''Tetrapygus'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

sea urchin Sea urchins or urchins () are echinoderms in the class (biology), class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body cove ...

s in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Arbaciidae Arbacioida are an order of sea urchins, consisting of a single extant family, the Arbaciidae. They are distinguished from other sea urchins by the presence of five separate plates around the anus. Unlike their close relatives, the Salenioida, all ...

. It is a

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...

genus and the only species is ''Tetrapygus niger'' which was first described by the Chilean naturalist

Juan Ignacio Molina Fr. Juan Ignacio Molina (; (June 24, 1740 – September 12, 1829) was a Chilean-Spanish Jesuit priest, natural history, naturalist, historian, translator, geographer, botanist, ornithologist, and linguist. He is usually referred to as Abate Moli ...

in 1782. It is found in the southeastern Pacific Ocean on the coasts of South America.

Description

The oral (under) surface of the

test Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film) ...

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.

Distribution

''T. niger'' 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 The Strait of Magellan (), also called the Straits of Magellan, is a navigable sea route in southern Chile separating mainland South America to the north and the Tierra del Fuego archipelago to the south. Considered the most important natura ...

, at depths down to about . It is the most common sea urchin on this stretch of coast.

Ecology

''T. niger'' is a herbivore and grazes on the

kelp Kelps are large brown algae or seaweeds that make up the order (biology), order Laminariales. There are about 30 different genus, genera. Despite its appearance and use of photosynthesis in chloroplasts, kelp is technically not a plant but a str ...

Lessonia trabeculata ''Lessonia trabeculata'' is a species of kelp, a brown alga in the genus '' Lessonia''. It grows subtidally off the coasts of Peru and northern and central Chile, with the closely related '' Lessonia nigrescens'' tending to form a separate zone i ...

'', 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. Starfish including '' Luidia magellanica'', '' Meyenaster gelatinosus'', '' Stichaster striatus'' and '' Heliaster helianthus'' prey heavily on grazing herbivores such as ''T. niger''. 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 EL, El or el may refer to: Arts and entertainment Fictional entities * El, a character from the manga series ''Shugo Chara!'' by Peach-Pit * Eleven (''Stranger Things'') (El), a fictional character in the TV series ''Stranger Things'' * El, fami ...

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

coralline algae Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls. The colors of these algae are most typically pink, or some other shade of re ...

References

{{Taxonbar, from=Q3519178, from2=Q3203595 Asteriidae Monotypic echinoderm genera