Starfish of the Mithrodiidae Family

Bradley’s Sea Star, Mitrodia bradleyi

The Mithrodiid Sea Stars are members of the Mithrodiidae Family of Starfishes and are in the Phylum Echinodermata (meaning spiny skin), a large Phylum that includes Brittle Stars, Sea Cucumbers, Sea Lilies, Starfish and Urchins. The Echinodermata are of great scientific interest because, via fossil records, they have been dated to the Cambrian Age (over 500,000,000 years ago) with 7,000 living and 13,000 extinct individual species.

The Starfish of the Echinodermata Phylum 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. 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 exist from intertidal zones to depths of 9,150 m (30,000 feet) with 1,500 species known globally. They all have flattened flexible bodies that are covered with colored skin. They are characterized by a larval stage with bilateral symmetry and an adult stage with 5-rayed radial symmetry.

Mithrodiid Sea Stars are in the Class Asteroidea. Asteroidea Sea Stars are either in the Class of Astereozoans or the Class Ophiuroidea.  Asteroideans are easily distinguished from Ophiuroideans by having arms with a broad attachment point where the arms join the disc. Their arms taper as they extend from the disc. Ophiuroideans have a narrow attachment point and their arms are relatively the same thickness their entire length with a narrow attachment point at the disc. All family members have a flattened body that are unsegmented and cucumber, disc, sphere or star in shape. They have a water vascular system, tube feet, and a complete digestive system, but they lack a head, eyes, nervous system, or excretory system. They have a central disc that is surrounded by 5 to 40 radiating arms (or rays). 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 Asteroideans have the ability to regenerate amputated limbs.

Mithrodiid Sea Stars are in the Order Valvatida. These starfish generally have 5 arms, with 2 rows of tube feet, with suckers, on the underside of each arm. They have conspicuous ossicles (calcareous particles that are part of the endoskeleton) along their margins. Their central disc is small with elongate arms that are somewhat round in cross section. In some species, the arms are constricted at their base. These Sea Stars shed their arms more readily that those in other families. Their aboral (dorsal) surface is covered by a thick skin that has rough granules, tubercles, or spinelets. Their pedicellariae are elongate and multi-valved. The tube feet have double ampullae (bulb-like appendages that push water into tube feet, causing them to extend). These sea stars are often colorful with bands or blotches of brown, orange, red or white.

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. They are found attached to rocks, within coral and other hard substrate at depths up to 143 m (470 feet). They feed on encrusting organisms, detritus and small invertebrates. In turn they are preyed upon by crabs, fish, and other sea stars.

The Mithrodiid Sea Stars are found in the tropical and subtropical regions of the Indian and Pacific Oceans. There are only 7 species in the Mithrodiidae Family with just 1 species, Bradley’s Sea Star, being found along the Pacific Coast of Mexico.