Sea Star Anatomy & Regeneration

Sea stars — often called starfish, even though they are not fish — are echinoderms, a group of spiny-skinned ocean animals that also includes sea urchins and sand dollars. A sea star has no head and no brain. Instead, its body is built around a central point with five (or more) arms, called rays, arranged in a wheel-like pattern. This body plan is known as radial symmetry.

The top of a sea star is the aboral surface. It is covered with bumpy spines that protect the animal. The underside is the oral surface, because the mouth sits right in the center. Running down the middle of each arm on the oral side is a groove packed with hundreds of tiny tube feet. These tube feet are powered by water, not muscle alone. Seawater enters through a small button-like plate on the aboral side called the madreporite and flows through a network of canals to push the tube feet in and out.

In the bays of Long Island Sound, the common sea star Asterias forbesi is a fierce predator of clams, mussels, and oysters — animals called bivalves. The sea star grips a clam with its tube feet, slowly pulls the shell open, then pushes its own stomach out through its mouth and into the shell to digest the soft body right inside. For a slow-moving animal with no teeth, it is a remarkably effective way to hunt.

One of the most amazing abilities of a sea star is regeneration — the regrowth of a lost body part. If a predator such as a crab or a gull bites off an arm, the sea star can slowly grow it back. Sea stars can even drop an arm on purpose to escape an attacker, a self-defense trick called autotomy.

Regrowing an arm is slow work. It can take many months, and the new arm grows only a few millimeters at a time. In many species, a single severed arm can grow into a whole new sea star — but only if it keeps part of the central disc attached. An arm with a piece of the disc that is busy regrowing four small arms is nicknamed a "comet," because it looks like a bright point trailing tiny rays.

In the early 1900s, oyster and clam farmers around Long Island Sound were losing fortunes to sea stars. Beds of Asterias forbesi swept across the shellfish grounds, prying open oysters faster than the farmers could grow them. The farmers fought back by dragging mops and chains across the bottom to scoop up the sea stars.

At first, many fishermen did something that seemed to make sense: they chopped the captured sea stars into pieces and threw them back into the water, sure they had killed the pests. They were wrong. Because a sea star arm carrying part of the central disc can regenerate into a whole new animal, cutting them up sometimes multiplied the population instead of shrinking it. The very ability that fascinates biologists made the sea star a far worse enemy of the oyster beds.

Eventually the farmers learned the science. Instead of cutting the sea stars, they hauled them onto land to dry in the sun, dipped them in boiling water, or spread quicklime over the beds — methods that actually killed the animals and stopped them from coming back.