Life Inside Animal Buttholes — Nature’s Strangest Hidden Homes

A Home Like No Other

Nature has always surprised us with its strange and creative ways of survival. But sometimes, the truth is stranger than fiction. In the deep sea, there are creatures that live in places so unexpected, so bizarre, that many people refuse to believe it at first. Among the weirdest examples? Some animals actually live inside the back ends of other animals — specifically, inside their rectal or cloacal cavities.

This is not a myth, not a joke, and definitely not made up. It’s a real biological phenomenon that scientists have studied for years. And it works — because in the animal kingdom, the best place to survive is not always the prettiest.

The Pearlfish and Its Very Specific Address

One of the best-known creatures to practice this lifestyle is the pearlfish, a thin, silvery fish that lives inside sea cucumbers. Yes — you read that correctly. It enters through the sea cucumber’s rear opening and makes itself at home in the soft internal cavity.

During the day, the pearlfish stays hidden inside, avoiding predators. At night, it may swim out to find food before returning to safety. This is a highly efficient strategy that keeps it alive in the harsh, open sea.

Most pearlfish don’t hurt the sea cucumber. They just use its body for protection. That kind of relationship is called commensalism — where one animal benefits and the other is unaffected. Some pearlfish species even do this with tunicates (sea squirts) or starfish, showing that this behavior isn’t rare — it’s just hidden.

The Sea Cucumber’s Open-Door Policy

So why sea cucumbers? Why not any other animal?

Sea cucumbers are soft, slow, and have a very special kind of anatomy. They breathe by taking in water through their cloaca — a shared opening used for both breathing and waste. Inside, the cavity is moist, full of oxygen, and large enough for a small fish to move around. For the pearlfish, this is perfect.

Some sea cucumbers can tighten their cloaca to prevent intruders. But others simply tolerate their unusual guest, even when more than one fish enters.

When the Rear Becomes a Nursery

For some species, these spaces aren’t just safe houses — they’re nurseries. Certain worms and tiny shrimp lay eggs inside these cavities, taking advantage of the stable temperature and protection the host provides.

Even small crustaceans like amphipods use these rear cavities as long-term homes. In the dark, enclosed space, their eggs are protected from currents, predators, and unstable conditions.

Not All Guests Are Harmless

While many species live peacefully inside their hosts, there are always exceptions. Some pearlfish have been observed biting or feeding on the internal tissue of their hosts. In those cases, the relationship becomes parasitic — one animal benefits, while the other is harmed.

Still, most of these relationships remain commensal. Sea cucumbers don’t show signs of stress, even when two or three pearlfish share the same body cavity. It might be strange to us, but it’s normal for them.

The Evolutionary Genius of It All

Living inside another animal’s body sounds outrageous to humans, but for evolution, it makes sense. It’s about survival, not comfort.

This strategy offers:

• Shelter from predators

• Constant moisture and temperature

• Flowing oxygen (in marine animals)

• Access to floating food particles

• A space with little competition

That’s why this has evolved in over 70 pearlfish species worldwide. These fish even have narrow, flexible bodies designed to slide in and out with ease.

Why Land Animals Don’t Do This

On land, this strategy doesn’t really exist. Here’s why:

• There’s no water flow to carry oxygen inside body cavities

• Most land animals move too much and have tighter anatomy
• There’s higher risk of injury for both the host and the intruder

Insects like carrion beetles may enter animal carcasses through openings to lay eggs, but no land animal is known to live inside another animal’s rear as a permanent home.

A Reminder That Nature Doesn’t Care What We Think

To us, it might sound disgusting. But nature doesn’t care how we feel. It only cares about survival. These animals are not doing this because they’re weird — they’re doing it because it works.

They don’t have shame or discomfort. They just follow instinct. What we find gross might actually be a brilliant evolutionary strategy.

Conclusion: Strange, Smart, and Entirely Real

Yes — some animals really do live inside other animals’ rear ends. But they aren’t monsters or mistakes. They’re survivors, using the safest, most reliable shelter they can find.

Nature has no rules about what’s "normal" — it only rewards what works. And sometimes, the most unexpected homes turn out to be the smartest.

So the next time you think, “That’s gross,” remember: to them, it’s just home.