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Description: Sea urchins are closely related to sea stars
and brittle stars. Urchins have three kinds of projecting appendages:
spine, tube feet and pedicellariae (pincers). The movable spines
are purple and the most conspicuous of the three.
Range and Habitat: Sea urchins inhabit rocks or mud along
the seashores. The purple urchin ranges from Alaska to Cedros
Island off the coast of Baja California. Although it is mostly
a subtidal species, the purple urchin is found intertidally on
almost barren rock.
Habits and Adaptations: All of the appendages are sensitive
to touch and may also respond to chemicals released into the water
by predators. Touching the urchin's body with a sharp object causes
all the spines to point to the region touched. The spines turn
away if a blunt object is used, allowing the pedicellariae to
be the primary mode of defense. They also show a shadow response,
making rapid movements with the spines when a shadow suddenly
appears.
Sea urchins exhibit a generalized light sensitivity all over
the body. Some urchins cover themselves daily with stones and
shells held in place by the tube feet. This occurs a few hours
after dawn and may protect light sensitive pigments in the body.
It has adapted well to the pounding surf by burrowing at least
half of its body into the rock on which it is found. The spines
of the urchin, aided by the motions produced by waves and tides,
account for the excavations. If a spine is damaged in the process,
it will be regrown. The purple urchin is not as sedentary as its
habit of living in holes might suggest. Many urchins do not live
in holes, especially where the rock is too hard. When sand is
piled up in their usual habitat, they may move upward.
Diet: Herbivorous, feeding on fixed algae like sea lettuce.
Breeding and Maturation: Sexes are separate. Both eggs
and sperm are shed into the water for chance fertilization. The
free-swimming larvae may live planktonic existence for several
months, and then metamorphose quickly into young urchins.
Miscellaneous: Preyed upon by sea stars, sharks, rays
and other animals. They are also hosts to a variety of parasites.
Ciliated protozoans (microscopic one-celled animals) live in the
intestines; two species of nematodes (roundworms), one of which
may exceed three feet in length, and a planarian are found in
the body; trematodes (flatworms) are parasites of the digestive
tract; and a specialized barnacle inhabits the body cavity of
sea urchins. Communalistic relationships in which the urchins
are not harmed also exist. Many copepods (small crustaceans),
bryozoans (moss animals), and sponges live on the spines of urchins.
One species of sea urchin is used for protection by two different
species of fish. The shrimp fish and the cling fish are found
swimming head down between the spines.
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