmospheric air. Carbon dioxide diffuses into the surrounding water due to its high solubility, while oxygen diffuses into the film as the concentration within the film has been reduced by respiration, and nitrogen also diffuses out as its tension has been increased. Oxygen diffuses into the air film at a higher rate than nitrogen diffuses out. However, water surrounding the insect can become oxygen-depleted if there is no water movement, so many such insects in still water actively direct a flow of water over their bodies.

The inorganic gill mechanism allows aquatic insects with plastrons to remain constantly submerged. Examples include many beetles in the family Elmidae, aquatic weevils, and true bugs in the family Aphelocheiridae, as well as at least one species of ricinuleid arachnid. [15] A somewhat similar mechanism is used by the diving bell spider, which maintains an underwater bubble that exchanges gas like a plastron. Other diving insects (such as backswimmers, and hydrophilid beetles) may carry trapped air bubbles, but deplete the oxygen more quickly, and thus need constant replenishment.