The fragile shells or tests of sea potatoes or heart urchins are regularly found on Assynt beaches, but I have never before seen one as large as this. It was collected from the eastern shore of Oldany Island (NC0934) by Wendy Glover, after she and Steve Collie had canoed across on 4th March 2018.
Sea potatoes are recorded up to 90mm, the length of this one. Much more usual are examples up to 40mm, like the other shown here, which, by coincidence, Gwen Richards and I found at Achmelvich the same day.
These tests have lost their ‘furry’ coating of recumbent spines, which in life help them to burrow down to 150mm beneath the sandy seabeds which are their natural habitat. They are one of the commoner examples, on local shores, of the echinoderms, a large group of exclusively marine organisms, uniquely 5-radiate in design, which also include starfish, brittle stars and sea cucumbers.