Mostly people just find the empty dry tests or shells of ‘Sea Potato’ sea urchins on the sea shore. Seeing a living specimen seems a lucky find and a real privilege to a beachcomber like myself. The specimen photographed in this blog was found at low tide on the sand at Port Eynon, Gower. It looks quite different from the empty tests you find on the strandline – especially where all the spines have dropped off (see the earlier post Sea Potatoes from Gower).
The Sea Potato, Echinocardium cordatum (Pennant), is also known as the Heart Urchin – partly because of the overall shape of the test but also, surely, because of the heart-shaped patch of specialised spines on its oral or lower surface. You can see this clearly in the above picture where the animal is lying upside down on the sand with its back end towards the left of the picture.
In several of these photographs you can see some small pink bivalved molluscs amongst the sea urchin spines. These are not there by accident. They are very special animals which only live in association with this sea urchin species. They are called Tellimya ferruginosa (Montagu) and are commensal with the Sea Potato or Heart Urchin. Commensal means that they have this close association because they gain benefit from it, probably mostly from an enriched food supply and water flow – but they are not parasites that gain advantage at a cost to the host.
All the spines are mobile. Spines are attached by skin and muscles to small lumps or tubercles on the test. There are several different shapes of spines and groupings of spines. Flat tipped spines occupy the heart-shaped patch on the under surface. These are used in excavating the burrow and for locomotion.
Most of the test is covered by longer, finer, sharply pointed spines. These form special groups in some parts of the body such as the backward directed circlet surrounding the anus at the posterior end of the animal – visible in many of the pictures.
Another special grouping of long spines is found on the upper or aboral surface of the sea urchin. This tuft often projects upwards through the entrance to the burrow occupied by the animal.
Between the groups of spines are shallow grooves with very modified, small and articulated spines called pedicellaria; and extensible tube feet like those found in starfishes. The grooves are the ambulacra and form a shape like a five petalled flower. The tube feet are pushed out by hydrostatic pressure from the inside of the test to the outside via small holes in the test. You can often see these places as reddened dots amongst the spines even if you cannot see the tube feet themselves.
The spiny mat that covers the animal is mostly found in the areas between the ambulacral grooves; these areas are the interambulacra.
For a short video clip of a Sea Potato sea urchin found at the water’s edge, actively moving its spines and still with two of the commensal bivalves in position amongst the peri-anal spines, click on the link below:
The final photograph shows a living Sea Potato in its correct orientation with the aboral or dorsal surface uppermost, and the anterior or head end facing away from the viewer.
Revision of a post first published October 2009
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