Sea Stars (Asteroidea)

The species group can often be identified without enlarging aids
  • Common Sea star (Asterias rubens). Small specimen on a VIRTUE disc
  • Common Sea star (Asterias rubens)

Although named as starfish for a long time, sea stars are not fish. They are invertebrates belonging to phylum Echinodermata under class Asteroidea. Like all echinoderms they are radially symmetrical with five arms or multiples of which. All sea stars are marine and live on coral reefs, on sand, rocks or on mussel beds. The upper side of sea stars is rough to the touch being made of calcium carbonate plates with tiny spines on their surface. The underside of sea stars is softer and lighter in colour. Here you can find the small mouth and the anus very close to each other on the central plate. Located in grooves along the middle of the arms of the sea stars are hundreds of tube feet which they use for crawling, feeding and breathing.

The ends of the tube feet are modified to act as suction cups and these help the sea stars to attach to surfaces. 
Sea stars reproduce sexually by external fertilization and the fertilized eggs undergo several planktonic larval stages before becoming adults. Other species of sea stars can reproduce asexually by fragmentation. They can regenerate lost body parts. 
Sea stars are greedy predators feeding on mussels, clams, small fish, snails and barnacles. They have the ability of pushing their stomach out through their mouth and thus they can perform digestion outside their bodies. This enables them to consume larger organisms which normally will not fit into their tiny mouths.  

Sea stars can “see”. Located at the tips of their arms are light sensitive dots which react negatively to light. They can also “smell”. They are equipped with chemoreceptors on their skin which can detect the presence of a prey nearby. Sea stars can “taste” the salinity of the water they are placed in. If put in the middle of two basins with differing salinities they will always go to the basin with water whose salinity matches the water where they live. Despite this ability, sea stars are poor osmoregulators.

The sea stars we find on the discs are there feeding on the barnacles and mussels.