The animal that eats its own brain: Strange life of the sea squirt

This larval stage lasts 1-3 days, powered by phototaxis and rheotaxis to find suitable settlement sites. Studies using electron microscopy show that the larval brain contains ~200 neurons involved in sensory processing.

Then, something bizarre happens: upon settling headfirst via adhesive papillae, metamorphosis triggers autophagy—self-digestion of ~80% of the central nervous system, including most of the brain, tail, and notochord tissues within hours.

Why would an animal do that?

As an adult, it becomes a stationary filter feeder, staying fixed in one place and feeding by pulling in water. It uses tiny openings in its body called gill slits to filter out plankton and nutrients. In fact, it can pump over 100 times its own body volume in water every day.

Since it no longer moves or searches for food, it no longer needs a complex brain. And brains are expensive to maintain—they can use up a significant amount of energy. So instead, the sea squirt redirects that energy toward survival and reproduction, producing hundreds of eggs in a single cycle.

At a microscopic level, scientists have found that genes linked to brain development become much less active once the animal settles down. At the same time, the body increases production of enzymes that help break down and absorb the unused brain tissue.

Even after all this, the sea squirt isn’t completely without a nervous system. It retains a very simple set of nerve cells—just enough to handle basic reflexes, like closing up if something touches it.

So is the brain actually replaced?

It’s not exactly “replacing” the brain, but more like a one-time radical rewiring. The sea squirt builds a simple brain during its free-swimming larval stage, then absorbs it completely once it settles down, essentially shedding traits it briefly shared with more complex chordates, a group it diverged from around 550 million years ago. This unusual life cycle challenges the idea that brains are always preserved across related species. In a way, the sea squirt offers a striking lesson in efficiency: intelligence is not always necessary, it’s situational. Scientists have even found that if the animal is prevented from settling, this brain resorption is delayed, proving that it’s an adaptive response rather than a fixed timeline. In nature, survival isn’t about holding on to complexity; it’s about keeping only what you truly need.