A Guide to Starfish

Starfish are star-shaped invertebrates that can be a variety of shapes, sizes, and colors. You might be most familiar with starfish that live in tide pools in the intertidal zone, but some live in deep water.

Classification

• Kingdom: Animalia
• Phylum: Echinodermata
• Class: Asteroidea

Background

Even though they are commonly called starfish, these animals are known more scientifically as sea stars. They do not have gills, fins, or even a skeleton. Sea stars have a tough, spiny covering and a soft underside. If you turn over a live sea star, you'll likely see its hundreds of tube feet wiggling.

There are over 2,000 species of sea stars, and they come in all sizes, shapes, and colors. Their most noticeable characteristic is their arms. Many sea star species have five arms, but some, like the sun star, can have up to 40.

Distribution

Sea stars live in all the world's oceans. They can be found in tropical to polar habitats, and from deep to shallow water. Visit a local tide pool, and you may be lucky enough to find a sea star!

Reproduction

Sea stars may reproduce sexually or asexually. There are male and female sea stars, but they are indistinguishable from one another. They reproduce by releasing sperm or eggs into the water, which, once fertilized, become free-swimming larvae that later settle to the ocean bottom.

Sea stars reproduce asexually by regeneration. A sea star can regenerate an arm and nearly its entire body if at least a portion of the sea star's central disc remains.

Sea Star Vascular System

Sea stars move using their tube feet and have an advanced water vascular system that they use to fill up their feet with sea water. They do not have blood but instead take in seawater through the sieve plate, or madreporite, located on top of the sea star, and use that to fill up their feet. They can retract their feet using muscles or use them as suction to hold onto a substrate or its prey.

Sea Star Feeding

Sea stars feed on bivalves like clams and mussels, and other animals such as small fish, barnacles, oysters, snails, and limpets. They feed by "grasping" their prey with their arms and extruding their stomach through their mouth and outside their body, where they digest the prey. They then slide their stomach back into their body.