Arbacia nigra

Arbacia nigra is a regular rocky-shore sea urchin native to the Pacific coast of South America, from northern Peru to southern Chile ^[1]. It was described in 1782 as Echinus niger, was long known as Tetrapygus niger, and since 2023 has been placed in Arbacia as the sister species of Arbacia dufresnii ^[2],[3]. It is a herbivore that feeds mostly on brown algae and keeps mussels from colonising the rock ^[4],[5].

Molina described the species in 1782 as Echinus niger ^[3]. Agassiz erected the genus Tetrapygus in 1841 for this single species, and in 1846 A. Agassiz had already proposed the combination Arbacia nigra ^[2]. Using mitochondrial (COI), nuclear (28S) and morphological data, Courville and colleagues returned the species to Arbacia in 2023, and the WoRMS record treats Tetrapygus niger as a superseded combination ^[2],[3].

It lives along the Pacific coast of South America from northern Peru to southern Chile, on rock and artificial structures in rocky habitats, especially where there are kelp forests and strong wave action ^[1]. A review places its range from the low intertidal down to 10 m depth, with juveniles mainly in crevices shallower than 2.5 m ^[1].

In central Chile, the shallowest part of the subtidal and the shallow edge of the Lessonia trabeculata forest served as recruitment zones between March and November 1990, and juveniles under 24 mm test diameter were associated with crevices ^[6]. Outside the kelp bed the animals appeared aggregated, and abundance declined toward the centre of the kelp forest ^[6].

It is a generalist herbivore whose diet consists mostly of brown algae, plus algal turfs, encrusting coralline algae, drift algae, mussel recruits and crustaceans ^[1]. In the intertidal of central Chile, Lessonia spp. made up 68% of the diet, and gonad development depended on the availability of drift kelp ^[4]. In Ancón bay in Peru, gut-content and isotope analyses showed that it also eats benthic macroalgae ^[5].

The larva is an unusual form of echinopluteus which, in culture at 14–18 °C, spent 94 to 120 days in the plankton and grew to more than 4.5 mm ^[7]. Roughly 40% of the larvae became juveniles, and the whole change from feeding larva to feeding adult lasted 4 to 6 days ^[7]. In controlled cultures, early development depends on temperature: reaching the early blastula took 14 hours at 15 °C, 45 hours at 10 °C and 164 hours at 5 °C ^[8].

In the low intertidal of Ancón bay, in central Peru, inclusion and exclusion experiments showed that this species prevents sessile organisms from colonising the substrate, including the dominant mussel Semimytilus algosus ^[5]. The authors suggest that by limiting mussels it may reduce the diversity of associated species, and that by keeping rock bare it benefits other grazers ^[5].

A review describes it as the most abundant urchin in Peru and Chile, with densities of up to 85 individuals per m2, and notes that at such densities barrens can form where algal growth is inhibited ^[1]. In Chile some individuals live on elevated surfaces and so avoid the predatory sea stars Heliaster helianthus and Meyenaster gelatinosus: in trials the urchin took 48% less time to break contact on elevated surfaces than on the bottom ^[9]. The experiments point to Meyenaster gelatinosus as a greater threat than Heliaster helianthus ^[9].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

Among 77 specimens from the central Peruvian coast, sampled at Pucusana, La Punta and Ventanilla, 905 microplastic-like particles were recovered, that is 11.75 per individual, mostly yellow, blue and red fragments and fibres ^[10]. In an earlier study of the species, the digestive tract held on average 3.22 microplastics per individual, with a range of 1 to 5, and fibres made up 75.9% ^[11].

At Caleta Horcón, an industrial zone in Chile, the gonads of urchins fed on algae cultivated there accumulated metals (Cu, As, Cd and Pb) at concentrations between 5 and 798 times higher than in the alga ^[12].

Embryos, larvae and sperm of this species have been used as laboratory material: one study used sperm nuclei to measure lipid peroxidation from oxidative stress with hydrogen peroxide ^[13], and another described embryonic development at 5, 10 and 15 °C ^[8]. Outside its native range, a 2007 record from an aquaculture facility in South Africa led to its listing as an introduced species, but a new survey in September 2014 found it at no site, and removal from invasive-species lists was recommended ^[1].

The Peruvian literature documents it in the rocky low intertidal of Ancón bay, at 11°46′S, where it lives alongside the snail Tegula atra ^[5]. A study published in 2026 gathered specimens at Pucusana, La Punta and Ventanilla, on the central littoral ^[10]. A review indicates that it occupies the low intertidal down to about 10 m depth, and that it is found particularly where there are kelp forests and strong wave action ^[1].