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This distinctive sea urchin is found only in the wave impact zone where it is subject to the full force of the surf. The rounded shape and flattened, shingle-like spines are adaptations for withstanding the force of the incoming waves. Its low profile also presents little resistance as the water surges back and forth, so that the urchin is not pulled off the rocks. Sucker-tipped tube feet on the urchin’s bottom surface and moveable spines around the body edge also help the urchin cling to the shoreline.
The shingle urchin is an herbivore, grazing on the cement-like red coralline algae (Porolithon) which forms a pink band on the rocks around mean tide level.
Sea urchins belong to the animal grouping, or Phylum, called the Echinoderms, the “spiny-skinned” animals. Their relatives include the seastars, brittlestars, crinoids (featherstars) and sea cucumbers. Echinoderms all share basic similarities in their habits and body design: they are all totally marine; all bottom-dwelling (benthic); are unique in the animal kingdom in having a five-part body symmetry; and a water-vascular system of muscular internal canals which provides hydraulic power for hollow, muscular tube feet that used in movement, attachment, feeding, and often gas exchange. Their large internal body cavity is also fluid filled, and circulating fluids carry gases and nutrients. Echinoderms have an internal skeleton of calcite, a form of calcium carbonate. Their skeletal plates, called ossicles, are developed to different extents in the different groups. In many echinoderms, like the sea urchins, extensions of the skeleton form the spines for which this group is named.
Looking past their spines, sea urchins have a circular to oval shape. But their basic five-part symmetry is evident in the five zones of tube feet, alternating with five zones of spines, which run from the mouth on the urchin’s underside to the top of the body. In urchins, the skeletal ossicles are flattened plates, fused into a rigid test (skeleton). The spines sit on knobs on the test, attached by skin and muscles; the water-filled tube feet extend from inside the test out through holes in the plates. The many, muscular tube feet are used for attaching to the seafloor, for moving, and sometimes food capture. The urchins’ moveable spines are important in defense, movement, and food capture.
Indo-Pacific, including Hawai‘i
to 3 inches diameter