Cushion star at Long Beach

Sea life: Cushion stars

When I was a child, I thought all starfish looked like this.

Cushion star at Long Beach
Cushion star at Long Beach

Cushion stars are a kind of starfish, but instead of radiating limbs, they have legs that are so short that often only the tips are distinguishable from their bodies. They are extremely common in the rockpools along the Cape coastline, and we used to love picking them out among the algae and seaweed when we visited the beach. They are often very well camouflaged.

Cushion star on the Clan Stuart
Cushion star on the Clan Stuart

Like many starfish, cushion stars are sensitive to light, and in shallow water will often hide under a handy piece of sea lettuce. They are usually found on hard surfaces such as rocks, or the pipeline at Long Beach. They eat algae and anything else they can get hold of, including (yum) carrion and decaying organic matter in the ocean.

Cushion star on the Clan Stuart
Cushion star on the Clan Stuart

In the two photos above and the one just below you can see the greyish, roughly circular region just off centre on the top of the cushion star known as the sieve plate (or madreporite, if you want to be fancy). It’s made of pourous calcium carbonate. It is the entrance to the vascular system of the starfish: instead of blood and bones, sea stars draw water into their bodies and use this for a skeleton. It’s a handy adaptation that makes them pretty much incompressible, which is handy in an underwater environment. The water is also pumped to the tube feet underneath the creature, which enables it to move about.

The anus is roughly top and centre on the body of a starfish. You can just discern it in these images, most clearly on the one below (look for the tiny blue-grey circle with a black speck in the centre, to the left and above the sieve plate).

Cushion star at Long Beach
Cushion star at Long Beach

Both times I have visited Sodwana (October 2010 and April 2011) we have seen very large cushion stars. Here’s a close up of one we saw at Stringer. It was about the size of a birthday cake and its pentagonal shape was almost hidden by the bulge of the top of its body.

Close up of the giant cushion star
Close up of the giant cushion star

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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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