The Urchin is Lurchin’

“Look but don’t touch”. Who hasn’t heard that before? The phrase applies especially well to those scuba diving or snorkeling the Meso-American Barrier Reef located off the coast of Belize. Why? Well, healthy coral reef structures are highly productive ecosystems teeming with biodiversity that spark the kind of curiosity that “killed the cat”, so to speak. The long-spined sea urchin, Diadema antillarum is just one perfect example that proves these statements true.

To avoid falling prey to animals such as otters, snails, several species of fish, the spines of sea urchins contain a mild toxin that act as a defense mechanism. The toxin is not usually venomous.
Like the other 700 species of sea urchins inhabiting marine environments around the world, Diadema antillarum belongs to the Echinoderm family. The external features of this species consists of a collection long, black spines attached to a “test.” In this case, tests are the hard shells that contain the internal organs of the organism. Thin, tube feet tipped with suckers that grasp the floor are present amidst all those spines. These are difficult to see, but function in grabbing and pulling prey towards its mouth, which is located underneath the urchin’s test. 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.

The feeding strategy that Diadema antillarum displays plays two important ecological roles. First, urchin grazing reduces the total amount of algae on a reef, similar to a lawnmower keeping the grass short. This enables corals (which compete with algae for space and sunlight) to grow better. Second, when urchins scrape algae from rocks, they create vacant spaces that can then be colonized by the larvae of other marine animals (corals, sponges, gorgonians). This helps to keep the diversity of reef animals high. In the absence of urchin grazing, coral reefs may become overgrown with algae, and the diversity of reef animals may be reduced (Toller, 2003). The predators of Diadema antillarum include otters, snails, several species of fish. To avoid falling prey to animals such as these, the spines of sea urchins contain a mild toxin that acts as a defense mechanism. Although the toxin is not usually venomous, it does however discharge a violet colored fluid that stains the wound. Camouflage is another defense mechanism of sea urchins. This ability allows them to hide themselves within rocks and other crevices. Indeed, Diadema antillarum is one of the many creatures that leave us awestruck and potentially stinging if we aren’t careful. Should you or anyone you know suffer from a sea urchin injury, do not hesitate to remove the spines with tweezers. Applying a weak acid, such as vinegar, lemon juice, or ammonia may help to help prevent infection in the wound.

Reef Brief is a weekly column published in the San Pedro Sun

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