It has an average length of 9.0 cm, but can reach lengths of 14.0 cm. It has 12 dorsal spines, and 15 to 17 dorsal soft rays. It also has 2 anal spines and 12 to 14 anal soft rays. Adults are generally brown, with the dorsal parts of the head and nape being darker, grading to tan on the lower parts of the head and breast. The scales have darker brown margins. The lips are whitish, the suborbital is mostly blue, and the preopercle and opercle scales have blue centers. The median and pelvic fins are brown, the pectorals are dusky, and sometimes there's a well-defined dark brown or blackish spot at the base of the posteriormost dorsal rays, which distinguishes it from the S. lividus, where the spot is diffuse. When males are in courtship or guarding their eggs they have a broad white bar across the middle of the body and a pale blue stripe from the mouth to the upper part of the pectoral fin.
S. nigricans is most frequently found across the coast of East Africa and around Madagascar; in the British Indian Ocean Territory and the southern coasts of India; across Southeast Asia, the northern coast of Australia; and Melanesia, Micronesia and Polynesia. They can also be found less frequently in the Red Sea, the Persian Gulf, the Gulf of California, and across the Western coast of Central America.
Adults inhabit reef flats and lagoon reefs. They frequently occur in colonies associated with live or dead branching staghorn coral (Acropora). They feed on algae, gastropods, sponges and copepods. They are territorial, and they maintain and "weed" filamentous algae patches growing on dead coral. They attack human intruders without hesitation, sometimes taking painful nips. They're particularly aggressive during reproductive periods. During aggressive encounters, they emit clicking noises. Males guard and tend the nest, visited by several females.
S. nigricans practices a form of agriculture with a species of red algae. The fish will claim a patch of "brown carpet algae" which it defends by chasing away other fish and sea urchins. The fish also pulls up other bits of algae that attempt to grow in the patch and swims outside of its territory to spit the invading algae out. When the fish claiming a patch is removed, the patch is eaten up within a few days. When a patch of the brown carpet algae is caged to keep both S. nigricans and other fish out of the patch, other species of algae quickly overwhelm the patch. This seems to indicate the brown carpet algae are dependent on S. nigricans for its survival.
- Capuli, Estelita Emily (n.d.). "Stegastes nigricans". fishbase.sinica.edu.tw. Retrieved 2016-02-05.
- Hata, Hiroki; Kato, Makoto (December 22, 2006). "A novel obligate cultivation mutualism between damselfish and Polysiphonia algae". Biology Letters. 2 (4): 593–6. PMC . PMID 17148297. doi:10.1098/rsbl.2006.0528. Retrieved 2007-01-15.
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