Ophiocomidae Family of Brittle Stars

Phylogeny: The Brittle Stars are members of the Ophiocomidae Family, a family that dates to the Cambrian Age (over 500,000,000 years ago) with approximately 20,000 species of which more than half are extinct.  They are in the Phylum Echinodermata (meaning spiny skin), which includes the Basket Stars, Sea Cucumbers, Sea Stars and Sea Urchins that are of great scientific interest because, via fossil records. Brittle Stars, along with Basket Stars and Serpent Stars, are in the Class Ophiuroidea.  Their name comes from the Greek words for snake tail, referring to their thin, flexible, wiggly, arms. They are agile relatives of the sluggish sea stars with anatomical structures that includes a flattened central disk and most have five slender arms, though a few have up to ten arms. Brittle stars owe their name to the notorious capability of voluntarily severing arms which occurs as a self defense mechanism while under attack by predators. Complete regeneration of missing parts takes from two and eight weeks.

Distribution:  Brittle Stars encompasses over two thousand individual species that have been placed in two hundred and twenty genera and thirteen families. There are at least one hundred and twenty-five species of Brittle Stars along the Pacific Coast of Mexico, ninety-seven of which are in the Sea of Cortez. Brittle Stars are one of the most abundant animals found in any habitat and can number over 15,000 individuals per square meter. They are are found in all oceans, from the shallows to depths of  6,100 m (20,000 feet), including deep sea thermal vents. Ophiocomidae Brittle Stars are the dominant species of Brittle Stars in the tropics and are found in all subtropical and tropical seas globally. There are one hundred and three known species in the Ophiocomidae Family of which four are found along Mexico’s Pacific Coast. Ophiocomidae are primarily found in shallow waters, though some species are found as depths up to 370 m (1,210 feet).

Morphology:  Identifying Brittle Stars can be challenging. Many species cannot be determined without close examination of the shape of various plates and mouth parts. Because the arms are fragile, and difficult to measure, Brittle Stars are often measured by diameter of the disk. They all have radial symmetry and they are large and colorful and are characterized by a clump of dental papillae at the jaw apex with numerous papillae along the edges of the oral plates. They have relatively stout arms, inserted ventrally, that can be wider away from their insertion at the disk. The arms have tube feet on their underside, but unlike the tube feet of Sea Stars, these feet lack suction cups. The spines along their arms are long and erect and are used primarily for feeding and movement and to a lesser extent for protection from predation. There is a mouth on the underside of the disk and consists of five “jaws”. They have five triangularly shaped jaws that frame a centrally placed mouth which is found on the ventral side. The arms are used for locomotion as they can rapidly “row” or pull themselves along the ocean floor. The dorsal surface of the disk has a granular texture. Some species within this family will change color from daytime to nighttime. The Ophiocomidae Brittle Stars can have disk diameters that exceed 3.2 cm (1.3 inches).

Ecosystems Roles:  Ophiocomidae Brittle Stars are found in a variety of habitats including algae bases, coral, gravel, rock, rubble, sand and sponges. They are primarily nocturnal  omnivores being depositional feeders, suspension feeders, grazers, predators and scavengers collecting particles with their arms which they transport to their mouth. They prey on algae, crustaceans, detritus, fish, and fish eggs. Unlike Sea Stars, Ophiocomidaes cannot extend their stomach out though their mouth to digest prey externally. In turn they are preyed upon by crabs, fishes, sea stars, shrimps and other brittle stars.  Brittle Stars lack eyes and nocturnal animals that take shelter during daylight hours. Brittle stars have never been an important item of human commerce.