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Cape sea urchin

Cape sea urchin

A wide diversity of creatures find their homes in the jungle of stems and fronds of the kelp forests that are prolific off South Africa’s west coast. Sea urchins and rock lobsters are among the animals that feed and take cover among the root-like hold fasts of the kelp.

Cape sea urchins live in vast numbers on shallow reefs, where they graze on kelp and algal debris. In fact there was a time when the density of urchins was so great along the southwest Cape coast that you couldn’t walk comfortably to the water’s edge at low tide without feeling like you were treading on sharp needles! Then, in 1994, the urchins did a mysterious disappearing act – vanishing almost completely.  In just two years they were virtually extinct in certain areas.

The disappearance of the urchins had marine scientists baffled. What could have caused this strange phenomenon? Like detectives, the scientists had to unravel the clues until they discovered that, prior to the disappearance of the urchins, the number of rock lobsters had increased considerably. It turned out that these creatures were responsible for the disappearance of the urchins – the lobsters had simply devoured them!

So the lobsters ate the urchins – big deal? Yes, it is a big deal; as with all life on our planet, if something happens to one kind of animal or plant, it has a ripple effect on many other animals and plants. In this case the disappearance of the urchins had an impact on abalone (perlemoen).

Although Cape sea urchins alone are of little value to humans, they provide important nurseries for abalone (perlemoen), which have significant commercial value.  The spines of the urchins protect juvenile abalone from predators such as octopus, klipfish and rock lobsters. The dramatic disappearance of the urchins left juvenile abalone in the open, making them an easy target for hungry predators.

Sharp spines provide a safe haven from predators for young abalone
In 1994, sea urchins in the southwest Cape were almost wiped out by lobsters