Almost every animal on Earth depends on oxygen to power its cells, but a few remarkable exceptions break this rule entirely. In 2020, researchers confirmed that a tiny parasite of salmon called Henneguya salminicola has completely lost the ability to use oxygen for energy, making it the first known animal that genuinely does not need to breathe. Beyond that extreme case, a surprising number of animals can survive days, weeks, or even months without any oxygen at all, using biochemical workarounds that scientists are only beginning to understand. The full picture of animal life and oxygen is stranger and more varied than most biology classes suggest.
The Animal That Threw Away Its Mitochondria
Every cell in your body contains mitochondria, the structures that use oxygen to produce energy. For a long time, biologists assumed every animal had them and used them. Then came Henneguya salminicola, a microscopic parasite that infects salmon muscle tissue. Using deep sequencing, researchers found that this organism has no mitochondrial genome at all and has lost nearly all the nuclear genes needed to replicate or transcribe one. It simply cannot perform aerobic cellular respiration. A closely related species, Myxobolus squamalis, tested with the same methods, does have a mitochondrial genome, confirming that H. salminicola specifically lost its own rather than the researchers making a technical error.1PubMed Central. A cnidarian parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome
H. salminicola belongs to the Myxozoa, a group within Cnidaria (the phylum that includes jellyfish and corals). These organisms are so reduced and specialized for parasitic life that they consist of just a handful of cells. The prevailing explanation is that because H. salminicola lives inside the tissue of its host, bathed in nutrients and shielded from the outside world, it no longer needed the expensive machinery of aerobic metabolism. Over evolutionary time, those genes degraded and disappeared. This is not just a theoretical loss; fluorescence imaging showed mitochondrial DNA lighting up in the related species but going dark in H. salminicola.
How does it get energy? The paper found that genes for other mitochondrial pathways were still present, suggesting the organism retains a remnant organelle that handles some functions but not oxygen-based energy production. It likely depends on anaerobic pathways or may even siphon energy directly from its host’s cells, though the precise mechanism remains an open question.
Multicellular Animals in Permanently Oxygen-Free Water
H. salminicola is a parasite embedded in another animal, so you might wonder whether any free-living animal can survive without oxygen. The answer appears to be yes, though the evidence comes from an environment so extreme that confirming it took extraordinary effort. In the L’Atalante basin at the bottom of the Mediterranean Sea, the water is so salty and dense that it forms a permanently anoxic brine lake on the seafloor. Researchers discovered three new species of Loricifera, tiny animals smaller than a grain of sand, living in the basin’s sediments. These organisms lacked mitochondria entirely and instead contained structures resembling hydrogenosomes, organelles that generate energy without oxygen. Using radioactive tracers, the team confirmed the loriciferans were metabolically active, not just dead bodies that had drifted down.2PubMed Central. The first metazoa living in permanently anoxic conditions
Hydrogenosomes have been well documented in single-celled organisms for decades. They evolved from ancestral mitochondria through reductive evolution: organisms living in low-oxygen environments gradually lost the genes for aerobic respiration, and the organelle shrank in function until it no longer needed oxygen at all.3PubMed Central. Diversity and reductive evolution of mitochondria among microbial eukaryotes Finding them in an animal, though, was a genuine shock. The Loricifera discovery and the H. salminicola findings together overturned the idea that aerobic respiration is universal in the animal kingdom. Independent lineages have converged on the same solution: ditch the oxygen-dependent parts of the mitochondrion and keep or replace only what you need.4PubMed Central. Convergent Evolution of Hydrogenosomes from Mitochondria by Gene Transfer and Loss
Vertebrates That Survive Months Without a Breath
The animals described so far are microscopic parasites or deep-sea oddities. But some familiar vertebrates can endure astonishingly long periods without any oxygen, even if they ultimately still require it over their full life cycle.
The western painted turtle holds the record among four-limbed vertebrates, surviving more than 170 days of anoxic submergence at 3°C. Painted turtles spend winters buried in the mud at the bottom of frozen ponds, where oxygen drops to zero. Their trick is partly chemical: the shell and skeleton release calcium and magnesium carbonates into the bloodstream while simultaneously absorbing lactate and hydrogen ions, preventing the lethal acid buildup that would kill most animals within minutes.5PubMed. Changes in the material properties of the shell during simulated aquatic hibernation in the anoxia-tolerant painted turtle Comparing several North American freshwater turtle species, researchers found that the more anoxia-tolerant species had higher shell carbonate concentrations and greater buffering capacity, suggesting that shell chemistry is a genuine adaptation for oxygen-free survival rather than a coincidence.6PubMed. Comparative shell buffering properties correlate with anoxia tolerance in freshwater turtles
Crucian carp and goldfish take a completely different approach. These fish can survive days to weeks in frozen-over Scandinavian lakes where dissolved oxygen plummets to zero. They evolved a pathway strikingly similar to brewer’s yeast: duplicated genes code for an enzyme that converts pyruvate to acetaldehyde, which is then turned into ethanol by a muscle-specific alcohol dehydrogenase. The ethanol diffuses out through the gills, preventing toxic buildup. In effect, the fish brew alcohol in their own muscles to stay alive.7PubMed Central. Extreme anoxia tolerance in crucian carp and goldfish through neofunctionalization of duplicated genes creating a new ethanol-producing pyruvate decarboxylase pathway During winter at cold temperatures, crucian carp also dramatically slow their metabolism, which reduces how much energy they need and stretches their anaerobic fuel supply further.8PubMed. Metabolic depression in winter-acclimatized crucian carp (Carassius carassius L.)
The Naked Mole-Rat’s Fructose Switch
Naked mole-rats live in crowded underground burrows in East Africa where oxygen levels drop well below what would kill a mouse. When oxygen runs out entirely, their brains switch to an emergency fuel: fructose. Most mammalian brains run exclusively on glucose, and when oxygen vanishes, glucose metabolism stalls because a key enzyme gets shut down by its own byproducts. Naked mole-rat tissues express high levels of a fructose transporter and the enzyme needed to feed fructose into the energy-production pipeline at a point that bypasses that bottleneck. The result is that their cells keep producing energy even when oxygen is unavailable.9PubMed. Fructose-driven glycolysis supports anoxia resistance in the naked mole-rat
This is not indefinite survival without oxygen. Naked mole-rats do need oxygen in the long run. But they can tolerate conditions that would cause fatal brain damage in other mammals within minutes, buying themselves time to reach better-ventilated parts of the burrow or wait for air quality to improve.
Tardigrades and the Art of Shutting Down
Tardigrades, often called water bears, are famous for surviving conditions that would destroy almost any other animal: vacuum, radiation, extreme temperatures. Oxygen deprivation is on that list. When oxygen vanishes, certain tardigrade species enter a state called anoxybiosis, essentially a reversible shutdown of normal metabolism. During this transition, their bodies physically swell; researchers have documented visible expansion in body length and volume within about 25 minutes of anoxic exposure.10bioRxiv. High resolution live imaging of tardigrade response to anoxia
But tardigrades are not invincible. In controlled experiments, two species showed strong recovery after 6 to 12 hours without oxygen, but survival dropped sharply after 24 hours. In one species, Ramazzottius cf. coronifer, a very high proportion returned to normal activity after 6 and 12 hours of hypoxia, but longer exposures were frequently lethal.11PubMed Central. An experimental study on tolerance to hypoxia in tardigrades So tardigrades tolerate oxygen loss for hours by essentially pausing, not by running on a different fuel. They are survivors, not true anaerobes.
Parasitic Worms and Low-Oxygen Living
Many parasitic worms spend part of their life cycle in the gut or tissues of a host animal, where oxygen can be scarce or absent. These organisms have adapted by retaining a molecule called rhodoquinone, which allows their mitochondria to run certain reactions in reverse compared to the typical aerobic pathway. This lets them use fumarate instead of oxygen as the final acceptor in their energy chain, a trick shared with the well-studied lab roundworm C. elegans.12PubMed. Rhodoquinone in bacteria and animals: Two distinct pathways for biosynthesis of this key electron transporter used in anaerobic bioenergetics In animals, the pathway for making rhodoquinone derives from tryptophan, an amino acid, through a route distinct from the bacterial version. This is a good example of convergent evolution: bacteria and animals arrived at the same molecule through completely different biochemical routes.
Some nematodes are remarkably durable under anaerobic conditions even without the parasitic lifestyle. The pine wood nematode Bursaphelenchus xylophilus, for instance, maintained survival rates above 60% after a full 14 days without oxygen in lab tests, while C. elegans and other species had zero survivors over the same period.13PubMed Central. Assessment of the behaviour and survival of nematodes under low oxygen concentrations Even within closely related animals, anoxia tolerance varies enormously.
How Animal Cells Detect Falling Oxygen
For the vast majority of animals that do need oxygen, the body has sophisticated systems for detecting when levels drop. In mammals, specialized cells in the carotid body, a tiny structure near the fork of the carotid artery in the neck, act as the body’s primary oxygen sensors. These glomus cells detect falling oxygen in the blood and trigger rapid responses: faster breathing, increased heart rate, and activation of the sympathetic nervous system.14PubMed Central. Molecular Mechanisms of Acute Oxygen Sensing by Arterial Chemoreceptor Cells. Role of Hif2α
At the cellular level, virtually all animal cells respond to low oxygen through a family of proteins called hypoxia-inducible factors, or HIFs. Under normal oxygen conditions, these proteins are rapidly broken down. When oxygen drops, HIFs stabilize and switch on a wide array of genes involved in cell survival, the growth of new blood vessels, and a shift from oxygen-dependent to oxygen-independent energy production.15PubMed Central. Cellular adaptation to hypoxia through hypoxia inducible factors and beyond The mechanism by which cells sense the change involves a chemical modification called hydroxylation: enzymes that require oxygen tag HIF proteins for destruction, so when oxygen disappears, the tagging stops and HIF accumulates.16PubMed. Hydroxylation of HIF-1: oxygen sensing at the molecular level This system is ancient and conserved across a huge range of animals, underscoring just how central oxygen is to animal life.
Human Populations Adapted to Thin Air
Humans are no exception to the rule that animals need oxygen, but some populations have evolved to thrive with considerably less of it than most people are used to. Tibetans, who have lived at elevations above 4,000 meters for thousands of years, carry genetic variants in the HIF pathway, specifically in the genes HIF2A (also called EPAS1) and PHD2 (EGLN1), that alter how their bodies respond to chronic low oxygen.17PubMed Central. Human high-altitude adaptation: forward genetics meets the HIF pathway Rather than producing excess red blood cells the way most people do at altitude, which thickens the blood and raises the risk of stroke, Tibetans maintain relatively normal hemoglobin levels.
Sherpas, many of whom share Tibetan ancestry, show additional metabolic differences. Muscle biopsies reveal a lower capacity for fat burning but greater efficiency in how oxygen is used, better muscle energetics, and stronger protection against oxidative stress. These changes are linked in part to a variant of the PPARA gene that is more common in Sherpas than in lowland populations.18PubMed Central. Metabolic basis to Sherpa altitude adaptation The lesson is not that these people do not need oxygen; they absolutely do. Their bodies have simply become more efficient at extracting and using the oxygen available in air that contains roughly 40% less of it than at sea level.
Oxygen and the Origin of Animals
The deep relationship between oxygen and animal life goes back to the very beginning. The Cambrian explosion, the burst of animal body plans that appeared roughly 540 million years ago, has long been linked to rising oxygen levels in the oceans and atmosphere. Research on ocean chemistry across the Ediacaran-Cambrian transition shows that fluctuations in ocean oxygenation closely tracked the appearance and disappearance of major animal groups, with more oxygenated intervals corresponding to radiations of new species and oxygen crashes corresponding to extinctions.19Nature Communications. Coupling of ocean redox and animal evolution during the Ediacaran-Cambrian transition
One mechanism linking oxygen to animal diversification is carnivory. Predation is energy-intensive, and the aerobic metabolism that oxygen enables produces far more energy per unit of food than anaerobic alternatives. The emergence of oxygen levels sufficient to support active predators may have kicked off an ecological arms race: prey evolved shells, spines, and burrowing behavior, while predators evolved speed, jaws, and more complex nervous systems. The recognition of this physiological control on who could eat whom provides an integrated explanation for both the pattern and timing of Cambrian animal diversification.20PubMed Central. Oxygen, ecology, and the Cambrian radiation of animals
What Anoxia-Tolerant Animals Could Teach Medicine
When a human has a heart attack or stroke, the damage is not caused only by the loss of oxygen. A large part of the injury happens when oxygen returns, in a process called reperfusion injury. Returning oxygen generates a burst of reactive molecules that tear through cell membranes and proteins. Animals that routinely survive anoxia seem to have evolved ways to prevent this. Painted turtle hearts, for example, maintain a stable ratio of energy molecules and accumulate very little succinate during oxygen deprivation, and succinate is one of the key triggers of reperfusion damage in mammalian hearts.21Scientific Reports. Metabolic adaptations during extreme anoxia in the turtle heart and their implications for ischemia-reperfusion injury
More broadly, the strategies these animals use, including metabolic suppression, protective protein activation, and maintenance of mitochondrial function during oxygen starvation, overlap with pathways involved in human diseases where oxygen delivery fails. Researchers studying anoxia-tolerant species have argued that understanding how their cells survive without oxygen could improve treatments for conditions involving hypoxia or oxidative stress.22PubMed Central. Forever young: mechanisms of natural anoxia tolerance and potential links to longevity Mitochondria from turtles, crucian carp, and other tolerant species have become model systems for studying how to keep human mitochondria from self-destructing during and after oxygen loss.23PubMed. Mitochondria from anoxia-tolerant animals reveal common strategies to survive without oxygen
The naked mole-rat’s fructose pathway has attracted particular attention. If human brain cells could be coaxed into using fructose as a backup fuel during a stroke, even briefly, the window for treatment could expand significantly. That work remains early-stage, but the fact that an existing mammal already uses the trick makes it biologically plausible rather than purely theoretical.
Insects and the Problem of Breathing Dry Air
Insects face a unique oxygen challenge that has nothing to do with low supply. They breathe through a network of tubes called tracheae that deliver air directly to tissues, and every time those tubes open, water escapes. Many insects have evolved a breathing pattern where spiracles, the external openings of the tracheal system, remain shut for extended periods, opening only in controlled bursts. Research across multiple insect species found that the duration of these gas-exchange cycles increases in hotter, drier habitats, supporting the idea that this discontinuous breathing pattern primarily functions to reduce water loss while still getting enough oxygen in.24PubMed Central. Evolutionary responses of discontinuous gas exchange in insects Insects need oxygen just as urgently as any vertebrate, but they have had to balance that need against the constant threat of drying out.
Vent Animals and the Oxygen Paradox
Deep-sea hydrothermal vents are sometimes described as oxygen-free oases of life, but the reality is more nuanced. The giant tube worms, mussels, and shrimp that cluster around vents do live near scalding, anoxic fluid pouring from the seafloor. However, these animals are not anaerobic. They position themselves precisely at the boundary where oxygen-rich deep-sea water mixes with the vent fluid. Their symbiotic bacteria oxidize hydrogen sulfide or methane using that available oxygen (or sometimes nitrate) and feed the host with the energy produced.25Trends in Microbiology. Chemosynthetic symbioses The animals still need oxygen; they have just found a way to pair it with chemical energy sources rather than sunlight-derived food. It is a reminder that proximity to anoxic environments does not mean independence from oxygen.