Sea Star - Orthasterias kohlerkeri - (20 cm across)

Sea Star

Sea stars, or starfish, belong to one of the two major phyla with radial symmetry - their body parts are arranged in a symmetrical pattern that repeats along the rays of a circle. Usually, for sea stars and most other members of this group, the symmetry is 5-fold.

The group is Phylum Echinodermata, which means 'spiny-skinned'. Despite their adult radial symmetry, echinoderms give away their ancestral symmetry in their larval stages. A sea star goes through several stages with bilateral symmetry, including the bipinnaria stage shown on the Marine Larvae Gallery Page. From these observations and other evidence, biologists have concluded that echinoderms probably evolved from an ancestor with bilateral symmetry - perhaps a worm allied to the bryozoans or their ancestors.

Most echinoderms spawn directly into the sea. The fertilized eggs develop through several microscopic swimming stages, and in manny species these larvae feed on smaller plankton for several weeks before the animal is ready to settle to the bottom. At this time, the larva undergoes a drastic metamorphosis, changing from a bilateral worm-like swimming animal, to a radially-symmetrical animal than sually crawls slowly along the ocean floor.