Starfish

Generally starfish have five arms which are hollow and covered with short spines. Starfish have the ability to take calcium carbonate from sea water, from this it forms a skeleton connected to calcareous plates implanted into the skin. Starfish skeletons are commonly sold as souvenirs in seaside shops.

Some starfish species predate commercial species of molluscs such as oysters. If starfish numbers are high enough they can annihilate oyster beds. In the past when caught by fishermen they would be chopped up and throw overboard in an attempt to reduce numbers. This did not have the desired affect as starfish have the phenomenal ability to regenerate. Losing an arm it will regenerate it, but if a single chopped arm contained a part of the main body it would form a new starfish, hence numbers just increased. Fortunately for the starfish each arm contains extensions of the vital organs.

As with the sea urchin, star fish movement is driven by a water vascular system. This system enables the tube feet on the underside of the arms to contract and elongate. The tube feet have suckers at their end, on contraction the suckers grip while on contraction they release their grip. By coordinating the gripping and releasing of the numerous tube feet, movement is possible.

Starfish do not possess jaws and depending on the species digest food in different ways. Some species turn outward their stomach through their mouth and completely envelop their prey (shellfish) swallowing it whole, digesting it, and later regurgitating the shell.

The common starfish has perfected the skill of opening bivalves. It coordinates its tube feet to constantly apply a pull on each valve. Eventually the bivalve tires and the valves part enough for the starfish to insert its stomach into the shell. The bivalve’s body is partially digested and later pulled into the starfish for complete digestion.

Species of Starfish Found In Cornwall

     Common Starfish Asterias rubens

  Cushion Star Asterina gibbosa

      Spiny Starfish Marthasterias glacialis

Common Starfish Asterias rubens

Primarily an offshore animal but is found on rocky shores on the lower shore under rocks. It can reach sizes well over a foot across, but this is only at depths, those found on shore at generally much smaller.

It has thick fleshy tapering limbs, the upper side of which are covered by blunt spines. The colour varies from yellow to orange brown, with a paler underside.

The common starfish is a voracious predator feeding primarily on bivalves. This starfish can be a serious problem to commercial beds of oysters, made worse as periodically there are sudden breeding booms when numbers reach epidemic proportions.

Cushion Star Asterina gibbosa

The cushion star is the most common starfish found on Cornwall's rocky shores, typically under rocks around the low tide mark. It is a small creature reaching only 2 inches.  

It has small stubby limbs. The colour varies depending on the actual habitat but is often green, orange to brown.

The upper body bears whisker like spines.

Spiny Starfish Marthasterias glacialis

The stiff limbs are covered by thick sharp spines. The spines are arranged in rows and are fastened to small calcareous plates.

The colouring varies from blue, green to purple with the underside being yellow.

This starfish can reach over a foot across, however onshore specimens are typically smaller. It feeds on shellfish and smaller starfish.