Photo by Sonke Johnsen


Form & Function


      People often consider brittle stars and sea stars to be the same type of animal. They are very similar, however, brittle stars have arms that are longer and skinnier, and their tube feet do not have suckers on them like those of sea stars.

      The brittle star, Ophiopsila riisei, has a disk diameter of about 12 millimeters. The disk has a gray color with black spots, with a blotchy appearance to it. This species is like other ophiocomids, in that they have both oral papillae and dental papillae (EOL 2003).

     Ophiopsila riisei can have up to to seven flat arm spines that are about 165 millimeters (about 16 centimeters) long each. The arms of this brittle star are irregularly banded with an off-white color and different shades of gray, red, maroon, and purplish-brown (EOL 2003, Bocas 2003). These arms are also unbranched and are not able to coil tightly, while other brittle stars are capable of extreme coiling when other animals disturb them. The arms of Ophiopsila riisei may coil slightly, but not nearly as much as other species; their arms primarily move side to side (Pomory 2007). The image above shows the irregular banding and color of the arms of Ophiopsila annulosa, a brittle star very closely related to Ophiopsila riisei.

      The tube feet of an echinoderm are located on the oral surface of sea stars, and they function in locomotion, respiration/gas-exchange. food gathering, and sense-reception (Hajduk 1992, Mooi 1986). These feet are the exterior distensions of the star's water-vascular system (Hajduk 1992). Hemipholis elongata, a brittle star of the same class as Ophiopsila riisei, has tube feet containing several layers. These layers consist of: a ciliated epithelial layer lining the lumen of the foot, a layer of longitudinal muscle, a connective tissue layer, and finally a cuticle bilayer for a covering. The cells of the iner ciliated epithelium layer could potentially have the job of circulating fluid in the water-vascular system. The connective tissue layer is made of collagen fibers arranged in longitudinal and circular directions; nervous tissue components were found throughout the matrix of this layer (Hajduk 1992). The outer epithelium layer of each tube foot is covered by a thin cuticle layer, and has microvilli extending from its outer surface (Hajduk 1992, Cobb and Moore 1986). These microvilli have been observed and some seem to have been adapted for mucus secretion (Hajduk 1992).

      Retraction of each tube foot in ophiuroids is accomplished by contracting the longitudinal layer in the foot. When this muscular layer contracts, the fluid that fills the lumen of the tube foot is pushed into the tenacular extensions of the foot's wall. These extensions become limp and soft int he extended parts of the tube foot, and the appear enlarged or swollen in the contracted portion. The repition of this fluid transposition, although very little fluid is displaced, accounts for some of the locomotion of these brittle stars (Hajduk 1992).


For information about the reproduction process of Ophiopsila riisei, visit the Reproduction page!