Dusky Gregory (species: Stegastes nigricans) in taxonomy (Lizard Island Field Guide)
Stegastes nigricans
Dusky Gregory

©Andy Lewis: An adult Dusky Gregory in pale colouration

©Andy Lewis: An adult Dusky Gregory above its algal farm on Staghorn Acropora coral

©Andy Lewis: A Dusky Gregory territory adjacent to a Giant Clam (Tridacna gigas). The white area on the clam mantle is damage caused by the fish.
Kingdom Animalia
Phylum Chordata
Class Actinopterygii
Order Perciformes
Family Pomacentridae
Genus Stegastes
Species Stegastes nigricans



Distinguishing features

A small to medium sized herbivorous damselfish with a dark brown body, although the colour can range from very dark brown to a light tan. A small dark spot is usually visible at the base of the posterior dorsal fin. Juveniles are yellowish posteriorally, grading to light brown anteriorally, with the dark spot prominent. Lives in colonies amongst branching coral, where it farms long filamentous algae.


  • Up to 14 cm (Standard length)

Depth range

  • Depth range data is not yet available.



©Atlas of Living Australia: Australian distribution

Distribution and habitat preferences

Shallow reef flats and tops of lagoonal and backreef patches.

Found in most locations around the Island.


The Dusky Gregory is a common herbivorous damselfish at Lizard Island. These fish usually form colonies of 3 to more than 10 individuals, located among the branches of hard corals such as staghorn Acropora and Porites cylindrica. They "farm" algae on the dead branches of the coral, by weeding out undesirable algal species and through their defense of the farm, encouraging the growth of desirable taxa. In Japan, the algae grown by the fish is not found anywhere else - hence a mutualism exists. It is not known whether the algae inside the fish territories at Lizard Island is a mutualist or a generalist species. The farming of the fish has a major impact on the benthic community, and research has shown these fish can modify both algal, invertebrate, and coral communities by their feeding activities.

Web resources


  • Arnal, C. (2000). Ecologie comportementale de la symbiose poisson nettoyeur/ poisson client: motivations et honnĂȘtetĂ©, Ph.D. thesis, University of Perpignan, France. LIRS catalog number 637.
  • Barneche, D.R., S.R. Floeter, D.M. Ceccarelli, D.M.B. Fresnel, D.F. Dinslaken, H.F.S. Mario and C.E.L. Ferreira (2009). Feeding macroecology of territorial damselfishes (Perciformes: Pomacentridae), Marine Biology, 156: 289-299. LIRS catalog number 1252.
  • Bay, L.K. (2005). The population genetic structure of coral reef fishes on the Great Barrier Reef, Ph.D. thesis, James Cook University. LIRS catalog number 967.
  • View all references