The Echinoid Directory

Lifestyle - heart urchins

Heart urchins are rarely seen because most live buried within the sediment of the sea floor. The one illustrated below (Echinocardium) is shown dissected out in life position about 10 cm below the sediment-water interface. Heart urchins have a dense coating of curved, overlapping spines which have the appearance of hair. The role of these spines is to keep the sediment away from the surface of the test. In this way the animal can maintain a water-filled space around itself that is crucial for respiration when living infaunally. Those living within fine silts and muds may in addition produce a mucous coat to prevent finer particles from clogging the burrow. In finer sediments water circulation can be very sluggish and respiration becomes a problem so these species typically construct and maintain an open passageway from the surface down to their burrow (this is visible in the photograph below). This passageway is constructed using specialised aboral tube-feet in the frontal ambulacrum, and the basal opening is maintained by apical spines that form a conical tuft.

Heart urchins feed on organic material found within the sediment that they burrow through, or on surface detritus drawn down into the burrow via the respiratory funnel by tube-feet and water currents. Their life span can be as much as 12 years, but is usually shorter. They have few predators when buried, but subtidal species are preyed upon by shorebirds, and some marine snails also feed on them.

The sexes are separate, although they cannot usually be distinguished by external appearance.