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Sea Urchin

Introduction to the Sea Urchin


Photo: NOAA

The sea urchin

Photo: NOAA

Sea Urchins, a prickly-looking marine animal, spherical in shape and covered with long movable spines. It somewhat resembles a hedgehog and is sometimes called the “hedgehog of the sea.” Sea urchins are found on the ocean bottom, usually near rocky shores. Sea urchins may be brown, black, purple, green, white, or red. Most are about two to four inches (5 to 10 cm) in diameter, including the spines.

In addition to its spines, the sea urchin also has pedicellariae (three-jawed pincers atop slender stalks) and tiny tube feet projecting from its body surface. The movable spines (which in some species are solid and in others hollow and filled with poison) are used for locomotion and protection. The pedicellariae (which in some species contain poison glands) are used for defense and for cleaning the body by removing larval animals and small crustaceans. The tube feet are hollow, muscular projections ending in suckers. They are flexible and can be extended beyond the spines to grip objects on the ocean floor.

The sea urchin feeds on seaweed and other organic matter. On its undersurface is a mouth with five strong teeth used in feeding. Some sea urchins bore holes with their teeth in rocks along the shore and then use the rocks as hiding places. Sea urchins reproduce sexually by means of eggs and sperm. The eggs are used as food in many European and Asian countries. The life cycle of a sea urchin starts by spawning directly into the sea. First, the male spurts out his sperm. Then the female comes and spurts out her eggs. If the sperm and eggs touch, then a new baby sea urchin will form. After the male spurts out his sperm, more males and females come and spurt out their sperms and eggs. As a group thing there is a good chance that more baby sea urchins will form.

Sea urchins live in the mid-level or low-level of a tide pool. Sea urchins eat algae off of hard surfaces. Most of the time they eat seaweed. Sometimes they eat bits of plants and small animals. Sea urchins have hundreds of tiny, tubed feet. They have five teeth in the middle of their back side. Sea urchins use these teeth to pull, tear and rip off algae on the rocks.These teeth continue to grow throughout the sea urchin's life. A sea urchin's size is about 4 inches and they come in many different colors. The most common are purple and light pink.

Sea Urchins have radially symmetrical bodies divided into 5 equal parts.  They move about using articulating spines and tube feet.  Jawed tube feet called pedicellariae are employed for defense along with sometimes venomous spines.  Urchins feed upon algae, sponge, or detritus.  Their hard jaws and stout spines are capable of eroding coral or rock in some species.  Urchin roe is a delicacy in Japan and supports a fishery in the North Pacific.

Like most creatures, sea urchins are vital for the survival of other living creatures surrounding them.  They have many predators and due to this, if the sea urchin population decreased, the sea creatures that feed on them might begin to die out as well. A few predators that feed on sea urchins are sea otters, star fish and humans. However for there to be a healthy balance in their environment, it’s very important that the population of sea urchins do not decrease or increase all that much.  This problem occurred in the 1980’s in the Caribbean - South Atlantic when the sea urchin population began to increase at an extremely rapid rate.  At one point this immense number of sea urchins began to eliminate the seaweed which lived in the same area.  They were also eroding the coral reef.

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