Animal Diversity Web

There are approximately 940 species of echinoids distributed worldwide in marine habitats from the intertidal to 5000 meters deep. Their fossil record is extensive due to their test (an internal skeleton), and dates back to the middle Ordovician period.

Echinoids are commonly grouped as regular or irregular, with the greatest differences pertaining to the oral structure, shape of the organism, and location of the anus. Regular echinoids are the sea urchins; they are generally found on rocky substrates. Irregular echinoids are the sand dollars, which are generally found on sandy or soft ground.

Like all echinoderms, echinoids are pentaradially symmetrical, have a water-vascular system, and have an internal skeleton made of calcitic ossicles (plates). A distinguishing feature of the echinoids is that the ossicles imbricate (overlap) and are fused into a globular or discoidal test; its flattened or concave oral side faces the substratum and the aboral side is arched in most species. The mouth, in the peristomal membrane, contains a powerful chewing apparatus called the Aristotle's lantern. The lantern is composed of five jaws and is capable of extending through the mouth of some urchins. The mouth leads to the intestine and anus, which is located in the center of the aboral surface in regular echinoids. The anus is either posterior or on the oral surface of irregular echinoids.

Spines and tube feet surrounding the peristome function in locomotion, burrowing, and food-gathering. Generally, urchins have longer spines; sand dollars have shorter spines which give them a fuzzy appearance. Tube feet are a part of the water vascular system characteristic of all echinoderms. Pincers located between spines are called pedicellariae. Some types of pedicellariae and specialized spines of urchins contain venom used in self-defense.

In regular urchins, the ossicles, or plates, of the test are arrayed in ten longitudinally oriented columns. Two adjacent columns each form one of five ambulacral series. These are the plates through which tube feet extend. On the aboral side, the tube feet function in respiration and sensation. The ambulacral series of plates are conspicuous in the cleaned test of a sand dollar: restricted to the aboral side, they are arrayed in a petaloid pattern. At the aboral end of the interambulacral series of regular urchins are the five (sometimes four) genital plates, through which the gonopores open. One of the genital plates serves as the sieve plate, or madreporite, for the water vascular system. Together the madreporite, the anus, and the gonopores make up the periproct.

Most echinoids have five conspicuous gonads arrayed interambulacrally. The sexes are separate. In some species, gametogenesis is regulated by photoperiod so that spawning of most or all members of a population occurs during the same time. Some female urchins brood their young externally, within the protection of their spines or tube feet. In species with indirect development, an echinopluteus larva is produced. Such a larva is bilaterally symmetrical, and undergoes metamorphosis to attain the pentaradial symmetry of the adult.

Echinoids graze on just about anything they come across, plant or animal. This includes algae, bryozoans, and dead animals.

Members of this class are food for crabs, sea stars, fish, birds, otters, and other mammals. Probably the single most important contribution of these animals to scientific knowledge is their embryological development. Researchers investigate the development of deuterostomes using sea urchin eggs, due the clear radial cleavage during a zygote's development. Echinoids of economic importance for the U.S. are the red (Strongylocentrotus franciscanus), the purple (S. purpuratus), and the green (S. droebachiensis) sea urchins. These urchins are harvested for their roe and are exported to Japan; the roe, called uni, is used in sushi.

References:

Brusca, R.C. and G.J. Brusca. 1990. Chapter 22: Phylum Echinodermata. Invertebrates. Sinauer Associates, Inc., Sunderland, Massachusetts.

Hyman, L.H. 1955. The Invertebrates: Echinodermata: The Coelomate Bilateria. Volume IV. McGraw-Hill Book Company, Inc. New York and other cities.

Kozloff, E.N. 1990. Chapter 21: Phylum Echinodermata. Invertebrates. Saunders College Publishing. Philadelphia and other cities.

Price, R.J. and P.D. Tom 1995. Sea urchins. Sea Grant Extension Program Publication. http://seaurchin.org/Sea-Grant-Urchins.html

Contributors

Judy Follo (author), Daphne G. Fautin (author).