Black Sea Urchin

The oral (under) surface of the test is flattened while the aboral (upper) surface is shallowly domed. There is a small apical disc and the ambulacral areas are straight. There are up to five large primary tubercles in rows in the inter-ambulacral areas, interspersed with smaller secondary tubercles. The mouth is surrounded by a sunken subpentagonal peristome which is half as wide as the test. The primary spines are moderately long while the secondary spines are short. The colour of this sea urchin is purplish-black.

Arbacia nigra is found in the southeastern Pacific Ocean on the coasts of Peru and Chile, its range extending from northern Peru to the Strait of Magellan, at depths down to about 40 m (130 ft). It is the most common sea urchin on this stretch of coast.

Arbacia nigra is a herbivore and grazes on the kelp Lessonia trabeculata, the main constituent of the kelp forests on the rocky coasts of Chile. Excessive grazing by the sea urchin causes lack of recruitment of the kelp because the sea urchin completely consumes young plants, while it only feeds on the stipe (stalk) of older plants.