Saltwater Aquarium Guide -- Sea Urchins

Sea Urchins:

Sea Urchins can be classified into two larger groups by their morphology. The "regular" Urchins have a spherical to oval body shape, while the "irregular" Urchins that live in sandy substrates have flattened bodies. Sand dollars are "irregular" Urchins. While they congregate for sexual reproduction, they are generally solitary creatures.

Sea Urchins are approximately 2"-3" in diameter, with the spines extending possibly twice that distance. Urchins found in fast-moving water or inter-tidal zones usually have shorter spines, while those found in reefs have much longer spines, and those that traverse the substrate have the shortest spines.

The body consists of five double rows of two types of stationary plates, with tubular feet extending through only one type of these plates. Smaller, tube feet (pedicellariae) are distributed over the body and help clean the spines, transport food, and even defend the Urchin, sometimes with a poisonous gland. Sea Urchins can often be seen carrying pieces of shells, rocks, or even food in the form of macro algae. Care must be taken to prevent the Urchins from accidentally overturning rocks in the reef tank causing damage to themselves and other reef inhabitants such as live corals. Intolerant of poor water quality, they may lose all of their spines when stressed. If proper water quality is provided, Urchins can regenerate their spines.

Sea Urchins make great additions to marine aquariums, consuming undesirable algae and detritus within the tank. The Sea Urchin is actually an omnivore, eating both plant matter and meaty foods, but most prefer algae. In the aquarium, they will slowly graze over the rocks and glass, and can be given an occasional piece of meaty food.

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