Only one animal has been confirmed to survive its entire life without any oxygen at all: a tiny parasite of salmon called Henneguya salminicola, which has completely lost the cellular machinery for aerobic respiration. But the question opens up quickly from there. Dozens of animal species can endure days, weeks, or even months without oxygen by switching to alternative metabolic strategies, and a handful of deep-sea species may spend their entire lives in permanently oxygen-free waters. The spectrum from “technically still needs a little oxygen” to “genuinely oxygen-independent” turns out to be far more crowded than most people realize.
The Salmon Parasite That Ditched Oxygen Entirely
Henneguya salminicola is a myxozoan, a group of microscopic parasites that belong to the same broad animal phylum as jellyfish and corals. In 2020, researchers using deep-sequencing techniques confirmed that this organism has no mitochondrial genome whatsoever and has lost nearly all of the nuclear genes normally responsible for copying and reading mitochondrial DNA.1PubMed Central. A cnidarian parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome In plain terms, the part of the cell that almost every other animal uses to burn fuel with oxygen simply does not exist in this species. It is the first and, so far, only animal known to have permanently abandoned aerobic respiration.
How it actually generates energy is still being worked out. The researchers found that H. salminicola retains some mitochondria-like structures, but these organelles no longer function as oxygen-consuming powerhouses. The parasite lives embedded inside the muscle tissue or cysts of its salmon host, where nutrients are abundant and the metabolic heavy lifting has essentially been outsourced. Over evolutionary time, it shed the genes it no longer needed, much the way cave-dwelling animals lose their eyes. The discovery overturned a long-standing assumption in biology that aerobic respiration is a universal feature of animal life.1PubMed Central. A cnidarian parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome
Loricifera in the Deep Sea
Before Henneguya made headlines, the most dramatic example of oxygen-free animal life came from the floor of the Mediterranean Sea. In 2010, researchers reported finding three new species of Loricifera, a phylum of tiny animals barely visible to the naked eye, living in the sediments of the L’Atalante basin. This basin is both hypersaline and completely devoid of oxygen, conditions previously thought to exclude all multicellular life.2PubMed Central. Anaerobic metazoans: no longer an oxymoron
What made the finding especially striking was the cellular evidence. These loriciferans lacked normal mitochondria altogether. Instead, they possessed organelles resembling hydrogenosomes, structures more commonly associated with single-celled organisms that thrive without oxygen, along with symbiotic bacteria living inside their cells.3PubMed Central. The first metazoa living in permanently anoxic conditions Radioactive tracer experiments indicated the animals were metabolically active, not just dead specimens that had drifted down from oxygenated water above.
The loriciferan findings remain somewhat contested. Some scientists have questioned whether the evidence fully rules out the possibility that the specimens were in a dormant or dying state. But the original team’s biochemical data, including signs of active metabolism and reproduction, was robust enough to reshape the conversation about where animals can and cannot live. At a minimum, these creatures demonstrate that the boundary between “needs oxygen” and “does not need oxygen” is blurrier than textbooks once taught.
Parasitic Worms and Their Anaerobic Lifestyles
If you expand the lens from “animals that never use oxygen” to “animals that can go long stretches without it,” parasitic worms dominate the list. Helminths, the large group that includes roundworms, tapeworms, and flukes, routinely inhabit parts of a host’s body where oxygen is scarce or essentially absent: the gut lumen, bile ducts, deep tissues. Their metabolic flexibility is extraordinary.
Parasitic flatworms are a good case study. Because they lack a circulatory system, gas exchange happens by passive diffusion through their body wall. This creates a steep oxygen gradient: the outer surface may see moderate oxygen levels from the host’s tissues, while the interior runs mostly anaerobic.4PubMed Central. Evolutionary Adaptations of Parasitic Flatworms to Different Oxygen Tensions The worm’s body essentially operates in two metabolic zones at once, aerobic on the outside and anaerobic at the core.
Many parasites go further, actively choosing anaerobic pathways even when some oxygen is available. The roundworm Ascaris suum, a common pig parasite, and the malaria parasite Plasmodium falciparum both rely on a modified version of the mitochondrial enzyme complex II, which runs in reverse compared to the way it works in a host mammal. Instead of feeding electrons into an oxygen-consuming chain, the enzyme acts as a fumarate reductase, enabling energy production without oxygen.5PubMed. Role of complex II in anaerobic respiration of the parasite mitochondria from Ascaris suum and Plasmodium falciparum This is not a stopgap; it is the primary energy pathway for these organisms during key life stages.
A special molecule called rhodoquinone is central to how many of these worms pull off anaerobic respiration. Animals like Caenorhabditis elegans (a free-living nematode often used in labs) and parasitic helminths synthesize rhodoquinone from the amino acid tryptophan, using a pathway that diverges from the one their hosts use.6PubMed Central. Hydrogen Sulfide Induces Oxidative Damage to RNA and DNA in a Sulfide-Tolerant Marine Invertebrate Rhodoquinone allows the electron transport chain to use fumarate instead of oxygen as the final electron acceptor, keeping ATP production going when oxygen drops to zero.
Bivalves That Toggle Between Modes
Mussels, oysters, and clams are not the first animals that come to mind when you think about surviving without oxygen, but many species in this group are remarkably good at it. Estuarine and intertidal bivalves regularly face low-oxygen conditions, whether from tidal exposure, sediment burial, or seasonal dead zones. Rather than dying, they shift their metabolism from aerobic to anaerobic pathways, accumulating end products like alanine and succinic acid in their tissues.7PubMed Central. Estuarine bivalve metabolic response mediated by environmental drivers
Not all bivalves handle this equally well. Pacific oysters (Crassostrea gigas) appear to be among the more tolerant species, showing a blunted inflammatory and cell-death response to oxygen deprivation lasting up to six days.8PubMed. Effects of intermittent hypoxia on cell survival and inflammatory responses in the intertidal marine bivalves Mytilus edulis and Crassostrea gigas Blue mussels (Mytilus edulis), by contrast, ramp up stress and cell-death signals much faster under the same conditions. Intertidal species that evolved with the daily rhythm of tidal exposure tend to handle the oxygen-on, oxygen-off cycle better than subtidal species that rarely encounter it.9PubMed. Species-specific metabolic responses to intermittent hypoxia in marine bivalves revealed by (1)H-NMR metabolomics
The trick for bivalves is not just surviving the low-oxygen period but managing the return of oxygen afterward. When oxygen floods back into tissues, it can generate a burst of damaging reactive molecules. The species that tolerate hypoxia best are often the ones that handle this reoxygenation phase most gracefully, clearing the metabolic waste products accumulated during the anaerobic stint without triggering widespread cell damage.
Vertebrates That Push the Limits
True oxygen independence has not been found in any vertebrate. But a few fish and at least one shark come strikingly close, surviving hours or even months in water with almost no dissolved oxygen.
Crucian carp and goldfish, both members of the genus Carassius, are the champions of vertebrate anoxia tolerance. They survive prolonged periods of complete oxygen deprivation by converting the end product of sugar breakdown into ethanol, which they then excrete through their gills. This is essentially the same chemistry that yeast uses to make alcohol, repurposed by a fish.10PubMed Central. Extreme anoxia tolerance in crucian carp and goldfish through neofunctionalization of duplicated genes creating a new ethanol-producing pyruvate decarboxylase pathway The ethanol pathway prevents the toxic buildup of lactic acid that would kill most vertebrates under the same conditions. In Scandinavian lakes that freeze over completely in winter, crucian carp can survive for months beneath the ice in water with effectively zero oxygen.
The epaulette shark (Hemiscyllium ocellatum), a small bottom-dwelling species from Australian reef flats, takes a different approach. Rather than producing ethanol, it suppresses its own metabolism dramatically during low-oxygen events, reducing energy demand to match the meager supply. Research has shown that its brain blood flow stays steady even after two hours of severe hypoxia, whereas blood pressure drops by about half, suggesting a compensatory widening of blood vessels in the brain.11PubMed Central. Succinate-mediated reactive oxygen species production in the anoxia-tolerant epaulette (Hemiscyllium ocellatum) and grey carpet (Chiloscyllium punctatum) sharks The epaulette shark’s mitochondria also produce far less of the harmful reactive oxygen species that typically surge when oxygen returns after a period of deprivation. Its closely related cousin, the grey carpet shark, tolerates similar conditions but uses a different strategy and generates roughly twice as much oxidative damage in the process.11PubMed Central. Succinate-mediated reactive oxygen species production in the anoxia-tolerant epaulette (Hemiscyllium ocellatum) and grey carpet (Chiloscyllium punctatum) sharks
Tardigrades, Nematodes, and Other Tiny Survivors
Tardigrades, those famously indestructible microscopic animals, can tolerate a remarkable range of extremes, but oxygen deprivation is not actually their strongest suit. When oxygen drops, tardigrades enter a state called anoxybiosis, a form of suspended animation. However, experimental work has shown that survival drops sharply with longer exposure: most tardigrades tested failed to recover after 24 hours in severely low-oxygen conditions.12PubMed Central. An experimental study on tolerance to hypoxia in tardigrades Their impressive tolerance to drying, freezing, and radiation does not translate into the same kind of metabolic flexibility seen in parasitic worms or deep-sea loriciferans.
Among nematodes, there is wide variation. The pine wood nematode Bursaphelenchus xylophilus can survive more than 14 days under completely anaerobic conditions, with over 60% of individuals still alive at the end, whereas the lab workhorse C. elegans reaches total mortality well before that mark.13PubMed Central. Assessment of the behaviour and survival of nematodes under low oxygen concentrations Interestingly, the survivors among B. xylophilus tended to be young larvae rather than adults, hinting that developmental stage plays a role in anoxia tolerance. The practical significance is real: B. xylophilus is a destructive forest pest, and its ability to survive oxygen-deprived environments complicates efforts to kill it during wood treatment and transport.
Worms That Farm Their Own Bacteria
Some of the most creative solutions to oxygen-poor living involve partnerships. Gutless marine oligochaetes, small worms found in ocean sediments, have lost their digestive systems entirely and instead harbor a consortium of bacterial symbionts beneath their skin. These bacteria include both sulfide-oxidizing and sulfate-reducing species that pass sulfur compounds back and forth between them in a tightly coordinated cycle.14PubMed Central. Symbiosis reshapes metabolism of sulfate-reducing bacteria in gutless marine worms
The worm Olavius crassitunicatus, found in oxygen-depleted sediments off the coast of Peru, hosts at least five distinct bacterial types simultaneously, including two gamma-proteobacteria, two delta-proteobacteria, and a spirochete.15PubMed Central. Coexistence of bacterial sulfide oxidizers, sulfate reducers, and spirochetes in a gutless worm (Oligochaeta) from the Peru margin The sulfide-oxidizing bacteria provide the worm with organic carbon (essentially food), while the sulfate-reducing bacteria regenerate the sulfide the oxidizers need. The worm provides the real estate. This arrangement lets the worm thrive in sediments where oxygen is negligible and where most animals could not feed themselves even if they could breathe.
Why Oxygen Became the Norm in the First Place
If some animals can get by without oxygen, why did aerobic metabolism become so dominant? Part of the answer is historical. Rising oxygen levels in the oceans and atmosphere during the Ediacaran and Cambrian periods, roughly 600 to 500 million years ago, appear to have paced the diversification of early animal life.16Biogeosciences. Oxygen increase and the pacing of early animal evolution Animals that could exploit oxygen had access to a more energetically efficient way of burning fuel, which supported larger bodies, more complex tissues, and faster movement.
But the advantage is not as lopsided as it sounds. While oxygen allows cells to extract more energy from each molecule of sugar, building and maintaining cellular biomass is actually cheaper in the absence of oxygen because oxygen is so reactive. The very property that makes it a good electron acceptor also makes it destructive, constantly threatening to oxidize the molecules cells are made of. Some researchers have estimated that maintaining a cell costs roughly thirteen times more energy per cell in an oxygen-rich environment than in an anoxic one, simply because of the constant repair work needed to counteract oxidative damage.17PubMed Central. Energy metabolism in anaerobic eukaryotes and Earth’s late oxygenation Oxygen is a tradeoff, not a universal upgrade.
Drug Development Targeting Anaerobic Worm Metabolism
The metabolic differences between oxygen-dependent hosts and their oxygen-independent parasites represent a practical opportunity. Because parasitic helminths rely so heavily on glycolysis and anaerobic pathways like malate dismutation for energy, those pathways are targets for new drugs. Existing anthelmintics already work partly by disrupting glucose uptake and mitochondrial function in the worms, and researchers are now developing more specific inhibitors aimed at the glycolytic enzymes and fumarate reductase systems that the parasites depend on but that human cells do not use in the same way.18PubMed. Decoding the metabolic checkpoints in helminths: Opportunities for novel anthelmintic drug development
This matters because drug resistance is a growing problem in livestock and human helminth infections worldwide. If researchers can identify and block metabolic steps that are essential for the worm but irrelevant to the host, the resulting drugs should be both more effective and less toxic. The anaerobic quirks of parasitic worm metabolism, once merely a curiosity in comparative physiology, are now central to the pipeline for next-generation treatments.
Living in Toxic Brines
Surviving without oxygen is often just one piece of a larger puzzle. Many oxygen-free environments are also laden with hydrogen sulfide, a gas that poisons mitochondrial respiration in most animals. Species that inhabit these zones need to deal with sulfide toxicity on top of oxygen deprivation. The marine worm Urechis caupo, for example, tolerates sulfide concentrations that would rapidly kill most other invertebrates, but not without cost: exposure causes dose-dependent oxidative damage to both RNA and DNA in the worm’s cells, even though the animal survives and continues to function.6PubMed Central. Hydrogen Sulfide Induces Oxidative Damage to RNA and DNA in a Sulfide-Tolerant Marine Invertebrate
The deep-sea loriciferans, the gutless oligochaete worms, and many intertidal bivalves all face versions of this double challenge. It is rarely enough to simply turn off the oxygen-burning pathway; the animal also needs defenses against the chemical hazards that come with the environments where oxygen is absent. Understanding how these animals manage that balancing act continues to be an active area of research, with relevance to fields from conservation biology to the search for life on other worlds where oxygen may never have been part of the equation.