A seashore favourite of mine is the Heart Urchin, also known as a Sea Potato, Echinocardium cordatum (Pennant). Here is one from the strandline at Whiteford Sands, Gower, South Wales. This photograph shows the under surface of the empty test or shell of the animal with the spines still attached.
The photograph below shows the upper surface of the sea urchin test – again with the spines still attached but you will notice that the individual spines are a different shape from the ones on the lower surface.
The spines easily fall off after the sea urchin dies because they are held in place by the skin which rapidly disintegrates. The picture below (taken on Rhossili beach) shows the upper surface of the empty test without any spines. You can see the pattern of tubercles with which the spines articulated.
Without the spines, five broad grooves are visible. Sea urchins, in common with other Echinodermata like starfish and brittle stars, have pentamerous symmetry – that is, the body design is based on fives.
In the picture below an empty Sea Potato test is shown without spines and the lower surface in view. In life, these seashore creatures burrow between 8 and 15 cm down in the wet sand at low tide level and in the sediments permenantly under water just beyond that (the shallow sublittoral zone).
You can find more information about this species on the marLIN web site.
Revision of a post first published 7 May 2009
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