Stegastes nigricans or dusky farmerfish is a species of damselfish

Pomacentridae

Pomacentridae are a family of perciform fish, comprising the damselfishes and clownfishes. They are primarily marine, while a few species inhabit freshwater and brackish environments . They are noted for their hardy constitutions and territoriality...

 found around coral reef

Coral reef

Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

s at a depth of 1 to 12 meters.

S. nigricans practices a form of agriculture

Agriculture

Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

 with a species of red algae

Red algae

The red algae are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species  of mostly multicellular, marine algae, including many notable seaweeds...

. The fish will claim a patch of "brown carpet algae" which it defends by chasing away other fish and sea urchin

Sea urchin

Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...

s. The fish also pulls up other bits of algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

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 that the brown carpet algae is dependent on S. nigricans for its survival.