The Heart Urchin (Echinocardium cordatum) is found in sub-tidal regions in temperate seas around the world and lives buried in the sandy sea floor.
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. It is a fawn colour but the tests that are found on the strandline have often lost their spines and are white. The spines trap air which helps prevent asphyxiation for the buried urchin, they also act and an anchor and aid in burrow burying. The ambulacrum forms a broad furrow in a star shape extending down the sides of test. There are two series each of two rows of tube feet. The test reaches from six to nine centimetres in length.
The Heart Urchin has a discontinuous cosmopolitan distribution. It is found in 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. It is very common round the coasts of the British Isles in the neritic zone.
The Heart Urchin buries itself in sand to a depth of ten to fifteen centimetres. It occurs in sediments with a wide range of grain sizes but prefers sediments with a size of 200 to 300 µm and a low mud content. It makes a respiratory channel leading to the surface and two sanitary channels behind itself, all lined by a mucus secretion.The location of burrows can be recognised by a conical depression on the surface in which detritus collects. This organic debris is used by the buried animal as food and is passed down by means of the long tube feet found in the front of the ambulacrum.
The sexes are separate in the sea potato and the males and females both liberate gametes into the water table in the spring. The echinoplutei larvae that develop after fertilisation have four pairs of arms and are laterally flattened. In late stage larvae, tube feet may be seen developing round the skeleton. The larvae are pelagic and form part of the zooplankton. Metamorphosis take place about 39 days after fertilisation with the larvae settling out and burrowing into the substrate. The lifespan of the sea potato is thought to be ten or more years.