It is a monotypic genus and the only species is Tetrapygus niger which was first described by the Chilean naturalist Juan Ignacio Molina in 1782.

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.

[6] Starfish including Luidia magellanica, Meyenaster gelatinosus, Stichaster striatus and Heliaster helianthus prey heavily on grazing herbivores such as T.

[7] In 1998–1999, T. niger had a population explosion in northern Chile, possibly linked to a decrease in the numbers of predatory starfish, L. magellanica and M. gelatinosus, which decrease was probably caused by failure of the starfish to recruit as a result of the previous year's El Niño.

[8] This resulted in a tendency to form "urchin barrens" with no macro-algae and limited biodiversity, the rocks being covered with encrusting coralline algae.