Echinoderms: Sea Stars (Asteroidea)

Sea stars, commonly known as Starfish, are found all around the coast of the Balearic Islands. They have some interesting features. As echinoderms, all of them have a water-vascular system (used for movement) and possess Mutable Collagenous Tissue (MCT) which is used to stiffen their bodies as a stress response and also for autotomy- the shedding of limbs when under attack or, in some cases, for asexual reproduction; a new animal can grow from the autotomised limb. Another common (but not universal) feature of Sea Stars is stomach inversion, which allows them to digest food outside their bodies.

There are four other species commonly found in the islands but which are a less conspicuous:

Astropecten spinulosus: endemic to the Balearic Islands with a grey, brown or pinkish colour, These bury themselves in the sand, making them difficult to see. I have watched one of these bury itself and it is a fascinating sight- the entire process took perhaps twenty seconds, after which it was entirely buried and invisible. Of the sea-stars described in this page, this is the only one that does not evert its stomach; instead, it consumed small molluscs whole, digests the soft tissues then ejects the hard parts (shells and radulae). In addition to the Ocelli (simple compound eyes at the end of the limbs, which all of these species possess), Astropecten spinulosus has individual photosensitive cells scattered all over it body, including the tube feet.

Blue Spiny Starfish (Coscinasterias tenuispina): This usually lives under rocks and seagrass meadows where it feeds on other echinoderms and bivalves. This species is bluish-white colour with and brown papulae and usually has 7 limbs, with small spines but specimens have been found with up to 12 limbs. Their reproduction mode is seasonal, possibly temperature-dependent.

Starlet cushion star (Asterina gibbosa): small in size (between 2-4 cm) with short arms and pentagonal shape. Greenish grey or reddish, it can be found under rocks, sand and Posidonia meadows at shallow depths. It feeds mostly on algal films and everts its stomach to do so. I have not found any information on autotomy in this species.

Spiny starfish (Marthasterias glacialis): the largest starfish of our coasts is greenish brown and with three rows of hard spines. It is particularly voracious and is active at night.

These last four species exhibit negative phototaxis. That is, they move towards shadow or away from light, either to hide in/under rocks and weed or to regulate nocturnal behaviour.

Two species, the Mediterranean Starfish (Echinaster sepositus) and the Purple Starfish (Ophidiaster ophidianus) are easily seen due to their bright coloration and contrast with the substrates. They appear similar but the differences are clear on close inspection:

The Mediterranean starfish (Echinaster sepositus) has a bright red colour and is found in shallow rocky environments. On its limbs are dispersed respiratory papulae.

The Purple Starfish (Ophidiaster ophidianus) has a darker red colour than the Mediterranean starfish is also found in shallow rocky areas but is usually found in the warmer waters it has a deeper colour than the Mediterranean Starfish – a purple-red- and its limbs become slightly narrower near the central disc. Instead of papulae, it exhibits linear respiratory ridges along its limbs.

Photographs by Sophia CHAPMAN

(Click on images to enlarge them.)

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