Ophiuroidea

Lore & Background

The ophiuroids diverged in the Early Ordovician and can be found today in all major marine provinces, from the poles to the tropics. Basket stars are usually confined to deeper parts of this range, while brittle stars are common in reef communities, hiding under rocks and within other organisms. A few species can tolerate brackish water, an ability otherwise almost unknown among echinoderms. The skeleton is made up of embedded ossicles.

Of all echinoderms, Ophiuroidea may have the strongest tendency toward pentaradial symmetry. The body outline is similar to that of starfish, with five arms joined to a central body disk, but the disk is sharply marked off from the arms. The disk contains all viscera; internal organs of digestion and reproduction never enter the arms. The mouth has five toothed jaws, and the madreporite is usually located within one of the jaw plates. The coelom is strongly reduced.

The water vascular system ends in tube feet, generally with one madreporite, though some forms have one per arm or none at all. Suckers and ampullae are absent from the tube feet. The nervous system consists of a main nerve ring around the central disk, with radial nerves running to the ends of the limbs. Most ophiuroids have no eyes, but they have sensitive nerve endings in their epidermis and can sense chemicals, touch, and light, especially at the ends of their arms.

Reader's Guide

Ophiuroidea are significant as a diverse and widespread class of echinoderms, with over 2,000 living species found in all major marine provinces, from the poles to the tropics and from shallow reefs to abyssal depths greater than 6,000 m. Their notable features include a skeleton of fused calcium carbonate plates forming a test, and arms supported by vertebral ossicles that articulate via ball-and-socket joints. The two large clades, Ophiurida and Euryalida, exhibit distinct movement patterns: Ophiurida move horizontally and quickly when disturbed, using one arm as a lever while the other four act as opposite levers, while Euryalida move vertically, can coil their arms around objects, and have larger vertebrae and smaller muscles.

Ophiuroids are generally scavengers or detritivores, but many are carnivorous or omnivorous. Basket stars may use suspension feeding, trapping plankton with mucus-coated arms. Digestion occurs in pouches of the stomach, and respiration and excretion occur through cilia-lined bursae. Most species have separate sexes, with external fertilization, though many brood larvae in bursae or give live birth. A few species have free-swimming ophiopluteus larvae. The class is also notable for its ability to tolerate brackish water, an unusual trait among echinoderms.

Did You Know?

• The name 'ophiuroid' comes from Ancient Greek words meaning 'serpent' and 'tail', referring to the serpent-like arms.
• More than 1,200 species of brittle stars are found in deep waters greater than 200 m deep.
• Ophiuroids have no eyes but can sense light, chemicals, and touch, especially at the ends of their arms.
• Some brittle stars, such as Amphipholus squamata, are truly viviparous, with embryos receiving nourishment from the mother through the wall of the bursa.