Starfish Have No Blood ? They Run on Seawater

Starfish (or sea stars, if you want to be precise) have no blood. None at all. Instead, they circulate seawater through an elaborate internal plumbing system called the water vascular system. This salty fluid delivers nutrients, removes waste, and powers movement — all the things blood normally handles, except with considerably fewer red cells and far more ocean.

The system works by drawing seawater in through a tiny sieve-like plate on top of the starfish called the madreporite. From there, water travels through a network of canals and reaches hundreds of small tube feet on the underside of each arm. By controlling the pressure in those feet, the starfish can grip, move, pry open shellfish, and even flip itself right-side up. It is essentially hydraulics, but make it animal.

This also means that if you remove a starfish from seawater for too long, it quite literally runs dry. Their internal mechanics depend on a constant supply of the ocean itself. They are not just living in the sea — they are running on it.

Biologists have studied the water vascular system for centuries, and it still inspires modern soft-robotics research: imagine a robot that moves using fluid pressure alone, with no rigid joints whatsoever. The starfish worked all of that out roughly 500 million years ago, which is honestly showing off.