Stegastes nigricans
A territorial damselfish that farms algae on coral reefs.
Stegastes nigricans, the dusky farmerfish, is a damselfish that lives on tropical coral reefs, typically in waters between one and twelve meters deep and between latitudes 30°S and 30°N. It is best known for cultivating single-species patches of algae, including cyanophores and rhodophytes.
The species was first described in 1985 by Gerald Robert Allen and Alan R. Emery. Its genus name, *Stegastes*, comes from the Greek word for "covered," while *nigricans* is Greek for "swarthy" or "black-skinned." It belongs to the damselfish subfamily Pomacentrinae. Adults average 9.0 centimeters in length but can grow up to 14.0 centimeters. They have 12 dorsal spines and 15 to 17 dorsal soft rays, along with 2 anal spines and 12 to 14 anal soft rays. Their bodies are mostly brown, with darker shading on the top of the head and nape, fading to tan on the lower head and breast. The scales have darker brown edges. The lips are whitish, the area below the eye is mostly blue, and the scales on the preopercle and opercle have blue centers. The median and pelvic fins are brown, and the pectoral fins are dusky. A distinct dark brown or blackish spot at the base of the rearmost dorsal rays helps tell it apart from *Stegastes lividus*, which has a blurrier spot. During courtship or egg-guarding, males develop a broad white bar across the middle of their body and a pale blue stripe running from the mouth to the upper part of the pectoral fin.
These fish are found naturally on coral reefs, especially on reef flats and in lagoon reefs where staghorn coral grows. They often live in colonies. Their range is limited by water temperature and diet, and their populations have been negatively affected by changes in climate and ocean composition due to global warming and fossil fuel use, which are shrinking coral reef habitats. They are most common along the coast of East Africa and around Madagascar, in the British Indian Ocean Territory, along southern India, across Southeast Asia, the northern coast of Australia, and through Melanesia, Micronesia, and Polynesia. They are less common in the Red Sea, the Persian Gulf, the Gulf of California, and the western coast of Central America.
Adults live on reef flats and in lagoon reefs, often forming colonies around live or dead branching staghorn coral (*Acropora*). They eat algae, gastropods, sponges, and copepods.
Quick Facts
- Taxon
- Stegastes nigricans
Facts from the source article.
Lore & Background
Stegastes nigricans was first described and named by Gerald Robert 'Gerry' Allen and Alan R. Emery in 1985. The genus Stegastes is derived from the Greek 'stegastos,' meaning on or covered, while nigricans is Greek for swarthy and black-skinned. Recent research has placed S. nigricans in the damselfish subfamily Pomacentrinae. Adults are generally brown, with darker dorsal parts of the head and nape 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. A well-defined dark brown or blackish spot at the base of the posteriormost dorsal rays distinguishes it from S. lividus, where the spot is diffuse. Males in courtship or guarding eggs display 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 found naturally in and around coral reefs, inhabiting reef flats and lagoon reefs in colonies in areas with staghorn coral. Its distribution is limited by water temperature and diet, and its populations have been negatively impacted by climate and ocean composition changes due to global warming and fossil fuel use, which alter coral reef habitats. The species 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. It 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.
Reader's Guide
Stegastes nigricans is significant for its unique agricultural behavior: it claims a patch of algae, defends it by chasing away other fish and sea urchins, and removes invading algae by pulling it up and spitting it outside its territory. Because S. nigricans lacks cellulases in its stomach, it disposes of algae it cannot digest, limiting its diet to cyanophores and rhodophytes. It farms nine algae species, removing competitively superior late-colonizing algae and faster-growing species that would overwhelm its preferred delicate algae. When a fish claiming a patch is removed, the patch is eaten within a few days. Caging a patch of 'brown carpet algae' (Womersleyella setacea) to exclude S. nigricans and other fish results in other algae species quickly overwhelming the patch, indicating the brown carpet algae depend on S. nigricans for survival. The presence of S. nigricans greatly increases the primary productivity of the area by boosting oxygen, nitrogen, and phosphorus concentrations. The species is also notable for its aggressive territoriality, attacking human intruders without hesitation and emitting clicking noises during aggressive encounters. Males guard and tend the egg nest, visited by several females.
Did You Know?
- It has an average length of 9.0 cm but can reach 14.0 cm.
- The species does not have cellulases in its stomach, so it disposes of algae it cannot digest.
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