Seeing stars in the Mediterranean Sea

by B. N. Sullivan

If you go diving or snorkeling in coastal areas of the Mediterranean region, you are very likely to see this Red Sea Star (Echinaster sepositus), which is the most common starfish species in that area.  Divers and snorkelers have a good chance to spot this sea star resting on the bottom along rocky coastlines.  Although there are records of Mediterranean Red Sea Stars found at depths of more than 200 meters, they are seen most often in relatively shallow water -- less than 10 meters.

Mediterranean Red Sea Stars are actually red-orange in color, rather than pure red.  As sea stars go, they are relatively large.  The one in the top photo on this page was about 20 cm (8 in) in diameter.  The individual in the photos below was even larger -- nearly 30 cm (12 in) across.

Like many common sea star species, Echinaster sepositus has five rays, or arms.  One time, however, we found a six-armed sea star that looked just like E. sepositus, except that it had an extra arm.  At first I was not certain that it was indeed the same species, so I took the time to photograph it carefully.  Someone who knows much more than we do about Echinoderms looked at the photos and assured us that the six-armed sea star was a less common, though well-known, morphological variant of E. sepositus.

The two macro photos below are of the unusual six-armed Mediterranean Red Sea Star.  In these macro photos you can see the characteristic surface of the sea star, which is uneven -- sort of dimpled.  In the first of the two photos below, you can see the tiny structures, called papullae, which protrude from the 'dimple' indentations on the sea star's surface. In the second photo below you can see the animal's tube feet protruding from grooves on the underside of each ray.

Both the tube feet and the papullae facilitate water exchange, and are involved in the animal's respiration and excretion.  The sea star 'breathes' by extracting oxygen from sea water.  It excretes some dissolved waste matter through the papullae and tube feet, too.

The tube feet also aid the sea star in locomotion.  The tube feet in each row move successively, in a wave, using hydraulic pressure from the animal's water vascular system.  This allows the critter to crawl along.  By a similar process, the tube feet also can be used to pass bits of food from the distal ends of the rays to the sea star's mouth, which is at the center of its underside.

If you look closely at the second macro photo below, you can see that there is a little suction cup at the tip of each tube foot.  These structures help the animal stay put after it situates itself on a rock or other hard surface.  If a sea star gets overturned, it can right itself by twisting one or more of its rays so that the tube feet can grab hold of the surface and turn itself right-side up again.  (This process can take quite awhile and consumes a lot of the critter's energy, so if you pick up a sea star to look at it, be kind enough to return it to its normal position!)

The sea star lounging in an algae bed in the photo at the top of this page was photographed in the shallows of Aedipsos Bay, on the coast of the Greek island of Evia.  The two macro images below, of the six-armed Mediterranean Red Sea Star, were photographed at Cape Greco, Cyprus.