Why can fish survive out of water?

Why can fish survive out of water?

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Obviously, humans can survive underwater for a short while. I've assumed this is because at some point it benefited us to stop breathing momentarily, and so we evolved a respiratory system that was able to take short breaks.

Similarly, fish can survive outside of water for a short while. However, I can't think of a single instance for most fish where they may have needed to evolve that ability. Even their ancestors originated from aquatic environments, so I don't think it's a residual ability they no longer need.

Why did fish evolve a respiratory system that could take breaks like ours can?

Do not have too a "panselectionist" view of evolution!

You can survive in a bath of mercury for a little while. You can survive naked in the outer space for a while (see here). Yet none of your ancestors where exposed to such conditions. We can be tolerant to certain conditions without having been selected to tolerate it.

Similarly, you managed to survive in your very specific environment. Yet none of your ancestors ever encountered this specific environment.

In other words, evolution is more than just natural selection. Not every phenotype in every environment you can think of is the result of a direct selective pressure acting on it. A classical and easy to read paper on the subject is Gould and Lewontin (1979).

Why fish not die suddenly outside of the water?

The main reason why fish die outside of water is that they cannot intake oxygen from air (see Breathing under water; not considering lungfish). So once fish are exposed to air only, they stop intaking oxygen, consume the oxygen that they have left in their circulatory system and tissues and slowly die of asphyxie but there is no reason for the death to be direct and sudden.

The answer is simple. A fishes gills are exposed to air that is oxygen rich. Gills can take oxygen from the air. We do not inhale water, so humans under water must hold their breath. I am assuming the question is not referring to lungfish, who have modified their swim bladders (homologous to our lungs) to take up oxygen.

Correct that they cannot take in "sufficient" amounts, but a bit is all it takes them from asphyxia for an hour or so. The question was not why can't fish live their entire lives out of water, but why they don't die immediately.

Actually there is a huge group of fish that need to breath air, swamp living fish, stagnant water is very low in oxygen that is why many fish gulp air (air contains a lot more available oxygen than stagnant water) even house goldfish will do this is poorly oxygenated bowls (see below). in fact the early ancestors of terrestrial vertebrates show a lot of adaptations for swamp living, legs also work better in swamps than fins (at least above a certain size) that is why so many swamp fish develop leg like fins, such as the snakehead fish. These are not adaptations for land but for shallow stagnant water, air gulping for hypoxic water in particular.

Note most of these fish are not lungfish (or lobe finned fish) they are ray finned fish, they are using their swim bladder to extract oxygen. Aquatic surface respiration is actually a very widespread behavior in fish, and can be triggered in a very large number of ray finned fish. The behavior is quite well studied and has lead to better developed air breathing multiple times.

Fish that cannot do this (like sharks) will die very quickly on land as the suffocate, since gills don't work in the air. The risk for normal fish is dehydration of their breathing tissue, of while they are IN water this has a easy solution.

1. Do Fish Actually Need Oxygen

As humans we need oxygen to breathe fish are no different. Instead of taking oxygen from the air like us, they collect dissolved oxygen from the water as they swim.

There’s a lot more oxygen in the air than there is in water. This makes it much easier to get oxygen when living on land, but fish have evolved gills to let them survive in water.

    absorb a large amount of salt while respiring because it’s in the water. Consequently, they need a specialized mechanism to remove excess salt from their body.
  • Freshwater fish have the opposite problem. There isn’t much salt in the water, so they need a mechanism to hold on to as much of it as they can.

Fish need oxygen for the same reason as humans. Without it they’ll struggle to respire, and eventually die.

Oxygen gets combined with other elements to create proteins and even form new cells.
When breaking down food in the body, oxygen is used to convert and store its energy. This energy is used in all active processes within the body.

Fish don’t need quite as much oxygen as us since they’re cold blooded. Warm blooded animals need extra energy to keep the body heated. Fish don’t need this energy, so they can survive with less food and oxygen.

There’s a finite amount of oxygen in a body of water. The larger the body of water, the larger surface area and as a result the more oxygen.

In an aquarium this can be a big problem. There’s only a small amount of water, meaning it can quickly run out of oxygen. This is just one reason why tank size is so important.

Aquarists can use plants to introduce a some oxygen into the tank as they photosynthesize. Air pumps are useful too as they bring oxygen depleted water up to the surface, forming a small circulation.

Podcast: How do mRNA vaccines work and why were they developed so fast?

G enetics Unzipped is back for 2021 with a new series of stories from the world of genes, genomes and DNA, from the history of genetics to the latest cutting-edge research. In the first episode geneticist Dr Kat Arney takes a look at the discovery of messenger RNA (mRNA) and finds out how mRNA has been pressed into service as a COVID-19 vaccine at breakneck speed.

There were some big names involved in the discovery of mRNA in the 1960s – Francis Crick, Sydney Brenner, Francois Jacob and more – but who actually discovered this vital molecular messenger? And why did nobody win a Nobel Prize for it?

As scientist and author Matthew Cobb explains: “Who discovered mRNA? It is complicated. No wonder the Nobel Prize committee did not try and reward the discovery. Naming just three (or even six) people would be invidious — mRNA was the product of years of work by a community of researchers, gathering different kinds of evidence to solve a problem that now looks obvious, but at the time was extremely difficult.”

From the 1960s we come right up to the present day to look at mRNA vaccines for COVID-19, which have been developed at breakneck speed to tackle the pandemic. We explore the key breakthroughs that turned the languishing field of mRNA therapeutics into a game-changing medical technology, take a closer look at how mRNA vaccines work and why they were developed so fast for COVID-19, and explore how this new technology might change the face of immunization and public health in the future.

Evolution: Out Of The Sea

Thursday 26th July saw the launch of, a new English language science blog network., the brand-new home for Nature Network bloggers, forms part of the SciLogs international collection of blogs which already exist in German, Spanish and Dutch. To celebrate this addition to the NPG science blogging family, some of the NPG blogs are publishing posts focusing on “Beginnings”.

Participating in this cross-network blogging festival is’s Soapbox Science blog, Scitable’s Student Voices blog and bloggers from,, Scitable and Scientific American’s Blog Network. Join us as we explore the diverse interpretations of beginnings – from scientific examples such as stem cells to first time experiences such as publishing your first paper. You can also follow and contribute to the conversations on social media by using the #BeginScights hashtag. – Bora

In the beginning, the earth was without form, and void and darkness was upon the face of the deep, as a giant cloud of gas and dust collapsed to form our solar system. The planets were forged as the nebula spun, jolted into motion by a nearby supernova, and in the center, the most rapid compression of particles ignited to become our sun. Around 4.5 billion years ago, a molten earth began to cool. Violent collisions with comets and asteroids brought the fluid of life - water - and the clouds and oceans began to take shape. It wasn't until a billion years later that the first life was brought forth, filling the atmosphere with oxygen.

Over the next few billion years, single-celled organisms fused and became multicellular body plans diversified and radiated, exploding into an array of invertebrates. Yet all this abundance and life was restricted to the seas, and a vast and bountiful land sat unused. Around 530 million years ago, there is evidence that centipede-like animals began to explore the world above water. Somewhere around 430 million years ago, plants and colonized the bare earth, creating a land rich in food and resources, while fish evolved from ancestral vertebrates in the sea. It was another 30 million years before those prehistoric fish crawled out of the water and began the evolutionary lineage we sit atop today. To understand life as we know it, we have to look back at where we came from, and understand how our ancestors braved a brand new world above the waves.

It was a small step for fish, but a giant leap for animalkind. Though, looking at modern fish species, it's not so hard to envision the slow adaptation to life out of the sea. Just the other day, I was feeding my pet scorpionfish Stumpy, and he surprised me with this slow, deliberate crawl towards his food:

A number of fish exhibit traits which are not unlike those of the first tetrapods: the four-limbed vertebrates that first braved life on land, direct descendants of ancient fish. Many of Stumpy's relatives, including the gurnards, are known for their "walking" behaviors. Similarly, mudskippers have adapted anatomically and behaviorally to survive on land. Not only can they use their fins to skip from place to place, they can breathe through their skin like amphibians do, allowing them to survive when they leave their shallow pools. Walking catfishes have modified their respiratory system so much that they can survive days out of water. But all of these are only glimpses at how the first tetrapods began, as none of these animals has fully adapted to life on land. To understand how tetrapods achieved such a feat, we must first understand the barriers that lay between their life under the sea and the land above that awaited them.

Living in air instead of water is fraught with difficulties. Locomotion is one problem, though as evolution in a number of lineages has shown, not as big a problem as you might think. Still, while mudskippers and catfish seem to walk with ease, the same cannot be said of our ancestors. Some of the earliest tetrapods, like Ichthyostega were quite cumbersome on land, and likely spent most of their time in the comfort of water. These first tetrapods came from an ancient lineages of fishes called the Sarcopterygii or Lobe-Finned Fish, of which only a few survive today. As the name implies, these animals have meaty, paddle-like fins instead of the flimsy rays of most modern day fish species. These lobe fins, covered with flesh, were ripe for adapting into limbs.

But these early tetrapods had to develop more than a new way to walk - their entire skeletons had to change to support more weight, as water supports mass in a way that air simply doesn't. Each vertebrae had to become stronger for support. Ribs and vertebrae changed shape and evolved for extra support and to better distribute weight. Skulls disconnected, and necks evolved to allow better mobility of the head and to absorb the shock of walking. Bones were lost and shifted, streamlining the limbs and creating the five-digit pattern that is still reflected in our own hands and feet. Joints articulated for movement, and rotated forward to allow four-legged crawling. Overall, it took a long 30 million years or so to develop a body plan fit for walking on land.

At the same time, these cumbersome wanna-be land dwellers faced another obstacle: the air itself. With gills adept at drawing oxygen from water, early tetrapods were ill-equipped to breathing air. While many think that early tetrapods transformed their gills into lungs, this actually isn't true - instead, it was the fish's digestive system that adapted to form lungs. The first tetrapods to leave the water breathed by swallowing air and absorbing oxygen in their gut. Over time, a special pocket formed, allowing for better gas exchange. In many fish, a similar structure - called a swim bladder - exists which allows them to adjust buoyancy in the water, and thus many have hypothesized that tetrapod lungs are co-opted swim bladders. In fact, exactly when tetrapods developed lungs is unclear. While the only surviving relatives to early tetrapods - the lungfishes - also possess lungs (if their name didn't give that away), many fossil tetrapods don't seem to have them, suggesting that lungfish independently evolved their ability to breathe air. What we do know is that it wasn't until around 360 million years ago that tetrapods truly breathed like their modern descendants.

The other trouble with air is that it tends to make things dry. You may have heard the statistic that our bodies are 98% water, but, as well-evolved land organisms, we have highly evolved structures which ensure that all that water doesn't simply evaporate. The early tetrapods needed to develop these on their own. At first, like the amphibians that would arise from them, many tetrapods likely stuck to moist habitats to avoid water loss. But eventually, to conquer dry lands and deserts, animals had to find another way to keep themselves from drying out. It's likely that many of the early tetrapods began experimenting with ways to waterproof their skin. Even more important was the issue of dry eggs. Amphibians solve the dryness issue by laying their eggs in water, but the tetrapods which conquered land didn't have that luxury.

The solution to land's dry nature was to encase eggs in a number of membrane layers, in what is now known as an amniote egg. Even our own children reflect this, as human babies still grow in an amniotic sac that surrounds the fetus, even though we no longer lay eggs. This crucial adaptation allowed animals to cut ties with watery habitats, and distinguishes the major lineage of tetrapods, including reptiles, birds and mammals, from amphibians.

These crucial adaptations to tetrapod skeletons and anatomy allowed them to conquer the world above the waves. Without their evolutionary ingenuity, a diverse set of animals, including all mammals, would not be where they are today. Yet still we barely understand the ecological settings that drove these early animals out of the sea. Did dry land offer an endless bounty of food not to be passed up? Perhaps, but there is evidence that our ancestors braved the dry world very early on, even before most terrestrial plants or insects, so it's possible earth was barren. Were they escaping competition and predation in the deep? Or was land important for some yet undetermined reason? We may never know. But as we reflect upon our beginnings, we have to give credit to the daring animals that began the diverse evolutionary lineage we are a part of. While we may never understand why they left the water, we are thankful that they did.

Other Posts in the Evolution Series:

Photo: A model of Tiktaalik rosea, one of the earliest tetrapod ancestors. Photo courtesy of Tyler Keillor.

Invasion of the Snakeheads

The scene is a sheriff’s office near a mountain lake, where a hunter and his dog have been found dead. The sheriff sets a bright orange hunting vest on his desk in front of an anxious woman. She nods, identifying it as her husband’s. “He loved that dog,” she says, crying.

“Listen, Norma,” the sheriff says. “If there’s anything at all that I can do, you tell me.”

“You can find the animal that did this and send it straight to hell. You can do that.”

The culprit in the Sci Fi Channel’s made-for-TV movie Snakehead Terror turns out to be a lakeful of monster fish. This star turn is fitting for the toothy “Frankenfish” that has generated many hair-raising newspaper and television news stories—the northern snakehead.

In addition to inspiring filmmakers, the snakehead’s appearance in North American waters in the past few years has worried wildlife biologists and commercial and sport fishermen. They fear that it will invade new rivers, multiply rampantly and edge out other species.

The northern snakehead is native to Asia and is one of 29 snakehead species. It made its national news debut in 2002, after an angler at a pond behind a strip mall in Crofton, Maryland, caught a long, skinny fish, about 18 inches from end to end, that neither he nor his fishing buddy recognized. They photographed the fish before throwing it back a month later, one of them took the picture to the Maryland Department of Natural Resources (DNR). An agency biologist e-mailed the picture to fish experts, who told Maryland it had a snakehead on its hands.

It was after another angler caught a snakehead in the same pond and netted some babies that all hell broke loose. National newspaper and TV news reports described snakeheads as vicious predators that would eat every fish in a pond, then waddle across land to another body of water and clean it out. A reporter from the Baltimore Sun called it “a companion for the Creature from the Black Lagoon.” The scariest reports, fortunately, turned out to be mistaken. While some species of snakeheads can indeed wriggle long distances across the ground, the northern snakehead—the only species found in the Crofton pond—appears not to be one of them. But northern snakeheads do like to eat other fish, and a heavy rain could conceivably wash one or more from the pond into a nearby river that runs through a National Wildlife Refuge and into the Chesapeake Bay, the largest estuary in North America. To eliminate the snakehead menace, Maryland wildlife officials dumped the pesticide rotenone into the Crofton pond, killing all of its fish. Six adult snakeheads went belly up—as did more than 1,000 juveniles. Problem solved. Or so it appeared.

Two years later, northern snakeheads fulfilled biologists’ worst fear and showed up in the Potomac River. Experts worried that snakeheads in the Potomac, by eating other fish or out-competing them for food, could drive down numbers of more desirable species, such as shad or largemouth bass. You can dump poison in a little, enclosed pond, but you can’t poison the Potomac. It’s a wide, shallow river that originates in West Virginia and runs 380 miles before emptying into the Chesapeake. The bay fuels the region’s economy through recreation and fishing. Snakeheads couldn’t survive in the mildly salty water of the bay, but they could scarf down shad, fish that spawn in the Potomac and other freshwater tributaries. Millions of dollars have already been spent on fish stocking, dam modifications and other projects to help the shad, which used to be plentiful enough to support a commercial fishery in the bay.

Besides Crofton and the Potomac, the fish have popped up in several other places in the United States. In 1997, one was caught in a Southern California lake. A couple more appeared in Florida waters in 2000. In Massachusetts, one was caught in 2001 and a second in 2004. And in July 2004, an angler caught two in a lake in a Philadelphia park. Like the Crofton fish, the Philadelphia ones had settled in and started reproducing. But unlike the Crofton fish, they had access to a river—the Schuylkill, which feeds into the Delaware. Moreover, tidal gates that normally keep fish in the park had been stuck open for two years. Philadelphia fisheries managers decided that poisoning or draining the park’s interconnected ponds would cause more harm to resident fish than the snakeheads would, and have resigned themselves to snakeheads becoming a new member of the park’s ecosystem. The most recent surprise appearance was this past October when a northern snakehead was pulled out of Lake Michigan. The catch has raised fears that the voracious predator might take over the Great Lakes.

The northern snakehead, which is native to parts of China, far eastern Russia and the Korean peninsula, may seem plug-ugly to the undiscerning eye—it has big, pointy teeth and, given its particularly heavy mucus covering, a slime problem. It can grow up to five feet long. Like its reptilian namesake, it’s long and slender and can sport blotchy snakelike patterns on its skin. Unlike most fish, the northern snakehead has little sacs above its gills that function almost like lungs the fish can surface and suck air into the sacs, then draw oxygen from the stored air as it swims. The air sacs are handy for surviving in waters that are low in oxygen, and even allow the fish to survive out of water for a couple of days, as long as it doesn’t dry out. A female lays thousands of eggs at a time, and both parents guard their offspring in a large nest they make in a clearing of aquatic plants.

Northern snakeheads are a popular food in their native range they’re said to be good eating, particularly in watercress soup, if a bit bony. They’re fished commercially and raised in fish farms in Asia. They’ve also been sold live in markets in the United States. The Crofton snakeheads were eventually traced to a Maryland man who’d bought two of the fish in New York City for his sister to eat. When she demurred, he kept them in his aquarium and later released them. The U.S. Fish and Wildlife Service soon banned the importation and interstate transport of snakeheads, a plan that had already been in the works precisely because of fears that some snakehead species could thrive in parks, rivers and lakes if they got loose. The ban made it illegal to import all live snakehead species, including the colorful tropical species that populate the odd aquarium. Virginia has outlawed the possession of all snakeheads.

But the bans haven’t stopped everyone. A Los Angeles grocer was arrested this past May for allegedly smuggling live northern snakeheads into the country from Korea and selling them in his store he pleaded guilty to importing an injurious species. U.S. fans of snakehead soup and other delicacies, however, may still legally obtain killed, frozen snakeheads, which are available in many of the Asian markets that once sold them live.

One day this past April, an angler caught a feisty northern snakehead in Pine Lake, in Wheaton, Maryland, outside Washington, D.C. Local officials drained the lake but found no more snakeheads. Then, like an ecological game of Whac-a-Mole, another northern snakehead reared its toothy head the very next week when a professional bass fisherman pulled a 12 1/2-incher from Little Hunting Creek, a Potomac tributary in Virginia about 15 miles south of the nation’s capital. Biologists tried using nets to capture snakeheads in the river, but eventually decided that a better way would be to let anglers go at the fish with plain old hooks and lines—which led to one of the odder fishing tournaments in recent memory.

On an overcast Friday morning in July, I joined a few dozen anglers at Columbia Island Marina in Arlington, Virginia, across a narrow channel from the Pentagon. The 2004 Snakehead Roundup was about to get under way. The roundup was sponsored by the Marina Operators Association of America to remind boat owners to take care not to transport unwanted species from one place to another—as hitchhikers on their boats or trailers, for example—and to let them know what northern snakeheads look like. Although 16 adult snakeheads had been caught in the Potomac by that time, no one knew whether they’d been born there or whether someone had just tossed them in—or even how common they were.

I tagged along in a 19-foot white-and-blue ski boat with three managers from a family-owned company whose boss didn’t seem to mind that the information technology division was running itself that day. “We’re conducting an offsite meeting,” software designer Brian Turnbull explained. Turnbull’s father-in-law, who is Vietnamese, asked him to bring a snakehead home. “He says if you catch one, you don’t have to hand it over to the state. It’s a delicacy.” Fortunately, Turnbull wasn’t required to choose between duty to family or to society because he didn’t catch a snakehead. Neither did anyone else on the boat, and neither, we found out when we later pulled up at the marina, did anyone else in the roundup.

A few weeks later, John Odenkirk, a biologist from the Virginia Department of Game and Inland Fisheries, seemed to be imitating the sheriff in Snakehead Terror, who kills his murderous lakeful of snakeheads by electrocuting them with a downed power line. Odenkirk, driving an aluminum boat through Dogue Creek, a Potomac tributary, was “electrofishing,” which involved running about 1,000 volts through a boom that protruded from the bow and trailed wires in the water like tentacles. “High voltage . . . The next best thing to explosives,” read the small print on the back of Odenkirk’s green “Snakehead Task Force” T-shirt, which he designed to sell to colleagues for $12 apiece.

Electrofishing, a common sampling method in fisheries research, isn’t meant to kill fish. But it may knock them out for a while. (It’s not considered sporting and requires a special permit.) Odenkirk nosed the boat in and out of the empty slips at the Mount Vernon Yacht Club a couple of miles downriver from Little Hunting Creek. Tiny fish leapt out of the water as others lolled gracelessly on their backs, stunned, just below the surface. Biologist Steve Owens and technician Scott Herrmann leaned over the bow clutching long-handled nets. Afish’s response to the electrical current depends on its skeletal structure, scales, size and how close it is to the wires. “Snakeheads are—they’re kind of bad-asses,” Odenkirk said. “They don’t like the juice and they try to avoid it.” Still, a snakehead that got close to the trailing wires would be stunned and surface, for Herrmann or Owens to snag. At least, that was the theory. We sped back up the Potomac past Mount Vernon to Little Hunting Creek, where the first Potomac snakehead was caught by a fisherman back in May. At the end of an hour and a half of electrofishing, the catch included many carp, several species of catfish, a bunch of goldfish, a long-nosed gar, a turtle— and zero snakeheads. Odenkirk said he’s always conflicted after an unsuccessful day of snakehead fishing. On the one hand, he said, he was disappointed he’d failed to catch one. On the other, “you’d be happy if you never saw one again.”

Though we didn’t see any snakeheads that day, Odenkirk says he’s sure the fish is established in the Potomac or soon will be. “It’s just not even an option that we’ve caught them all.” He says the fish probably nest in wide, shallow expanses of lily pads and wetlands. “We just can’t get back in those areas.”

But other officials say they’re not convinced the fish are here to stay. Steve Early, assistant director in the fisheries service at the DNR, worked on the Crofton pond in 2002 and has handled some of the Potomac snakeheads. He thinks the fish were only very recently dumped in the river, perhaps after Virginia’s 2002 ban on snakehead ownership. He points out that most of the snakeheads caught this year have been 2 to 6 years old, and that if they’d been living in the Potomac for years, surely someone would have caught one before. Early remained unpersuaded even after a baby snakehead was found in a Potomac tributary this past September. It was the 20 th northern snakehead caught in the Potomac watershed, and the first juvenile. “Well, it’s not good news,” he says of the discovery, but points out that if some snakeheads do manage to reproduce, they may never thrive in the big river. Their future also depends on whether other fish in the Potomac develop a taste for snakehead fry.

For now, scientists are working on figuring out how the adults got there. It’s a critical question—if the fish were just recently dumped in the river, there’s a chance they’ll die without having generated a self-sustaining population—but it will require more than a rod and reel or a stun gun to answer.

Behind a door at the National Museum of Natural History in Washington, D.C. rest specimens from the world’s largest fish collection. Smithsonian ichthyologist Thomas Orrell walked down an aisle between rows of gray metal shelves containing jars with labels such as “China 1924.” Orrell held up a jar marked Channa argus, the northern snakehead. “They’re really beautiful fish,” he said.

Orrell is trying to learn if the northern snakeheads caught this past summer in the Potomac were born there. He’s analyzing DNA from 16 fish if some of the Potomac specimens are closely related, it’s likely that the fish bred in the river. If they’re not kin, they were likely dumped in the river. Orrell is also comparing the DNA of Potomac fish with that of those caught in the Crofton pond, testing the idea that someone might have captured juveniles before the pond was poisoned and released them in the Potomac.

Orrell led me down a bare stairwell into the museum’s basement, past sandbags piled near an entrance in case of heavy rain and a walk-in freezer that smelled of long-dead fish, containing, among other things, an enormous tuna frozen since the 1960s. He lifted the top of a nearby freezer chest, rooted around and pulled out a long, black lump. “Watch out for flying debris,” he said, unwrapping a black garbage bag and scattering pieces of frozen blood. Inside was one of the most recent Potomac catches: a dark, diamond-patterned snakehead more than a foot long, now solid as a rock. After showing it off, Orrell shrugged, wrapped it up, laid it back in the freezer and washed his hands. He already knows whether the snakeheads are reproducing in the Potomac, but he isn’t telling adhering to scientific protocol, Orrell declines to share his data until they’ve been reviewed by other experts and published in a scientific journal.

If northern snakeheads do have some ecological impact in the Potomac, largemouth bass are likely to suffer, says U.S. Geological Survey fishery biologist Walter Courtenay, who in 2002 wrote a snakehead risk assessment for the agency. The two species have similar habitats and would probably eat each other’s young. Capt. Steve Chaconas, one of only a few full-time fishing guides on the Potomac, does not like snakeheads one bit. “Of course, I’m worried about what potential it could have to impact the fishery,” he says. “Also because I’m a businessperson and my business relies entirely on people coming here to fish.” Even now, he says, customers ask how much the snakeheads have hurt fishing. It’s hard to estimate the extent of the snakehead’s impact on largemouth bass and other Potomac species. The northern snakehead was introduced to rivers in Japan in the early 20th century, but there has been little study of its ecological effects there. (The largemouth bass, native to North America, was introduced to Japanese waters in 1925 and is reportedly terrorizing native fish and snakeheads alike.)

In southern Florida, a close relative of the northern snakehead, the bullseye or cobra snakehead, has been living for a few years in the canals of BrowardCounty. The fish, which is native to rivers in South Asia and Southeast Asia, can grow to four feet or longer, but there are not yet enough data to know what effect the bullseye snakehead has had or will have on Florida ecology. Courtenay says the fish probably first got into Florida waters through ritual animal release, a common practice in East Asia that some immigrants have continued in their new land. (A study conducted in Taiwan in the 1990s, for instance, found that 30 percent of Taipei citizens— most of them Buddhists—had released animals as part of a prayer.)

Florida is home to dozens of introduced fish. Paul Shafland, a fisheries scientist with the Florida Fish and Wildlife Conservation Commission, has worked with invasive fish for 30 years, but he isn’t as troubled by them as most biologists. “We have philosophically, largely determined that exotics are inherently bad, and that’s fine,” he says. But, he adds, some introduced fish might fill up some part of the food web that was previously unoccupied.

In fact, introduced fish are just about everywhere. Rainbow trout, native to the western United States, have been transplanted into cold waters all over the Midwest and East. In the Great Smoky MountainsNational Park, on the border between Tennessee and North Carolina, rainbows have taken over at least 70 percent of the native brook trout’s territory since the 1930s. In the late 1960s, the walking catfish, an Asian species that really can move over land, escaped into the Florida wild. They’ve walked their way into warm waters throughout the southern half of the state, without causing major damage so far, Shafland says.

Lake Michigan, says Philip Willink, an ichthyologist at Chicago’s FieldMuseum, is also infested with nonnative fish. “Out of eight species of salmon here, six are introduced,” Willink says. But, as in the Potomac, some native fish still hang on in the lake, and he says it’s worth fighting new invasions. “We’re just trying to preserve what is left, because once it’s gone, it’s gone.” Since the Lake Michigan snakehead was found in a fairly deep harbor with little vegetation—an unlikely snakehead habitat—Willink surmises that the fish was probably just tossed into the water. Scientists did some electrofishing in the harbor to look for more snakeheads but didn’t turn up any.

How do gills work?

As water passes over or is pumped over the gills, oxygen is absorbed by through the walls of the secondary lamellae and CO2 is released. The secondary lamellae contain blood with low levels of oxygen. As water flows over the lamellae oxygen is asborbed into the blood and then the blood pumped around the body by the fish’s heart. The large surface area of the secondary lamellae is also helpful for exchanging body heat, ions and water between the fish’s body and the surrounding water.

Having so many tiny secondary lamellae creates an enormous surface area for oxygen to be absorbed through. This is helped further by the fact that secondary lamellae have thin walls so gas can be absorbed into the blood stream easier. Dissolved oxygen is found in much lower concentrations in water than it is in air so gills need to be far more efficient with their absorption than lungs do.

Do you ever wonder what happens to the fish in a frozen lake?

It is winter in the Northern Hemisphere. The vicious cold has transformed the scattered blue lakes of the North Woods into white disks — barren wastelands of ice. The harsh winds rushing across the icy plains combined with average air temperatures that are just above freezing seem to offer a less-than-hospitable refuge for wildlife.

But a keen sportsman knows better. Cutting a hole in the ice and dropping a colorful lure down into the depths of the lake, a patient ice fisher knows that luck is on her side. Obscured from human eyes underneath the ice lies a healthy stock of fish, tantamount to populations in the warmest months of the year.

“They survive just fine under the ice,” says Jake Vander Zanden, Director of the University of Wisconsin–Madison Center for Limnology. “They are adapted to survive in these low temperatures it’s not that big of a deal.”

Fish survive quite well in the winter because they evolved experiencing the annual changes that take place in the Northern latitudes, which include big changes in temperature and the availability of oxygen throughout the seasons.

In the summer months, the water at the surface of a freshwater lake is heated by the sun, while the water at the bottom of the lake remains colder. Because cold water is more dense, it gets “locked in,” stuck underneath the warmer, less dense water.

As the months move by and the weather gets colder, the lake slowly moves toward an even temperature. Once the temperatures match between layers, the density differential dissipates and the water column flips over, in a process called fall mixing. The same mixing process happens again in the spring once the ice melts and the winds can churn the waters once again.

Following the fall water cycle, water temperatures across the lake reduce and the lake surface freezes. Because fresh water is maximally dense at 4 C, or 39.2 F, the water at temperatures below 4 C actually rise to the top of the water column, making the bottom layer the warmest, and the most attractive habitat for certain fish species to survive in during the winter.

Freshwater fish are “poikilotherms” that cannot regulate their body temperature except by their own actions, like swimming or basking. They are divided into two categories, warmwater and coldwater species.

“An example of a warmwater fish is a bass, they have their optimal temperature conditions on the warmer side,” Vander Zanden says. “They might be just found at around 25 C (77 F) and above, whereas coldwater fish may have their optimal conditions at 10 C (50 F).”

Outside of their optimal temperature range, fish must make adjustments to survive. One of the most common ways that fish adjust to the winter temperatures is by decreasing movement, thereby slowing down their metabolism to conserve energy, and diminishing their need to hunt or forage. And certain fish, like some species of catfish , will actually burrow into the soft silt down on the lake bed to stay warm.

“ Many fish species are low energy during the winter, they’re sitting there not moving around very much, and not feeding at all,” says Vander Zanden. “But, if you’re a fish swimming around, you still might get eaten by another fish.”

“The same predator prey interactions are happening under the ice,” he adds.

The consistency in the food chain under the ice assures that ice fishermen can secure a catch, knowing that hungry fish will be attracted to their lures. But food is one side of the survival coin for fish. On the other side are their oxygen needs.

“ From the perspective of a fish or any organism that needs oxygen, the aquatic environment is not a great place to be, because oxygen is in really low abundance in aquatic systems versus air,” Vander Zanden says.

When the water is not covered by ice, oxygen from the air is readily cycled into the water. But once that icy lid is placed over the top of the lake, that process largely stops. Some amount of oxygen is replenished through the photosynthesizing plants that survive under the ice, although light cannot get through the ice when heavy snow is packed on top. Underneath the ice, fish consume an ever-decreasing supply of oxygen.

According to Vander Zanden, Lake Mendota presents some additional challenges for fish looking for oxygen.

Due to farming runoff and pollution, algal blooms form and sink to the bottom of the lake. In the winter as the blooms decompose under the ice, the process sucks precious oxygen out of the water. An area where a larger mass of the blooms is decomposing can become anoxic, or oxygen-starved.

“We never have [anoxic events] in the atmosphere, we never say, ‘Oh yea, my backyard went anoxic today,’ that doesn’t happen,” laughs Vander Zanden. “But it is an issue for fish, so fish have a lot of adaptations for extracting oxygen from their environment.”

According to Vander Zanden, fish can extract oxygen through a variety of adaptations, not only through their gills. Different fish species do this either by absorbing oxygen into their skin, into the blood vessels in the walls of their swim bladders, stomach and gut, and some even inhale the air bubbles that form underneath the ice through their mouth.

However, sometimes anoxic events become too widespread for the fish populations to escape. When an entire lake becomes oxygen starved, winter-kill events take place. As the anoxic zone creeps upwards into the water column, fish cling to the under-surface of the ice as the oxygen is depleted, until they suffocate to death. This can lead to some alarming sights, like this photograph captured after a winter-kill event in South Dakota.

These fish suffocated in an anoxic zone in the Lake Andes National Wildlife Refuge in South Dakota, floating towards the surface and eventually getting trapped in the ice. When the ice was pushed up against shore it buckled, exposing these icy remains. (Taken by Kelly Preheim)

Winter-kill events are more common in lakes much smaller than Mendota, says Vander Zanden, where the volume of water makes those events unlikely.

“The fish are there in the fall and they are there again in the spring,” Vander Zanden says. “The whole food web is alive and kicking in the winter.”

Dissolved oxygen

The availability of dissolved oxygen in the water of a lake is vital for supporting lake ecosystems. Aquatic plants make oxygen available for animals and microbes through photosynthesis. Bacteria can deplete oxygen supplies which can lead to the death of fish, invertebrates and other organisms that depend on oxygen to survive.

Nutrient rich waters are at increased risk of becoming oxygen deprived. High levels of nutrients can support large algal blooms but as the algae dies, they are decomposed by bacteria which can use up all the available dissolved oxygen. The result can lead to mass death and a complete desolation of the ecosystem.


  • A lake is an area of land filled with water.
  • They are the largest source of available freshwater.
  • Lakes can be found on all continents including Antarctica.
  • They are hugely important as a source of freshwater, as a habitat, and for their recreational and cultural significance.
  • A lake can be formed in a variety of ways such as by volcanic eruptions, land slides, depressions carved out by glaciers and tectonic movement.
  • Salt lakes can form when a lake has no outlet.
  • Different concentrations of nutrients make lakes eutrophic, mesotrophic and oligotrophic. Nutrient levels affect the plants, animals and microorganisms that can survive in a lake.
  • The presence of dissolved oxygen in a lake's water is vital for supporting a healthy ecosystem.

Last edited: 16 January 2016

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Basic Biology: An Introduction

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How Do Fish Breathe Underwater?

The air-breathing lungs of mammals, including humans, must be dry and empty of fluids to work properly. When we take a breath, tiny air sacs in our lungs pull oxygen out of the air and carry it to our bodies' cells.

The lungs of mammals would not work very well for a fish, because one breath underwater would fill them with fluid and make them useless. Nonetheless, fish need oxygen to breathe, too. In order to remove oxygen from the water, they rely on special organs called "gills."

Gills are feathery organs full of blood vessels. A fish breathes by taking water into its mouth and forcing it out through the gill passages. As water passes over the thin walls of the gills, dissolved oxygen moves into the blood and travels to the fish's cells.

If fish can breathe underwater, then why do some fish, like dolphins and whales, swim to the surface of the ocean? Because dolphins and whales aren't fish at all! They are mammals, just like humans.

Dolphins and whales are similar to humans in many ways: They give birth to live babies instead of laying eggs, are warm-blooded and have lungs for breathing air. When a whale or dolphin surfaces, it breathes air through its nose (commonly called a "blowhole") on the top of its head.

  • There are more species of fish than all the species of amphibians, reptiles, birds and mammals combined.
  • Fish have been on the earth for more than 450 million years.
  • The largest fish is the great whale shark, which can reach 50 feet in length.

Wonder Contributors

jon and Jojo from AL
for contributing questions about today’s Wonder topic!

Gill Arches

Most fishes have three or more gill arches on each side of the body. These support the gill filaments and are cartilaginous or bony and shaped like a boomerang. Each gill arch consists of an upper and a lower limb that is joined in the back. Gill filaments and gill rakers are attached to the gill arches.

The gill arches offer support for the gills as well as the blood vessels.   Arteries that enter the gills bring blood with low oxygen and a high concentration of wastes. Arteries that leave the gills contain blood with little waste that's rich with oxygen.

Watch the video: How To DESTROY Algae in 30 Seconds Get Rid Of Aquarium Algae FAST (August 2022).