Sea urchin

Lore & Background

Sea urchins are members of the phylum Echinodermata, which also includes starfish, sea cucumbers, sand dollars, brittle stars, and crinoids. Like other echinoderms, they have five-fold symmetry and move by means of hundreds of tiny, transparent, adhesive tube feet. The term 'sea urchin' refers to the 'regular echinoids', which are symmetrical and globular, and includes several taxonomic groups, with two subclasses: Euechinoidea and Cidaroidea. Irregular echinoids include flattened sand dollars, sea biscuits, and heart urchins. Together with sea cucumbers, they make up the subphylum Echinozoa, characterized by a globoid shape without arms or projecting rays. Urchins typically range in size from 3 to 10 cm, but the largest species can reach up to 36 cm. They have a rigid, usually spherical body bearing moveable spines, which give the class the name Echinoidea. The name urchin is an old word for hedgehog, which sea urchins resemble. Like other echinoderms, sea urchin early larvae have bilateral symmetry, but they develop five-fold symmetry as they mature. The mouth is at the base of the animal and the anus at the top. Several sea urchins, including sand dollars, are oval in shape with distinct front and rear ends, giving them a degree of bilateral symmetry. The internal organs are enclosed in a hard shell or test composed of fused plates of calcium carbonate. The test is rigid and divides into five ambulacral grooves separated by five wider interambulacral areas. All plates are covered in rounded tubercles to which the spines are attached. The spines are used for defense and locomotion. Sea urchins move by walking using their tube feet, assisted by spines. The mouth of most sea urchins is made up of five calcium carbonate teeth, and the entire chewing organ is known as Aristotle's lantern.

Reader's Guide

Sea urchins hold significant importance in both ecological and scientific contexts. Ecologically, they are key grazers of algae on the seabed, and when unchecked by predators such as sharks, sea otters, starfish, wolf eels, triggerfish, and humans, they can create urchin barrens—damaged environments devoid of large algae and the animals associated with them. Scientifically, sea urchins have been studied since the 19th century as model organisms in developmental biology because their embryos were easy to observe. This has continued with studies of their genomes due to their unusual fivefold symmetry and relationship to chordates. As deuterostomes, they are closely related to chordates, making them valuable for understanding evolutionary development. Species such as the slate pencil urchin are popular in aquaria for controlling algae. Fossil urchins have been used as protective amulets. The structure of their mouth and teeth has been found so efficient at grasping and grinding that similar structures have been tested for use in mechanical applications.

Did You Know?

• Sea urchins convert aqueous carbon dioxide using a catalytic process involving nickel into the calcium carbonate portion of their test.
• The name 'urchin' is an old word for hedgehog, which sea urchins resemble; they have archaically been called sea hedgehogs.
• The entire chewing organ of sea urchins is known as Aristotle's lantern, from Aristotle's description in his History of Animals.
• The red sea urchin manages about 7.5 cm a day when there is ample food, and up to 50 cm a day where there is not.