Sea Urchins

Sea urchins are found on Cornish shores but they are not often seen. The largest numbers tend to be in the sublitoral zone hence not uncovered by the tides. Sea urchins in the intertidal zone are usually well hidden; by camouflaging themselves by attaching seaweed to their bodies or by burying themselves in the sand. When they die their shells or tests are washed ashore, however due to their fragility are very quickly destroyed.

Sea urchin shells are very decorative and unfortunately this has been the undoing of some species such as the edible sea urchin. This species has been collected to such a degree that their numbers have severely declined.

Physiology

Sea urchins have a shell or test which is comprised of calcite plates. The test appears to be an exoskeleton but it is really an internal one, as is it covered by a thin layer of living tissue.

Projecting from the test are spikes. The spines are used as a deterrent as well as an aid for movement. With some species such as Sea Potato Echinocardium cordatum, the spines have become modified for digging.

The spines if worn or lost are regenerated. When an empty test is found there will be no spikes, these falling off soon after death. Where the spines were, can be seen small nodules.

The tube like feet, protrude through pores in the test. The pores are arranged in a five double bands reflecting sea urchins’ five fold symmetry. If one holds an empty urchin shell and lifts it to the light, the bandings of small pin size holes will be seen. The feet do not only provide movement but some are used for sensing, attachment, feeding and breathing.

Located at the shells apex is the mouth which contains the Aristotle’s lantern. The Aristotle’s lantern controls the manipulation of the teeth. The teeth are used to graze the rockfaces. When finding a empty test it is worth shaking, as often the Aristotle’s lantern is trapped inside, this can be shaken out for study.

Internally the shell is primary taken up by the gut and the five gonads. The gonads of the Edible Sea Urchin Echinus esculentus are the parts of the sea urchin eaten.

Locomotion

Sea urchins have tubed feet, each foot ending in a sucker. These suckers are used to grip the surface, the feet then contract so moving the urchin. This locomotion is driven by a water vascular system. This is a hydraulic system water, is pumped to the tubed feet or from them, so enabling extension or contraction.

Reproduction

In sea urchins the sexes are separate. Spawning is synchronised in species, this mass spawning improves the chances of fertilisation and reduces the overall affect of predation. Eggs and sperm are ejected into the sea, the fertilised eggs developing into free swimming larvae. The larvae join the other plankton and are carried by the currents until they settle on the sea bed and metamorphose into a minute adult.

Species of Sea Urchin Found In Cornwall

    Shore Urchin Psammechinus miliaris

  Edible Sea Urchin Echinus esculentus

     Sea Potato Echinocardium cordatum

Shore Urchin Psammechinus miliaris

The Shore Urchin is the most common urchin found on Cornish rocky shores. They are a small urchin, rarely over an inch and a half. The green spines have a purple tip giving it its alternative name the purple tipped sea urchin.

The shore urchin is an omnivore, as well as grazing algae, will also take small sessile creatures such as barnacles, bryzoans and sea squirts.

Shore urchins are found in rock pools but are often missed as they camouflage themselves by gripping to the shell, pebbles, shells and seaweed. It has been said that this may not be for camouflage, but for when they are stranded in shallow rock pools, to shade them from the sunlight.

Edible Sea Urchin Echinus esculentus

The edible sea urchin are common but only offshore, but they can be found on the lower shore. The test is reddish while its spines are more purple.

Urchins' tests are sold in souvenir shops often being made into lamp shades. The meat is considered good eating in France. They are dived for, heavily reducing their numbers.

Sea Potato Echinocardium cordatum  

This is a heart shaped urchin reaching three inches across, which can be common on the lower shore of clean sandy beaches.

The upper side is covered by numerous fine partially flattened spines giving the impression of hairiness. The majority of the feet are also are on the upperside, the pores through which they protrude, form a star shape.

The under side has spines modified for burrowing, these have evolved to become longer with spatula shaped ends. It is a burrowing urchin, digging down as deep as eight inches. An indicator of its existence is a small hole at the top of its burrow.

The sea potato has specialized long feet for reaching up the burrow to keep it in a good condition. It is essential for there to be a clear flow of water drawn into the burrow for respiration. As the burrows can be in fine sand, the burrow is lined with mucus to prevent it from caving in.

The sea potato feeds on organic detritus it comes across while burrowing. The mouth has no Aristotle’s lantern or teeth.

When the tests are found, it can be in large numbers due to spats of high mortality, which is normally after a storm, as it takes such an event to remove them from their burrows. Their tests are extremely fragile and very quickly get smashed by wave action.