Sea urchin and cushion star at Fisherman's Beach

Sea life: Urchins

I used to love collecting sea urchin shells as a child – we’d go on holiday to Betty’s Bay, and I would collect handfuls of shells from tiny to huge. When we went home, I’d make strings of shells, arranged in size from largest to smallest.

Sea urchins and brittlestar at Fisherman's Beach
Sea urchins and brittlestar at Fisherman's Beach

The bare, empty shells look green, but at the aquarium we have looked at them under the microscope and they are actually far more detailed than just plain old army green. The live urchin has spines, tube feet, and little pincers, all of which protrude from different holes in its shell. The outer surface of the shell is ribbed, with lots of lumps and bumps and gradations of colour all the way from green to purple. And looking at a live urchin under a microscope is amazing indeed.

Sea urchin at Fisherman's Beach
Sea urchin at Fisherman's Beach

Sea urchins tend to like to live on rocks – we don’t see many of them at Long Beach (possibly also because they’ve been eaten by the rock lobsters) but they’re prolific at Fisherman’s Beach, Shark Alley and Sunny Cove. They can be found clustered on the rocky reefs, many of them with a piece of seaweed as a hat.

Sea urchins at Fisherman's Beach
Sea urchins at Fisherman's Beach

Juvenile abalone shelter among sea urchins, and disturbing the balance of the sea urchin population can have devastating consequences for abalone. One of the primary predators of sea urchins is the West Coast rock lobster (commonly, and incorrectly, referred to as crayfish). When the population of rock lobster gets out of control, they eat too many urchins, and the juvenile abalone have nowhere to hide. The abalone population can consequently collapse. This has been documented and studied at several sites in the Western Cape. This has implications for fisheries management.

Sea urchin and cushion star at Fisherman's Beach
Sea urchin and cushion star at Fisherman's Beach

Sea urchin spines are a pain (literally) if you get them in your foot or other body part. For this reason your buoyancy needs to be good if you plan to swim over urchin country. There are several kinds of urchin found in the Cape, but I’ve only seen the one depicted in these photos so far. Urchin beds are more active than they seem at first glance, and worth a look when you’re diving in a rocky area.

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Clare

Lapsed mathematician, creator of order, formulator of hypotheses. Lover of the ocean, being outdoors, the bush, reading, photography, travelling (especially in Africa) and road trips.

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