Clownfish

Nemo and his father Marlin are usually identified as ocellaris clownfish, the species on this page, because the ocellaris is the most common clownfish in aquariums and matches the film's look. Strictly speaking, the animated fish mix features of the ocellaris and its near twin, the orange or percula clownfish, which has thicker black borders on its white bands. Both live in the Indo-Pacific, but the ocellaris is found from the Andaman Sea to Japan and northwestern Australia, while the percula lives farther east, around Papua New Guinea and the northern Great Barrier Reef.

The ocellaris clownfish is small: adults measure about 8 to 11 centimeters long and weigh only a few tens of grams. In each group, size follows rank. The female is always the largest fish, the breeding male is the second largest, and the young males below them stay smaller, partly by growing more slowly so they do not challenge the fish above them. Among other clownfish species, the largest is the maroon clownfish, whose females can reach about 17 centimeters, while some species never grow beyond 8 centimeters.

The female lays from a few hundred to about a thousand eggs on a bare rock right next to the anemone, under the shelter of its tentacles, and the breeding male fertilizes them. From then on the male does most of the work: he guards the eggs, fans them with his fins to keep them oxygenated and removes any that go bad. After about 6 to 10 days the larvae hatch at night and drift away in the plankton for one to two weeks, before settling on the reef and searching for an anemone of their own, which they find partly by smell.

In the wild, hardly. The anemone is their only shelter, and a clownfish caught in open water is easy prey for groupers, lizardfish and other reef predators, so they almost never leave it. When anemones bleach and die during marine heatwaves, the clownfish living in them decline too. In aquariums the story is different: with no predators around, captive bred clownfish live perfectly well without an anemone, and many never meet one in their whole lives.

Adult clownfish are protected by their anemone, so few predators manage to catch them, but any that stray from it can be eaten by groupers, snappers, lizardfish, moray eels and other reef predators. The most dangerous time is early life: the tiny larvae drift in the plankton for one to two weeks, where they are eaten by fish, jellyfish and other filter feeders, and only a small fraction survive to settle on a reef. Eggs laid beside the anemone are also targeted by small fish, which is why the male guards them so closely.

Clownfish are omnivores, so they eat a mix of plant and animal matter. Their diet includes tiny crustaceans, copepods, algae, and leftovers from their host anemone's meals. They forage close to the anemone throughout the day, with feeding activity peaking in the morning. This flexible diet helps them handle seasons when food on the reef is harder to come by.

Yes, and it is one of the most fascinating things about them. All clownfish are born male. When the dominant female of a group dies, the breeding male changes sex and becomes the new female. The largest non-breeding male then steps up as the new breeding partner. This process is called sequential hermaphroditism and is quite rare among vertebrates.

The ocellaris clownfish is listed as Least Concern by the IUCN, and most other clownfish species are too. That said, they face real pressures. Coral bleaching driven by rising ocean temperatures also bleaches and kills the anemones they depend on, and a clownfish without an anemone is quickly eaten. Collection for the aquarium trade, which surged after the film Finding Nemo, has thinned some wild populations, although many aquarium clownfish are now bred in captivity. Healthy reefs are essential to their survival.

Clownfish produce a thick layer of mucus on their skin that protects them from the anemone's stinging cells. Scientists believe this mucus either masks the chemical signals that normally trigger a sting or directly stops the anemone's firing mechanism. It is not something they are born with. Young clownfish carefully acclimate to a new anemone through a gradual process of contact.

In the wild, clownfish can live anywhere from six to ten years on average, though in aquariums some individuals have lived for more than twenty years. Their lifespan depends heavily on the health of their host anemone and the surrounding reef. In aquarium settings, they often live longer due to the absence of predators and a steady food supply.

Pretty much, yes. Once a clownfish settles on a host anemone, it rarely leaves. The anemone provides food, shelter, and protection from predators, so there is little reason to wander. They defend their anemone aggressively against intruders, including fish much larger than themselves. This strong attachment to a single spot makes them one of the most site-faithful fish on any reef.