Arbacia nigra

Lore & Background

Arbacia nigra was first described by the Chilean naturalist Juan Ignacio Molina in 1782. Its test has a flattened oral surface and a shallowly domed aboral surface, with a small apical disc and straight ambulacral areas. The inter-ambulacral areas bear up to five large primary tubercles in rows, interspersed with smaller secondary tubercles. The mouth is surrounded by a sunken subpentagonal peristome half as wide as the test. Primary spines are moderately long, secondary spines short, and the overall color is purplish-black.

This sea urchin is found in the southeastern Pacific Ocean on the coasts of Peru and Chile, ranging from northern Peru to the Strait of Magellan, at depths down to about 40 m. It is the most common sea urchin on this stretch of coast.

Ecologically, Arbacia nigra is a herbivore that grazes on the kelp Lessonia trabeculata, the main constituent of kelp forests on the rocky coasts of Chile. Excessive grazing by the sea urchin prevents recruitment of the kelp by completely consuming young plants, while it only feeds on the stipe of older plants. Starfish such as Luidia magellanica, Meyenaster gelatinosus, Stichaster striatus, and Heliaster helianthus prey heavily on this sea urchin. In 1998–1999, a population explosion of T. niger occurred in northern Chile, possibly linked to a decrease in predatory starfish numbers, which decrease was probably caused by failure of the starfish to recruit as a result of the previous year's El Niño. This led to the formation of 'urchin barrens' with no macro-algae and limited biodiversity, the rocks covered with encrusting coralline algae.

Reader's Guide

Arbacia nigra is significant as the dominant sea urchin along the southeastern Pacific coast of South America, where its grazing behavior directly shapes kelp forest ecosystems. The article describes it as the most common sea urchin on this stretch of coast, indicating its ecological prominence. Its herbivory on the kelp Lessonia trabeculata can lead to a lack of kelp recruitment when grazing is excessive, as the urchin consumes young plants entirely. This dynamic is linked to broader ecological shifts: a population explosion of T. niger in northern Chile during 1998–1999, possibly caused by a decrease in predatory starfish (Luidia magellanica and Meyenaster gelatinosus) due to recruitment failure from the previous year's El Niño, resulted in 'urchin barrens'—areas devoid of macro-algae and with limited biodiversity, covered only by encrusting coralline algae. The legacy of this species is thus tied to its role as a keystone grazer and its capacity to trigger ecosystem state changes under altered predator regimes.

Did You Know?

• Its color is purplish-black.
• A population explosion in 1998–1999 in northern Chile led to the formation of 'urchin barrens'.