''Stegastes fuscus'', the dusky damselfish, is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

of bony fish 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 ...

Pomacentridae Pomacentridae is a family of ray-finned fish, comprising the damselfishes and clownfishes. This family were formerly placed in the order Perciformes or as indeterminate percomorphs, but are now considered basal blenniiforms. They are primaril ...

found near the seabed in shallow waters on the western fringes of the

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

.''Stegastes fuscus'' (Cuvier, 1830)
FishBase. Retrieved 2011-12-29.

Description

The colour of an adult dusky damselfish varies from brownish-olive to dark grey, with fine, darker-coloured, vertical stripes on the body. The fins are large with sometimes 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. The dusky damselfish can be distinguished from the very similar longfin damselfish ('' Stegastes diencaeus'') by the shorter, less angular dorsal and anal fins. The maximum length is , but is a more typical length.Territorial Behavior of the Dusky Damselfish, ''Stegastes fuscus'', at Champagne Reef
Michael Wachter. Retrieved 2011-12-29. The juvenile dusky damselfish is pearly grey with blue spots on the top of its head. It has a large, blue-rimmed, black eyespot where the dorsal fin spines meet the dorsal fin soft rays, and a smaller, similarly coloured eyespot on the caudal peduncle. Some juveniles off the coast of southeastern Brazil have a vivid yellow and orange band on the back and dorsal fin.

Distribution

''Stegastes fuscus'' occurs in shallow waters off Brazil, where it is common on both rocky and

coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in group ...

s at depths down to . It is also reported from the

Caribbean Sea The Caribbean Sea is a sea of the Atlantic Ocean, North Atlantic Ocean in the tropics of the Western Hemisphere, located south of the Gulf of Mexico and southwest of the Sargasso Sea. It is bounded by the Greater Antilles to the north from Cuba ...

, but another species, '' Pomacentrus dorsopunicans'', possibly was misidentified there. Another population is reported from

Senegal Senegal, officially the Republic of Senegal, is the westernmost country in West Africa, situated on the Atlantic Ocean coastline. It borders Mauritania to Mauritania–Senegal border, the north, Mali to Mali–Senegal border, the east, Guinea t ...

, but this lacks confirmation.

Biology

About 70% of the food consumed by the dusky damselfish consists of algae, mostly red, filamentous algae, but some calcareous algae are also eaten. The rest of their diet is small

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s. The feeding activities of the damselfish prevent coarser seaweeds, such as '' Jania'' spp., becoming dominant and greatly influences the structure of the benthic community.

Behaviour

The dusky damselfish is a

territorial A territory is an area of land, sea, or space, belonging or connected to a particular country, person, or animal. In international politics, a territory is usually a geographic area which has not been granted the powers of self-government, ...

species. It guards part of the seabed and repels intruders, sometimes chasing away fish several times its own size.Territorial Habits of ''Stegastes fuscus''
Megan Pugh. Retrieved 2011-12-30. The richer the site was in suitable algae for feeding, the more aggressive the fish became. Besides members of its own species, the main species chased away were

parrotfish Parrotfish (named for their mouths, which resemble a parrot's beak) are a clade of fish placed in the tribe Scarini of the wrasse family (Labridae). Traditionally treated as their own family (Scaridae), genetic studies have found them to be dee ...

, bluehead wrasse (''

Thalassoma bifasciatum ''Thalassoma bifasciatum'', the bluehead, bluehead wrasse or blue-headed wrasse, is a species of marine ray-finned fish, a wrasse from the family Labridae. It is native to the coral reefs of the tropical waters of the western Atlantic Ocean. In ...

''), bicolour damselfish ('' Stegastes partitus'') and ocean surgeonfish (''Acanthurus bahianus''). Certain other fish species, such as the smooth trunkfish ('' Rhinesomus triqueter'') and the

graysby The graysby (''Cephalopholis cruentata'') is a species of marine ray-finned fish, a grouper from the subfamily Epinephelinae which is in the family Serranidae which also includes the anthias and sea basses. It is found in the western Atlantic. I ...

(''Cephalopholis cruentata''), were not chased away. The author of the study surmised it was the food source rather than the geographical area being guarded, and these fish were not considered to be threats because they did not graze algae, instead feeding on small invertebrates. An alternative possibility was these fish benefited the dusky damselfish in some other way, such as by providing some protection against

predators Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...

or by removing invertebrates that feed on algae.

References

{{Taxonbar, from=Q2196282 fuscus Fish described in 1830 Taxa named by Georges Cuvier