Sea Potato

The sea potato is a heart-shaped urchin clothed in a dense mat of furrowed yellowish spines which grow from tubercles and mostly point backwards. The upper surface is flattened and there is an indentation near the front. This urchin is a fawn colour but the tests that are found on the strandline have often lost their spines and are white. During life, the spines trap air which helps prevent asphyxiation for the buried urchin. The ambulacra form a broad furrow in a star shape extending down the sides of the test. There are two series each of two rows of tube feet. The test reaches from six to nine centimetres in length.

The sea potato has a discontinuous cosmopolitan distribution. It is reported from temperate seas in the north Atlantic Ocean, the west Pacific Ocean, around Australia, New Zealand, South Africa and the Gulf of California at depths of down to 230 metres. A 2016 study revealed that it was a complex of at least 5 species, with three in Europe, one in Australia and one in the NW Pacific. It is very common round the coasts of the British Isles in the neritic zone.

In the sandy sea bed that it favours, the sea potato is often found in association with the bivalve molluscs Tellina fabula, Ensis ensis and Venus striatula.

The bivalve Tellimya ferruginosa is often found living inside the sea potato's burrow as a commensal. Up to fourteen have been found in one burrow with the young being attached to the spines of the urchin by byssus threads. Another species that makes use of the burrow is the amphipod crustacean, Urothoe marina.