What is a Starfish?

Starfish or sea stars are animals that live in the ocean. They belong to phylum Echinodermata. They are not actually fishes. They exhibit a superficially radial symmetry, typically with five or more "arms" protruding from a central body.

Echinoderms:

Echinoderms are invertebrates that are characterized by:

• A hard skeleton under its skin
• No head
• Body plan based on a five-pointed star shape with a mouth in the center
• Many special little tubes call "tube feet" to help them move or gather food

There are four main types of echinoderms: Sea stars (Class Asteroidea), brittle stars (Class Ophiuroidea), sea urchins (Class Echinoidea), and sea cucumbers (Class Holothuroidea). We are going to look at sea stars and brittle stars.

Anatomy:

Sea Stars have a flattened body with arms that form a star shape. Brittle Stars have a flat body with a small circle-shaped disk and very long, thin arms. Both Sea Stars and Brittle Stars exhibit a superficially radial symmetry, typically with five or more "arms" protruding from a central body. The arms often have bumps or spines on them. The mouth is underneath the body in the center of the disk.

Moving About:

Starfish do not have skeletons, but instead possess a hydraulic water vascular system. The water vascular system has many projections called tube feet, on the ventral (abdominal) face of the sea star's arms, which function in locomotion and feeding.

Diet and Digestion:

Starfish digestion is carried out in a sacklike stomach located at the center of the body. The stomach may be pushed out of their body and used to eat food. Some species take advantage of the great endurance of their water vascular systems to open the shells of molluscs, and inject their stomachs into the shells. Sea stars are predators, eating snails, oysters, clams, crabs, worms and other echinoderms. Brittle stars gather tiny bits of food with their tube feet and pass it to their mouths.

Reproduction:

Echinoderms have two kinds of life cycles: Sexual reproduction and regeneration. Most echinoderms make new echinoderms by sexual reproduction, while some can also reproduce by regeneration. Some sea stars and brittle stars break their bodies into two parts. Each new echinoderm grows back its missing parts.

Defending Themselves:

Since most echinoderms are small and slow, it is hard for them to escape danger. But they do have ways of protecting themselves:

• Spines - Many have hard bodies covered with spines.
• Camouflage - Some have coloring and patterns which make them hard to see.
• Hiding - Many hide in cracks or in burrows.
• Escaping - Brittle stars will break an arm off its body and escape.

Images from NOAA Photo Library.