We often think of oxygen as the one thing no living creature can survive without. After all, humans and most animals need to breathe to stay alive.
But did you know that some animals have evolved to survive in places where oxygen is almost nonexistent?
These animals can be found in some of the harshest environments on Earth, like the deep sea, muddy swamps, or even inside other animals’ bodies.
Their unique adaptations make them some of the most extraordinary beings on our planet.
1. Henneguya salminicola
Henneguya salminicola is a tiny parasite that lives inside salmon. What’s fascinating is that it doesn’t have mitochondria—the part of a cell that uses oxygen to create energy. Instead, it absorbs nutrients directly from its host fish, allowing it to survive without breathing. This discovery amazed scientists and proved that not all animals rely on oxygen to live.
2. Tardigrades
Also known as water bears, tardigrades are microscopic creatures famous for surviving in extreme conditions. While they normally use oxygen, they can survive without it by going into a state called cryptobiosis. In this state, tardigrades curl up into a tiny ball, losing almost all their water content, and essentially “pause” their life processes until conditions improve. This ability helps them survive in space, underwater, or anywhere oxygen levels drop.
3. Loricifera
Loricifera are tiny creatures that live in the deep, dark oceans where there’s no oxygen. These animals have found a way to survive by using hydrogen, instead of oxygen, to produce energy. They thrive in the mud of oxygen-free seabeds, proving that life can adapt to even the most hostile places on Earth.
4. Spiny-headed worms
These parasites live inside the intestines of their hosts, like birds, fish, and mammals. Since the intestines are an oxygen-free environment, these worms have evolved to absorb nutrients directly from their hosts’ food. This unique lifestyle means they don’t need oxygen at all, and they can live happily in places where most other animals cannot survive.
5. Hydra
Hydras are tiny freshwater animals related to jellyfish and sea anemones. While they usually use oxygen, they can survive in low-oxygen environments by switching to anaerobic respiration. This allows them to create energy without needing oxygen for short periods. Their simple bodies and unique biology make them incredibly resilient.