Starfish - Appearance

Appearance

Starfish are radially symmetric and typically express pentamerism or pentaradial symmetry as adults. However, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry. Modern starfish, as well as other echinoderms, exhibit bilateral symmetry in their larval forms.

Most starfish typically have five rays or arms, which radiate from a central disc. However, several species frequently have six or more arms. Several asteroid groups, such as the Solasteridae, have 10 to 15 arms, whereas some species, such as the Antarctic Labidiaster annulatus can have up to 50. It is not unusual for species that typically have five rays to exceptionally possess six or more rays due to developmental abnormalities.

The surfaces of starfish bear plate-like calcium carbonate components known as ossicles. These form the endoskeleton, which takes on a number of forms that are externally expressed as a variety of structures, such as spines and granules. These may be arranged in patterns or series and their architecture, individual shapes and locations are used to classify the different groups within the Asteroidea. Terminology referring to body location in starfish is usually based in reference to the mouth to avoid incorrect assumptions of homology with the dorsal and ventral surfaces in other bilateral animals. The bottom surface is often referred to as the oral or actinal surface, whereas the top surface is referred to as the aboral or abactinal side.

The body surfaces of starfish have several structures that comprise the basic anatomy of the animal and can sometimes assist in its identification. The madreporite can be easily identified as the light-coloured circle, located slightly off centre on the central disc. This is a porous plate which is connected via a calcified channel to the animal's water vascular system in the disc. Its function is, at least in part, to provide additional water for the animal's needs, including replenishing water to the water vascular system. Near the madreporite, also off centre, is the anus. On the oral surface there is an ambulacral groove running down each arm. On either side of this there is a double row of unfused ossicles. The tube feet extend through notches in these and are connected internally to the water vascular system.

Several groups of asteroids, including the Valvatida but especially the Forcipulatida, possess small bear-trap or valve-like structures known as pedicellariae. These can occur widely over the body surfaces. In forcipulate asteroids, such as Asterias and Pisaster, pedicellariae occur in pom-pon-like tufts at the base of each spine, whereas in the Goniasteridae, such as Hippasteria phrygiana, pedicellariae are scattered over the body surface. Although the full range of function for these structures is unknown, some are thought to assist in defence, while others have been observed to aid in feeding or in the removal of other organisms attempting to settle on the surface of the starfish. The Antarctic Labidiaster annulatus uses its large pedicellariae to capture active krill prey. The North Pacific Stylasterias forreri has been observed to capture small fish with its pedicellariae.

Other types of structures vary by taxon. For example, members of the family Porcellanasteridae employ additional sieve-like organs which occur among their lateral plate series and which are thought to generate currents in the burrows made by these infaunal starfish.