Fish
Damselfish
Pomacentridae · also called damselfishes, anemonefish
A damselfish is a thumb-length reef fish with the temperament of something ten times its size. It will ram a moray eel, a turtle, or a scuba diver that crosses its patch of algae. And one branch of the family, the anemonefish, took its aggression indoors, moving into the stinging tentacles of a sea anemone and defending that instead.
Overview
Damselfish are the family Pomacentridae, around 420 species, on tropical and warm-temperate reefs worldwide. They are small, brightly coloured, and famously territorial, and diet ranges from algae grazing to picking plankton from the water. The family includes the chromis, the sergeant majors, the California garibaldi, and the anemonefish — the clownfish.
Farmers and fighters
Some damselfish are algae farmers, weeding their territory of unwanted species, biting back invading coral, and cultivating a dense lawn of filamentous algae that survives nowhere else on the grazed reef. A territorial damselfish will drive off animals many times its size.
The clownfish
The anemonefish live among sea-anemone tentacles, protected by a mucus coat, trading shelter for cleaning and defence of the anemone. Each anemone holds a strict hierarchy — one big female, one smaller male, several juveniles — and all clownfish start male; when the female dies, the male changes sex to replace her. There are no anemonefish in the Atlantic.
Status
Damselfish as a family are Not Evaluated. Most are common and resilient, some even increasing on degraded, algae-covered reefs, but the anemonefish are more vulnerable because their host anemones bleach and die in the same heatwaves that kill coral, and the aquarium trade removes many wild clownfish and anemones — though clownfish are now widely captive-bred.
Damselfish: common questions
Is a clownfish a damselfish?
Yes. The anemonefish, of which the common clownfish is the best known, are a subgroup within the damselfish family, Pomacentridae. They share the family's small size, bright colour, and territorial aggression, and they simply added a partnership with large sea anemones. There are about 30 anemonefish species, all in the Indian and western Pacific oceans, and they are the only damselfish that live among anemone tentacles. Everything popular culture attributes to the clownfish, including the fierce defence of its anemone and the fact that all individuals begin as males, is damselfish behaviour taken to a specialised extreme.
Why don't clownfish get stung by the anemone?
The clownfish is protected by the mucus layer covering its skin, which either lacks the chemical signals that trigger the anemone's stinging cells or actively mimics the anemone's own mucus so the anemone treats the fish as part of itself. A clownfish introduced to an unfamiliar anemone does a careful acclimation dance over hours or days, brushing against the tentacles a little at a time and getting stung repeatedly at first while its mucus coat adjusts, until the anemone stops firing at it. If a clownfish is kept away from an anemone for a while and then returns, it has to re-acclimate. The relationship benefits both, giving the fish a stinging fortress and the anemone a bodyguard, a cleaner, and a supply of nutrients from the fish's waste and scraps.
Why is a small damselfish so aggressive?
Because on a crowded reef, a patch of algae or a safe crevice is worth fighting for, and a fish that does not defend its resources loses them. Many damselfish depend on a specific small territory, a coral head to hide in, a plot of farmed algae to eat, or a nest site where the male guards the eggs, and there is intense competition for these from other reef fish. Selection has therefore favoured damselfish that attack anything entering the territory, regardless of size, because a bluff charge is usually enough to move a larger fish along and the alternative is being eaten out of house and home. The famous case of a small damselfish repeatedly ramming a scuba diver is simply this instinct applied to an intruder it cannot possibly hurt.
Are clownfish and damselfish good aquarium fish?
Clownfish are among the most popular of all marine aquarium fish, being hardy, colourful, and now almost entirely captive-bred, which takes the pressure off wild populations, and a pair will often breed in a home tank. They do not strictly need an anemone in captivity and may adopt a coral or a corner instead. Other damselfish, such as the blue and green chromis, are also common in the trade and are among the toughest marine fish for a beginner, but many damselfish become extremely territorial as they grow and will harass and sometimes kill tankmates, so they are best kept singly or in a large system. Wild-caught anemones for clownfish are far harder to keep and are discouraged.