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Cushion Stars

Cushion Stars

The common cushion star (Asterina gibbosa)

A small flattened starfish, growing to 5 cm across. The dorsal surface is rough with projecting spines and most specimens are green in colour. T'here is often orange coloration along the edge of the arms.

It is occasionally found scavenging on dead crabs and often on bits of decomposing seaweed but is not able to catch live animals and the tube feet play no part in feeding. Asterina gibbosa's main source of food is bacterial diatom film (like A.phylactica) consumed by everting the stomach through the mouth and spreading it over the rock surface, using digestive enzymes to remove the film!  Maximum size is 40mm diameter and it is a protandrous hermaphrodite, male for the first three years and female for the next three, laying patches of 500-1000 unprotected eggs each year on the undersides of stones.

Cushion-stars are the only true starfish to be found on the intertidal seashore.

A type of starfish commonly found under boulders, rocks or in crevices on the rocky shores. It can be found in the middle or lower shore and below this into shallow water. It is found widely along the Atlantic and Mediterranean coast.

Asterina phylactica

Asterina phylactica, first described by Emson and Crump in 1979, is a second but rare species found here in Pembrokeshire. It is small (15mm max. diameter) pentagonal cushion star with short arms and bluntly rounded tips. It closely resembles Asterina gibbosa, (max.diam 50mm) but is differentiated by its small size and distinctive colour pattern of a red brown substar on a dark green background.

Asterina phylactica

The cushion star (Asterina phylactica) in a brooding aggregation with metamorphosing juveniles.

It has a completely different life history and reproductive strategy to A.gibbosa in that A.phylactica is a short lived species (usually two years) , being a functional male in the first year and a simultaneous hermaphrodite in the second. Individuals aggregate to breed in groups of 2-6 in May and the eggs are laid in early June. Self fertilisation is possible but cross fertilisation the norm. The eggs have direct development and are brooded (like a chicken) by the adult, for a period of three weeks. The newly metamorphosed juveniles crawl out from under the parent (see picture) which subsequently dies. Asterina phylactica is a microphagous feeder, everting the stomach through the mouth over the rock surface and digesting off the film of bacteria and diatoms . It is comparatively rare in Britain and normally lives on the sides and bottoms of stones in mid tide rock pools on semi exposed shores. The type locality is West Angle Bay in Pembrokeshire, where it almost became extinct due to severe oil pollution from the Sea Empress oil spill . The population, reduced to five individuals in 1997, has recovered to over 1000 in ten years, thanks in part to its ability to self fertilise and to protect the young. It is also known from sites in Devon , Cornwall, Anglesey , Eire, the west coast of  Scotland and the northern  mediterranean and ranges from midshore pools down into the sublittoral.

Thanks to Dr Robin Crump for the photograph and writing this piece on Asterina phylactica and comments on A. gibbosa.

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