The Cushion Starfish and the Pillars of Hercules

The Cushion Starfish, Asterina gibbosa

With its short arms projecting as short tips from its pentagonal body, the Cushion Starfish (Asterina gibbosa) is deserving of its name. Pincushion would be the more appropriate moniker though, as the animal, though capable of growing to 5 cm, is rarely seen bigger than 3 cm in diameter. It is found in the lower shore, often feeding on worms, brittle stars, sponges, star ascidians and hydroids in intertidal rock pools (Challinor et al., 2003 A Beginner's Guide to Ireland's Seashore p. 156), although it can be found to depths of 130 m (Hook, 2008 A Concise Guide to the Seashore p. 125). A. gibbosa shows a high variability in colour, ranging from red to yellow to green and is dispersed throughout the Atlantic and the Mediterranean.

Underside of Asterina gibbosa

A study on the relatedness of A. gibbosa in these areas (Baus et al., 2005 Molecular Ecology 14 pp. 3373 – 3382) revealed an interesting fact: that populations showed high levels of genetic differentiation between the Atlantic and the Mediterranean. This suggests that the Strait of Gibraltar represents a major barrier to dispersal for A. gibbosa dispersal. This so called “Pillars of Hercules” hypothesis has been reported for a number of other marine species (the spider crab Maja squinado (Neumann, 1998 Journal of Natural History 32 pp. 1667 – 1684), the sea bream Diplodus puntazzo (Bargelloni et al., 2005 Molecular Phylogenetics and Evolution 36 pp. 523-535), the barnacle Chthamalus montagui (Pannacciulli et al., 1996 Marine Biology 128 pp. 73-82)) and postulates that the Strait of Gibraltar represents a phylogeographic break, or a disruption in the geographical distribution of species. However Patarnello et al. (2007 Molecular Ecology 16 pp. 4426-44), on analysing numerous studies on genetic differentiation between the two regions suggest that observed differences are due to a combination of signature of vicariance, palaeoclimate fluctuation and life-history traits.