The Martasteria or Spiny Starfish, Marthasterias glacialis, is an echinoderm of the Asteriidae family common in the Mediterranean Sea. Also called a Tank star, due to the characteristic plates bristling with thorns that distinguish it from other starfish, it has a regular shape with five arms. The body is characterized by spines of variable color: white, gray, brown, red, blue. It appears to be the largest species in the Mediterranean, capable of reaching even 85 centimeters in diameter. stella marina spinosa
It lives in the western Mediterranean Sea, on rocky bottoms or in the Posidonia Oceanica meadows up to about 200 meters deep. We met it in the upper Tyrrhenian Sea in a splendid underwater reef 48 meters deep. The reef full of life, as you can see from the video, was home to various typical Mediterranean fish and a varied marine biology. The starfish Marthasterias glacialis can be confused with the Stella Coscinasterias tenuispina, from which, however, it is distinguished by the number of arms and the regularity of the shape; the variable starfish in fact has an irregular shape and a number of arms ranging from 6 to 12 even if it usually has 7.
Echinoderms possess a unique water vascular system. This is a network of fluid-filled canals derived from the coelom (body cavity) that function in gas exchange, feeding, sensory reception and locomotion. This system varies between different classes of echinoderm but typically opens to the exterior through a sieve-like madreporite on the aboral (upper) surface of the animal. The madreporite is linked to a slender duct, the stone canal, which extends to a ring canal that encircles the mouth or oesophagus. From this, radial canals extend along the arms of asteroids and adjoin the test in the ambulacral areas of echinoids. Short lateral canals branch off the radial canals, each one ending in an ampulla.
Part of the ampulla can protrude through a pore (or a pair of pores in sea urchins) to the exterior and is known as a podium or tube feet. The water vascular system assists with the distribution of nutrients throughout the animal’s body and is most obviously expressed in the tube feet which can be extended or contracted by the redistribution of fluid between the foot and the internal sac.