Fish
Clownfish
Amphiprion ocellaris · also called clown anemonefish, false percula clownfish, Nemo
The clownfish is a small, boldly striped reef fish that has built its entire life around a partnership with an animal that stings almost everything else. It spends its days within the tentacles of a sea anemone, rarely straying more than a metre, and it changes sex according to who else is home.
Overview
Clownfish, also called anemonefish, live on shallow coral reefs and in lagoons across the Indo-Pacific. Amphiprion ocellaris, the common clownfish, is the orange-and-white species made famous by film. Each clownfish or group of clownfish occupies a single host sea anemone and defends it fiercely; without an anemone, a clownfish on the open reef is quickly eaten.
The partnership
The relationship is a true mutualism — both partners benefit:
- The anemone gives the clownfish a stinging fortress to shelter in, a safe place to nest, and leftover food.
- The clownfish defends the anemone from tentacle-eating fish, cleans it, improves water flow and oxygenation around it by wriggling through the tentacles, and fertilises it with waste.
The clownfish survives the tentacles because of a mucus layer on its skin that the anemone does not recognise as foreign. A fish moving to a new anemone re-acclimates over several hours, brushing against it in gradually longer contacts until it can bury itself in safety.
A society in one anemone
An anemone typically houses a size-ranked group: one large female, one smaller breeding male, and several still-smaller non-breeders that are suppressed from maturing. All clownfish start life as males. When the female dies, the breeding male switches sex to replace her and the top-ranked non-breeder graduates to breeding male. This keeps a functioning pair in place without any fish needing to leave the safety of the anemone to find a mate.
Reproduction
Clownfish pairs breed year-round on warm reefs. The male clears a patch of bare rock next to the anemone, and the female lays a few hundred to over a thousand orange eggs on it. The male tends the nest, fanning the eggs with his fins and removing dead ones, for the six to ten days until they hatch. The larvae drift in open water for one to two weeks, then settle to the reef and must find and win an anemone of their own.
Conservation
Amphiprion ocellaris is listed as Least Concern and remains common, but clownfish are sensitive to the health of their hosts: warming seas that bleach anemones remove clownfish habitat directly. Localised over-collection for aquariums has been a problem in some regions, though captive breeding now supplies much of the trade.
Clownfish: common questions
How does a clownfish live among stinging tentacles?
A sea anemone's tentacles fire harpoon-like stinging cells at anything that brushes them, but not at a clownfish. The fish is protected by a thick mucus coat that partly mimics the anemone's own chemistry, so the anemone "reads" it as part of itself. A clownfish introduced to a new anemone performs a careful acclimation dance, touching it briefly and repeatedly over hours until the mucus adjusts and the stinging stops.
Are all clownfish born male?
Yes. Clownfish are sequential hermaphrodites of the "protandrous" type — born male, with the potential to become female. Each anemone holds a small group — a dominant breeding pair plus a few smaller non-breeding males in a strict size hierarchy. If the female dies, the breeding male changes sex and becomes the new female, and the largest non-breeder matures into the new breeding male. The change is permanent.
What does the anemone get in return?
It is a genuine partnership. The clownfish drives off butterflyfish and other species that nibble anemone tentacles, its droppings and the scraps it drops fertilise the anemone, its swimming circulates oxygenated water through the tentacles, and its bright colours may even lure in prey. Anemones with clownfish grow faster and survive better than those without.
Did "Finding Nemo" help or hurt clownfish?
Both. The 2003 film hugely increased demand, and in some reef areas wild clownfish were over-collected for aquariums. But it also drove a rapid expansion of captive breeding, and the ocellaris clownfish is now one of the most successfully farmed marine aquarium fish, which has taken pressure off wild populations.