Country diary: Eriskay

purple heart urchin (Spatangus purpureus), almost completely, Greece, Creta
Heart urchins are specialised burrowers that spend their days hidden under the sand beneath the sea’s surface. Photograph: Alamy

Lying on the silvery sand is an object the approximate size and shape of a small, slightly squashed jam doughnut. But the similarity ends there, for in colour and fragility it can only be compared to the best of meringues. Maybe it is its name that brings food similes so readily to mind, for what has washed up among the razor shells is the remainder of a sea potato, which despite its name is not a vegetable at all but a sea creature, a heart urchin.

Unlike other types of sea urchin, heart urchins do not roam the undersea rocks but lead a more secretive life. They are specialised burrowers that spend their days hidden under the sand beneath the sea's surface, breathing and feeding by means of a narrow mucus-coated tube. The remains usually found on the beach bear so little resemblance to the living animal that it is difficult to imagine how the creature got its name.

Only on the rare occasions when one is cast ashore, soon after death before the spines are lost, do we get any real idea of what lurks within the burrow. These yellowish brown spines all point in the same direction; they are also short and rather soft so the creature does have, with a stretch of the imagination, something of the appearance of a soggy furry potato.

What we more often find, though, is the white test or shell. A groove at one end gives the characteristic shape for which this type of urchin is named. One surface is pierced by five V-shaped bands of small double perforations radiating from the centre, marking where the creature's tube feet once emerged. The other surface reveals a smooth five-pointed star, its branches separated by the fine stippling of hundreds of tiny nodules. The fragile sea potato shells are easily shattered by the sea, but this morning I find several that are virtually intact; they are as lovely as fine matt-glazed ceramics.