The purple sea urchin (Strongylocentrotus purpuratus) is found in the Pacific Ocean ranging from Alaska to Cedros Island in Baja California (Ricketts and Calvin, 1948; Ebert et al., 1994). Although the purple sea urchin is abundant in low intertidal zones along open coastline, it can also be found in the mid and low-tidal zones of protected coastlines (Ricketts and Calvin, 1948). Reproduction most commonly occurs during winter and spring (Russell, 1987; Ebert and Russell, 1988; Ebert et al., 1994) and larvae need at least five weeks of development before settling (Ebert et al., 1994).
The sea urchin is an echinoderm, a type of marine animal that is typically several centimeters in length and has pentamerous radial symmetry. Pentamerous radial symmetry refers to a body with five equal parts originating from a center point. A classic example of pentamerous symmetry is the starfish. All echinoderms have an internal skeleton made of calcareous ossicles. In sea urchins, the ossicles are fused together into a structure called a test, or shell (Barnes, 1980).
Echinoids are a group of free-moving echinoderms that includes sea urchins, heart urchins and sand dollars. Characteristic of echinoids are the projections that cover the test, called spines. There are two types of spines that usually are equal in distribution across the test, longer primary spines and shorter secondary spines (Barnes, 1980).
Sea urchins are typically six to twelve centimeters in length and spherical in shape, although flattening can occur along the oral-arboral axis. Sea urchins use their spines for defense, prey capture, and locomotion. If a sea urchin is touched by a sharp object, the spines will point together in the direction of the stimulus to protect the test and cause injury to a potential predator (Rickets and Calvin, 1948). When prey swim into the spines of sea urchins, the spines will point outwards to reveal pedicellariae with poison glands capable of paralyzing small animals (Ricketts and Calvin, 1948; Barnes, 1980). These pedicellariae can also be used defensively against predators (Barnes, 1980). Spines can also be used in a slow form of locomotion. Spines in the front of the test are pushed into a substrate, while spines in back are used to propel the test forward (Ricketts and Calvin, 1948). Additionally, sea urchins use their spines in order to fit themselves into rock crevices (Edwards and Ebert, 1991).
Sea urchins have a unique feeding structure called Aristotle’s lantern. Aristotle’s lantern is made of five calcareous plates, called pyramids, which are attached by muscle fibers. Each pyramid has a tooth and the mouth is located at the center of the lantern (Barnes, 1980). Aristotle’s lantern is an adaptation for scraping and is used for scraping food off rocks (Barnes, 1980) in addition to widening rock crevices suitable for habitation (Ricketts and Calvin, 1948). The majority of the sea urchin’s diet is algae, although other plant and animal matter is consistently consumed (Barnes, 1980).
Reproduction occurs through external fertilization. Gametes are released into the ocean and fertilization occurs at random (Ricketts and Calvin, 1948; Barnes, 1980; Mead and Denny, 1985). The surf can both aid and hinder the chances of fertilization. A moderate surf increases mixing between eggs and sperm making fertilization more likely. However, if the surf is too strong, gametes can be damaged, diluted, or cause abnormal development of a fertilized egg (Mead and Denny, 1985).
Additional Information on the Purple Sea Urchin
This specimen was frozen before scanning, which accounts for the broken and unusual orientation of some of the spines.
Click on the thumbnails below for labeled images of the urchin in standard anatomical views.
Barnes, R. D. 1980. Invertebrate Zoology. Saunders College, Philadelphia, PA. pp 1089.
Ebert, T. A., S. C. Schroeter, J. D. Dixon, and P. Kalvass. 1994. Settlement patterns of red and purple sea urchins (Strongylocentrotus franciscanus and S. purpuratus) in California, USA. Marine Ecology Progress Series 111: 41-52.
Ebert, T. A., and M. P. Russell. 1988. Latitudinal variation in size and structure of the West Coast purple sea urchin: a correlation with headlands. Limnology and Oceanography 33(2): 286-294.
Edwards, P. B., and T. A. Ebert. (1991). Plastic responses to limited food availability and spine damage in the sea urchin Strongylocentrotus purpuratus (Stimpson). Journal of Experimental Marine Biology and Ecology 145: 205-220.
Mead, K. S., and M. W. Denny. 1995. The effects of hydrodynamic shear stress of fertilization and early development of the purlpe sea urchin Strongylocentrotus purpuratus. The Biological Bulletin 188: 46-56.
Ricketts, E. F, and J. Calvin. 1948. Between Pacific Tides. Stanford University Press, Stanford, CA. pp 365.
Russell, M. P. 1987. Life history traits and resource allocation in the purple sea urchin Strongylocentrotus purpuratus (Stimpson). Journal of Experimental Marine Biology and Ecology 108: 199-216.
Strongylocentrotus purpuratus page on the Animal Diversity Web (University of Michigan Museum of Zoology)
Strongylocentrotus purpuratus at the Minnesota Zoo
Strongylocentrotus purpuratus from EnchantedLearning.com
The Sea Urchin Genome Project