Stegastes fuscus
A territorial damselfish that influences benthic community structure.
The dusky damselfish, *Stegastes fuscus*, is a bony fish from the Pomacentridae family. It lives close to the seafloor in shallow waters along the western edge of the Atlantic Ocean.
Adults range in color from brownish-olive to dark gray, with faint, darker vertical stripes along the body. Their fins are large, and the dorsal and anal fins sometimes have a blue rim. The dorsal fin contains 12 spines and 15 to 16 soft rays, while the anal fin has 2 spines and 13 to 14 soft rays. This species can be told apart from the similar longfin damselfish (*Stegastes diencaeus*) by its shorter, less angular dorsal and anal fins. The fish reaches a maximum length of 15 cm (5.9 in), though 10 cm (3.9 in) is more typical.
Juveniles are pearly gray with blue spots on top of the head. A large, black eyespot with a blue rim sits where the dorsal fin spines meet the soft rays, and a smaller, similarly colored eyespot appears on the caudal peduncle. Off southeastern Brazil, some juveniles also show a vivid yellow and orange band across the back and dorsal fin.
The species is found in shallow waters off Brazil, common on both rocky and coral reefs down to depths of 40 m (130 ft). Reports from the Caribbean Sea may actually refer to *Pomacentrus dorsopunicans*, which was possibly misidentified there. A population has also been reported from Senegal, but this has not been confirmed.
About 70% of the dusky damselfish’s diet is algae—mostly red, filamentous types, though some calcareous algae are also eaten. The rest consists of small invertebrates. By feeding, the fish prevents coarser seaweeds like *Jania* spp. from becoming dominant, which strongly shapes the benthic community structure.
This fish is territorial. It guards a patch of seabed and drives away intruders, sometimes chasing fish many times its own size. The more suitable algae a site had for feeding, the more aggressive the fish became. Besides members of its own species, it mainly chased parrotfish, bluehead wrasse (*Thalassoma bifasciatum*), bicolor damselfish (*Stegastes partitus*), and ocean surgeonfish (*Acanthurus bahianus*). However, species like the smooth trunkfish (*Rhinesomus triqueter*) and the graysby (*Cephalopholis cruentata*) were not chased.
Quick Facts
- Taxon
- Stegastes fuscus
Facts from the source article.
Lore & Background
The adult dusky damselfish varies in color from brownish-olive to dark grey, with fine, darker vertical stripes on the body. Its fins are large, sometimes with a blue rim around the edge of the dorsal and anal fins. The dorsal fin has 12 spines and 15-16 soft rays, while the anal fin has two spines and 13-14 soft rays. It can be distinguished from the similar longfin damselfish (Stegastes diencaeus) by its shorter, less angular dorsal and anal fins. Juveniles are pearly grey with blue spots on the top of the head, a large blue-rimmed black eyespot where the dorsal fin spines meet the soft rays, and a smaller similar eyespot on the caudal peduncle. Some juveniles off southeastern Brazil have a vivid yellow and orange band on the back and dorsal fin.
Stegastes fuscus occurs in shallow waters off Brazil, common on both rocky and coral reefs at depths down to 40 m. It is also reported from the Caribbean Sea, but another species, Pomacentrus dorsopunicans, may have been misidentified there. A population reported from Senegal lacks confirmation.
About 70% of its diet consists of algae, mostly red filamentous algae, with some calcareous algae also eaten; the rest is small invertebrates. Its feeding prevents coarser seaweeds like Jania spp. from becoming dominant, greatly influencing benthic community structure. The species is territorial, guarding part of the seabed and repelling intruders, sometimes chasing fish several times its size. Aggression increased with the richness of suitable algae. Besides its own species, it chased parrotfish, bluehead wrasse, bicolor damselfish, and ocean surgeonfish, but not smooth trunkfish or graysby, which do not graze algae.
Reader's Guide
The dusky damselfish is significant for its role as a territorial herbivore that shapes the benthic community on shallow reefs off Brazil. By consuming mostly red filamentous algae and preventing coarser seaweeds like Jania spp. from dominating, it influences the composition of the seabed. Its aggressive defense of feeding territories, even against much larger fish, underscores its ecological impact. The study noted that the fish chased away species that graze algae, such as parrotfish and ocean surgeonfish, but did not chase species like smooth trunkfish or graysby, which feed on small invertebrates. This suggests the guarding behavior is tied to protecting its food source rather than a specific area. The species' distribution is primarily confirmed off Brazil, with unconfirmed reports from the Caribbean and Senegal, indicating potential misidentification or range uncertainty. Its maximum length of 15 cm and typical length of 10 cm make it a moderately sized damselfish. The juvenile coloration, including a blue-rimmed eyespot and occasional yellow-orange band, aids in identification. Overall, Stegastes fuscus is a key species for understanding reef dynamics and territoriality in the western Atlantic.
Did You Know?
- The dusky damselfish can be distinguished from the longfin damselfish by its shorter, less angular dorsal and anal fins.
- It sometimes chases away fish several times its own size.
More in Clownfish and Damselfish, Part 2 1-24
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