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Inhabitants of Oregon's Tidepools: Purple Sea Urchin

Though not especially sharp, purple sea urchin spines are strong enough to wear away bits of rock.

Over time, generations of urchins form the shallow pits they live in.  The urchin hangs on to the rock and to the bits of shell and rocks that you might see on the urchin, with tiny tube feet that make up part of the soft tissue between the spines.  The pits are great refuge from roiling water during high tide and can make mini-tidepools at low tide.  This protection makes the pits valuable real estate and many animals can be found living under and alongside the urchins.

Sea urchins are spheres, up to fist-sized, of richly purple spines.  The spines are long enough to take up perhaps a quarter of the overall height and width of the urchin—about ½ to 1½ inches long, depending on the size of the urchin.  Sometimes the urchin spines are tidy, arranged in neat rows radiating from the center top and down the sides to the bottom, other times the spines have pivoted about irregularly, giving the urchin a disheveled look.  The softer tissues beneath and between the spines is very dark purple—nearly black.  Some urchins have rocks or pieces of shells attached.  Most purple urchins on the Oregon Coast live in shallow, urchin-shaped pits.Purple sea urchins munch kelp with the five-toothed mouth on the underside, moving to dinner or pulling the meal in using the tube feet.  As with its relative the seastars, pulling urchins off the rock or out of their pits can rip off tube feet they need to stay put.

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