Regular sea urchins inhabit a broad range of environments from wave exposed rocky outcrops, crevices within rocks, rock pools, coral reefs, sandy lagoons to sea grass beds and kelp forests. The Antarctic species, Sterechinus neumayeri, has an energy-efficient metabolism that is physiologically adapted to freezing and food-deprived environments. Echinoids in warmer waters, where food is plentiful, have many predators and attempts to elude them involve highly evolved cryptic coloration to blending in with the background. The pencil-like spines of the pencil urchin, Eucidaris, for example, encourage encrusting algae to settle, making it virtually invisible during the day. This is not the case in the Galápagos Islands, where spines are not covered with epipthytes or epizotes, and the Eucidaris are completely exposed. Urchins inhabiting sea grass beds adopt a slightly different avoidance strategy; for example, some species cover their test with shell fragments, algae, and other types of debris to provide appropriate camouflage.
Heart urchins and sand dollars have a different mode of existence to sea urchins. Their flattened test is perfectly suited to life beneath the sediment surface. This instantly provides cover from visually orientated predators. Most species occupy sandy habitats and avoid muddy or silt-dominated sediments. Others such as the purple-heart urchin Spatangus purpureus prefer coarse gravel habitats.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.