Sea Urchin- Don't Tread on Them

For the past few weeks, Reef Brief has featured sea stars and sea cucumbers, two of the most common marine invertebrates. This week, yet another member of the Echinoderm family will be covered: sea urchins.

    Most snorkelers or divers have witnessed these spiny creatures tucked away in rocks and crevices, and some have even had the painful misfortune of stepping on one. Despite a built-in defense system that can unfortunately inflict pain on humans, sea urchins occupy an important niche in the marine ecosystem.

    More than 700 species of sea urchins can be found in various marine environments throughout the world, particularly in areas of seagrass beds and coral reefs. These creatures, which can grow to be three feet in diameter, are characterized by a spherical and rigid shell that is known as a test. The test is covered with sharp spines that in Belize are commonly found to be black in color. Like their close relatives, sea urchins have tube feet that allow them to be very mobile in their quest for food. The suctioning tube feet assist the sea urchin in grabbing onto and pulling prey towards its mouth, which is located on the underside of its body. The typical diet of the sea urchin includes algae, mussels, sponges and even dead sea urchins. This food is chewed and consumed by the sea urchin with the help of a complex system of muscles and teeth, with digestion occurring in a hole located at the top of its body.

Reef Brief is a weekly column published in the San Pedro Sun
    Because sea urchins are very slow, they often fall prey to crabs, snails and even sea otters (in colder climates). And like so many other marine creatures, sea urchins are incorporated into the diets of humans, particularly in Asian and South American cultures. To avoid predators, sea urchins have a built-in defense system of spines that are similar to sharp needles containing a mild toxin. This toxin, which is rarely venomous, is released after the spine penetrates the skin of the victim and results in a discharge of a violet colored fluid that stains the wound. Sea urchins are also equipped with pinchers that can puncture and deliver venom into the skin. A less aggressive defense mechanism of sea urchins is their adept ability to camouflage themselves within rocks or when attached to barnacles.

    Sea urchin populations are currently at a healthy level. Not too long ago, however, these invertebrates experienced a population explosion. When the sea otters of Northern California were on the verge of extinction, sea urchin populations grew out of control. Without the otters around as predators, huge numbers of sea urchins devoured enormous kelp forests, which hindered the biodiversity of this delicate ecosystem. Today, otters have made a comeback, and consequently, sea urchin levels are back in balance.

    Next time you are out snorkeling or diving, keep your eyes open for these unique creatures hiding within the coral reef environment. But remember: never touch a living sea urchin or you'll regret it!

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