Clownfish

Clownfish, or anemonefish, are saltwater fish of the genus Amphiprion found in the warm, tropical waters of the Indo-Pacific, mainly on coral reefs. They typically have white vertical bars on a red, orange, yellow, brown or black background. They shelter among sea anemones, which protect them from predators; in return they defend the anemone from anemone-eating fish, clean and fan it, and attract beneficial microorganisms with their waste. Omnivorous, they feed mostly on plankton. Each group has a size-based hierarchy topped by the breeding female, then the breeding male. If the female disappears, the male changes sex and takes her place.

About the clownfish

The ocellaris clownfish is a small, orange-and-white reef fish from the eastern Indian Ocean and western Pacific that lives among the stinging tentacles of sea anemones. It belongs to the damselfish family, Pomacentridae, and its closest relative is the orange clownfish. It is also called the false percula or common clownfish.

Clownfish are part of the anemonefishes, a group of about 28 species. Their ancestors could probably handle the stings of only one kind of anemone, but over time different species adapted to many different hosts. This species, Amphiprion ocellaris, is the fish behind Marlin and Nemo in the 2003 film Finding Nemo, and its popularity in the aquarium trade surged after the film came out.

The body is small, stocky and oval, flattened from side to side with a rounded profile and a rounded tail fin. Most are orange to reddish-brown with three white bands. The first band runs just behind the eye, the second crosses the middle of the body and bulges forward toward the head, and the third circles the base of the tail. Each band and every fin is edged with a thin black line.

Not all ocellaris clownfish are orange. A black form with white bands lives off Australia's Northern Territory. Fish reach at most 11 cm (4.3 in) and average about 8 cm (3.1 in), and in each group the female is the biggest fish.

Behavior and diet

Ocellaris clownfish are diurnal, active by day, and they are aggressively territorial. Each group depends completely on its anemone. A coat of mucus keeps the fish from being stung, so the anemone's stinging tentacles give it a safe home and shelter for its eggs. Because predators avoid the anemone, adult clownfish are rarely eaten.

The partnership pays off for the anemone too. Clownfish fan it and remove parasites, chase off fish such as butterflyfish that would nibble the tentacles, and may lure prey close enough to be caught. When clownfish are present, the anemone's tentacles grow bulb-shaped tips that are thought to catch more sunlight.

Clownfish eat zooplankton, copepods and algae in roughly equal measure, which makes them generalized omnivores. They also eat parasites off their host. Even feeding follows the pecking order: large, dominant fish forage farther out, though rarely more than a few meters from the anemone, while small, bullied fish stay close to home.

The social system is unusual. All clownfish start life as males, a pattern called protandrous hermaphroditism. A group in one anemone holds a breeding female, which is the largest fish, a breeding male, and several smaller non-breeding fish. If the female dies or is removed, the breeding male becomes a female and the next largest fish moves up to become the breeding male. The female uses aggression to keep any other fish from becoming female, and lower-ranked fish grow more slowly.

Pairs are monogamous. In tropical waters they can spawn almost year-round, usually in the morning and around the full moon. The male clears a nest on bare rock beneath the anemone's tentacles, then chases the female toward it. She lays 100–1,000 orange eggs over one to two hours, and the male fertilizes them. He does most of the egg care, fanning them and eating any that are infertile or fungus-damaged, until they hatch in 6–8 days.

The larvae drift near the surface as plankton for about 8–12 days, then sink to the reef to find an anemone. Settling in is hard: a newcomer starts at the bottom of the hierarchy and may be chased out to search for another host. Clownfish eggs are eaten by other damselfishes, wrasses and, at night, brittle stars.

Range and habitat

The ocellaris clownfish lives in the eastern Indian Ocean and western Pacific. Its range includes northwestern Australia and Southeast Asia, reaching as far north as the Ryukyu Islands of Japan.

It lives in small groups on outer reef slopes and in sheltered lagoons, from near the surface down to about 15 m (49 ft). In the wild it uses just three host anemones: Heteractis magnifica, Stichodactyla gigantea and Stichodactyla mertensii. In reef aquariums it will also accept other anemones, such as Entacmaea quadricolor, or even corals as substitute hosts.

The species lives in fully marine water in the wild, but in captivity it can be raised in brackish water at less than half normal sea salinity without known harm.

Clownfish vs orange clownfish

The ocellaris clownfish is easy to confuse with its sister species, the orange clownfish (Amphiprion percula), which has the same colors and banding at first glance. The two differ in the thickness of the black outlines on the bands, the height of the dorsal fin and the count of dorsal-fin spines.

Conservation status

The ocellaris clownfish is listed as Least Concern on the IUCN Red List. It is not considered threatened overall.

Collection for the aquarium trade has reduced numbers in some places, and certain rare colors are especially sought after. The species breeds easily in captivity, which supplies both the hobby and research labs. A longer-term concern is its habitat: coral reefs face sedimentation, nutrient pollution, overexploitation and rising sea temperatures, and the fish depends entirely on its reef anemones.