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The Hardy Sea Urchin

By Chris Doyle on March 27, 2013 in Other

Photo: George Evatt

If you’ve ever been snorkelling or walked across a rock platform around the Eastern Beaches, you have more than likely seen a sea urchin, if not dozens of them. And while they look very unassuming sitting on rocks or on the bottom of the sea floor, sea urchins play a very important role in the ecology of the ocean.

Sea urchins belong to a group of animals called echinoderms, which is a Greek word meaning ‘spiny skin’. Just one look at a sea urchin and it is easy to see why they get this name. The spines offer protection against predators, although some species of fish, such as blue gropers, porcupine fish and eels, are capable of eating them, spines and all. Humans are also predators of sea urchins, and they are regarded as a delicacy in many parts of the world. In some cultures they are even considered to be powerful aphrodisiacs.

Apart from being a food source for both fish and humans, sea urchins are critical for controlling seaweed growth in the oceans. Although they are known to eat other animals such as mussels and sponges, sea urchins feed predominantly on seaweed and by doing so they restrict its growth. In areas where sea urchins have experienced dramatic declines, such as from overfishing, seaweeds quickly flourish and grow to heights that make them inedible to other smaller animals. In contrast, in areas where sea urchins have had population explosions, patches of seaweed get decimated to the point where they become almost desolate. It is a population explosion of sea urchins that is thought to have brought about the current dramatic losses of seaweed from the giant kelp forests in Tasmania.

Sea urchins feed mostly at night and this is when they are most active. While they don’t look they can move, sea urchins are actually quite mobile and move by using little adhesive suckers that are situated on their underside (the side facing the rock or ocean bottom, which is also where their mouths are located). These suckers, called tube feet, also help them cling to rocks and seaweed. If you ever try to pick up a sea urchin you will notice they pack a firm grip and it takes a lot of effort to get them off the rocks they cling to. This is another mechanism for protecting them from predators and also against big powerful swells that may otherwise knock them around.

Sea urchins are unique creatures and, apart from the closely related starfish and sea cucumbers, they share very little in common with other animal species. In saying that, however, there are some similarities with humans and they have become a model organism for studying fertilisation and early embryonic development. In fact, it was with sea urchins that it was first observed in the 1800s that sperm were responsible for fertilising eggs, paving the way for the development of the human reproductive technologies that we have today. Sea urchins are also frequently used to monitor for pollution and habitat degradation as they are very sensitive to changes in their environment.

Sea urchins are abundant in the Eastern Beaches and are often seen in dense groups just below the low tide mark. The species found locally do not have poisonous spines, although the spines can still puncture skin if they are trodden on so it’s best to take care around them.