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Oxygen deprivation is a death sentence for most vertebrates — even fish. Without it, the brain starts failing within minutes, cells lose their ability to generate energy and vital tissues suffer irreversible damage. Yet, there’s one small freshwater fish that routinely survives conditions that would kill almost any other animal.
This fish is the crucian carp (Carassius carassius), and it can live for weeks or even months without oxygen, enduring entire winters sealed beneath ice-covered lakes. Here’s how, according to research.
This Fish’s Life Beneath The Ice
As research from the Journal of Experimental Biology explains, crucian carp inhabit ponds and lakes across northern Europe and Asia, many of which freeze in the winter months. When this ice starts to form, oxygen exchange with the atmosphere stops in turn. And as microbes and other organisms continue to respire, dissolved oxygen levels drop to zero.
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By mid-winter, these waters become completely anoxic: no oxygen, no aerobic metabolism and no margin for error. Most fish will typically either migrate, suffocate or die. But not crucian carp; they somehow manage to remain active beneath the ice, sometimes for months, waiting for spring to come.
For most animals, oxygen serves as the final electron acceptor in mitochondrial respiration. Without it, the electron transport chain stalls, ATP production collapses and cells are forced into anaerobic glycolysis. This process yields very little energy, and it also rapidly produces toxic byproducts.
Prolonged anaerobic metabolism, for most mammals, will lead to lactic acid accumulation, tissue acidosis, organ failure and, eventually, death. Even the species that are considered hypoxia-tolerant cannot survive very long without oxygen. Crucian carp break this rule by doing something that no other vertebrate can do at scale: instead of allowing lactate to build up during anaerobic metabolism, crucian carp convert it into ethanol.
This remarkable pathway was first documented in a 1980 study from Science, and was later clarified through molecular and enzymatic studies. Specifically, the study explains that when oxygen disappears, crucian carp reroute glycolytic end products away from lactate accumulation and toward ethanol synthesis using modified mitochondrial enzymes.
In turn, crucian carp release this ethanol across their gills into the surrounding water. In other words, they essentially turn their toxic metabolic waste into a diffusible substance, which then simply leaks away. It’s the same biochemical logic that yeast uses during fermentation. Crucian carp are the only known vertebrates to rely on this strategy for long-term survival.
How Long Can These Fish Really Survive?
Ethanol production isn’t enough on its own. Even with this sophisticated waste removal, energy demands must be drastically reduced. Because of this, a crucian carp will undergo profound metabolic depression during anoxia, as a 2021 study from Metabolites explains:
- Its heart rate slows
- Its movement decreases
- Its growth halts
- All of its non-essential cellular processes are completely shut down
The study notes that their metabolic rate can drop to less than 10 percent of normal levels. In turn, ion channels in neurons are regulated in order to prevent excessive firing. Protein synthesis is also minimized, while ATP consumption is tightly rationed.
This coordinated shutdown allows the limited ATP generated through anaerobic pathways to sustain its essential cellular functions for months. In laboratory experiments at low temperatures, crucian carp have been observed to survive for over four months without oxygen. But in natural settings, this timeframe will depend on temperature, energy reserves and body size.
Cold water is essential. The lower the temperature, the lower the metabolic demand; this is what makes long-term anoxia possible. In warmer temperatures, crucian carp will exhaust their energy reserves more quickly. Still, no other vertebrate comes close to this level of anoxia tolerance.
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Why This Fish Matters
From an evolutionary perspective, the crucian carp’s strategy makes sense. Northern freshwater environments are unpredictable. Winterkill events, where oxygen drops to zero, are common. So, rather than competing for oxygen or migrating, crucian carp evolved a biochemical workaround. By tolerating conditions that kill competitors and predators, they gain exclusive access to resources once oxygen returns.
Because of this, crucian carp have become a model organism for studying oxygen deprivation, stroke and metabolic suppression. Understanding how their brains avoid excitotoxicity could inform treatments for ischemic injury in humans. Their ability to reversibly shut down metabolism offers clues for organ preservation and trauma medicine.
Although humans should hopefully never need to ferment themselves through oxygen deprivation, the cellular principles at work in crucian carp challenge many of the assumptions we’ve maintained about the limits of vertebrate physiology. For biologists, it serves as a reminder that evolution often solves problems not by incremental tweaks, but by radically re-engineering species once the environment demands it.
Stories like this fish’s reveal how deeply life adapts to its environment. Measure your own connection to nature with this science-backed test: Connectedness to Nature Scale
Not every survivor fights. Some adapt, like this fish. Find out which animal mirrors your personality with this fun, science-inspired test: Guardian Animal Test
NOTE – This article was originally published in Forbes and can be viewed here
