The Echinoid Directory

The fertilised egg of most sea urchins develops into a characteristic free-swimming microscopic larvae termed the echinopluteus. This larvae migrates into the surface waters and feeds on microplankton which it captures using its ciliated arms. The arms are supported by fine calcite rods and there may be between two and six pairs of arms developed. The larva is perfectly bilaterally symmetric and shows no sign of the later pentaradiate symmetry that characterises echinoderms. Echinoplutei come in a wide variety of shapes, and each major group of echinoid has its own characteristic larval form. The echinopluteus lives for several weeks feeding in the surface waters before metamorphosis. Metamorphosis involves the formation of adult structures on the left-hand side of the larva and the resorption of larval features such as the arms.

The drawing above is of Tripneustes esculentus (Leske) and comes from Mortensen, T. 1921. Studies of the Development and Larval Forms of Echinoderms. G. E. C. Gad, Copenhagen (pl. 2).

For more information on echinoid embryology and development visit www.worms.zoology.wisc.edu/urchins