Mithrodiidae Family of Starfish

Bradley’s Sea Star, Mitrodia bradleyi. A representative of the Mitrodiidae Family of Starfish.

Phylogeny:  The Mithrodiid Sea Stars are members of the Mithrodiidae Family of Starfishes and are in the Phylum Echinodermata, a large Phylum that includes Brittle Stars, Sea Cucumbers, Sea Lilies, Starfish and Urchins. The Echinodermata are of great scientific interest because they date to the Cambrian Age (over 500,000,000 years ago) with 7,000 living and 13,000 extinct individual species via fossil records. They are in the Class Asteroidea and the Order Valvatacea.

Distribution: The Mithrodiid Sea Stars are found in the tropical and subtropical regions of the Indian and Pacific Oceans. There are seven species in the Mithrodiidae Family with just one species, Bradley’s Sea Star, being found along the Pacific Coast of Mexico. They are found attached to rocks, within coral and other hard substrate from the intertidal zone to depths up to 143 m (470 feet).

Morphology:  The Mithrodiid Sea Stars are characterized by a larval stage with bilateral symmetry and an adult stage with 5-rayed radial symmetry. They have a small central disc with a  round cross section that is surrounded by five elongated arms (or rays), with two rows of feet with suckers, on the underside of each arm. The arms have a broad attachment point where the arms join the disc and they taper as they extend from the disc. These Sea Stars shed their arms more readily that those in other families which they are able to regenerate. The disc contains most of the organs, with the mouth on the ventral side and the anus and madreporite (entry plate for the water vascular system) on the dorsal surface. All family members have a flattened flexible body that are unsegmented and cucumber, disc, sphere or star in shape covered with colored skin. Their aboral (dorsal) surface is covered by a thick skin that has rough granules, tubercles, or spinelets. They have conspicuous ossicles (calcareous particles that are part of the endoskeleton) along their margins. Their pedicellariae are elongate and multi-valved. They have a unique water-vascular system that uses hydraulic power to operate a multitude of tined tube feet that are used in locomotion and food capture. The tube feet have double ampullae (bulb-like appendages that push water into tube feet, causing them to extend). Some feed via inserting the stomach out through the mouth surrounding the prey, secreting enzymes to digest the food and then retracting the stomach when finished eating. They have a water vascular system, tube feet, and a complete digestive system, but they lack a head, eyes, nervous system, or excretory system. Mithrodiid Sea Stars are medium to large in size, with a maximum diameter or “wingspan” of 55 cm (21.5 inches) and 5.9 kg (13 lbs 0 oz) in weight. These sea stars are often colorful with bands or blotches of brown, orange, red or white.

Ecosystem Roles: The Mithrodiid Sea Stars feed on encrusting organisms, detritus and small invertebrates. In turn they are preyed upon by crabs, fish, and other sea stars.