Common starfish's gruesome hunting trick: Forcing stomach through mouth to digest prey

Walk along a British rockpool at low tide and you'll likely spot one: a rust-colored creature the size of a dustbin lid, five arms splayed across the rocks like a starfish should be. It is a starfish, and it is hunting. What makes this ordinary-looking predator extraordinary is the mechanism it uses to kill and consume its prey—a feeding strategy so alien it seems designed by something other than evolution, yet it has worked without meaningful change for more than 450 million years.

The common starfish, widespread across the lowest reaches of the British and Irish shores, looks deceptively simple. Those five arms are lined with barbed spines sharp enough to cause serious pain. But the spines are not the creature's most effective weapon. That distinction belongs to something far stranger: a stomach it can push out of its own mouth.

To understand how this works, you first need to know what a starfish is not. It has no brain. It has no blood. Instead, it draws seawater into its body and uses that water as a transport system, moving food and oxygen through itself the way other animals use circulatory systems. It moves slowly across the seafloor, but this slowness masks a ruthless efficiency as a predator. Mussels and other bivalve molluscs are its preferred prey.

When a common starfish finds a mussel, it does something that looks almost gentle at first. It wraps its five arms around the shell and grips tight with thousands of tiny tube feet—suction-cup-like structures that line the underside of each arm. Then it pulls. The starfish does not try to smash the shell or pry it open with brute force in the way you might imagine. Instead, it applies steady, relentless pressure, using those thousands of feet to maintain tension until the mussel's own muscles—exhausted from fighting—finally give way and the shells crack open just enough.

What happens next is the part that stops most people cold. The starfish forces one of its two stomachs out through its mouth and into the narrow gap between the mussel's shells. Inside that gap, in the darkness, the stomach begins to digest the mussel's soft tissue. The process is not quick, but it is thorough. When the meal is finished, the starfish simply pulls its stomach back inside itself, retracts it through its mouth, and moves on to find another victim.

This method of feeding—extruding the stomach to digest prey from the inside out—is not some recent evolutionary innovation. It is not a clever adaptation that starfish developed in response to some environmental pressure. It is ancient beyond measure. The fossil record shows that starfish were using this exact same technique to hunt and eat more than 450 million years ago, long before dinosaurs walked the earth, long before most of the animals we think of as modern even existed. In all that time, in all those millions of years, the strategy has barely changed. Evolution found something that worked and left it alone.

The next time you find a starfish in a rockpool, you are looking at a predator that operates by rules so different from our own that it barely seems like hunting at all. No speed, no aggression, no drama. Just patience, pressure, and a stomach turned inside out. It is a reminder that nature's solutions to the problem of survival are far stranger than anything we might invent.