Interesting facts on Heart urchin (Echinocardium cordatum).

A lot of different shells can be found along South African beaches. In Plettenberg Bay Pansy shells are the most famous shell, but the brittle shell of their cousins, the Heart urchins (Echinocardium cordatum), can also be found. Sea urchins (Echinoidea) belong to the phylum Echinoderms and are related to Sea stars (Asteroidea), Sea cucumbers (Holothuroidea) and Feather stars (Crinoidea). There are 3 types of sea urchins, spherical sea urchin (proper), heart-shaped sea urchins (sea potatoes) and flattened almost coin-shaped sea urchins (sand dollar). Sea urchins are herbivores and critically important as grazers to keep large seaweed species like kelp under control.

Like Sea stars urchins are pentaradial (five armed) and are formed into a rounded skeleton, known as a test. Urchins have moveable spines and tube feet in rows of five along the five “arms”. These tube feet are used both for movement and respiration. They pump water through the tube feet to the vascular system (gills) where oxygen is absorbed and carbon dioxide released. They have five pairs of teeth arranged in a circle creating a structure called Aristotle’s lantern, this structure is good at scraping food off hard surfaces and crushing sea shells.
The heart urchin has a heart- shaped shell around 6-10cm in length. This is covered with a large number of short, yellow-brown spines that are directed backwards making the heart urchin look hairy. They have two different type of spine;, spoon-shaped spines that look like shovels and are used to burrow themselves into the sand, and smaller spines that form a coat that protects them from becoming smothered by mud. The main difference between heart urchins and other sea urchins is that they have a clear front end (with the mouth) and back end (with the anus).

Sea urchins can be found in or on most substrates, from shallow lagoons and rocky shores to thousands of meters of depth. Heart urchins are found in sub-tidal regions in temperate oceans around the world and live buried in the sandy sea floor. Heart sea urchins feed by filtering organic material (plants and animals) from the sediment they live in. The material is transported to the mouth by the tube feet. They also have modified tube feet that create a channel up to the top of the sediment to keep a fresh flow of water to the animal.
Urchins have external fertilization, and then a planktonic larval phase which lives free in the water column before landing on the bottom and transforming into a juvenile urchin.

– Monks N. 2009 Echinoids. Deposits International rock and fossil magazine I. 20 P. 36-40
– Wisehart G.D. Rempala E.C. Leboffe M.J. 2012 A photographic atlas of marine biology ISBN: 978-089582-785-2

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