Clownfish or anemonefish are the sister family to the damselfishes, comprising 30 species. With adults reaching 15 cm, this is a family of small fish which display a very unusual partnership with some sea anemones.
Clownfish have developed a mucus coat mutation which allows them to live within the stinging tentacles of sea anemones. The clownfish, or anemonefish are protected from predators which cannot tolerate the stinging tentacles on the anemone. In return the clownfish defend the anemone from parasites and other fish who may prey on them ,such as butterflyfish.
Clownfish mostly feed on small plankton, with a small portion of their diet coming from algae and the undigested food from their host anemone. In return, fecal matter from the clownfish provide nutrients to the host anemone. The constant movement of the clownfish fins helps to provide water circulation and aerate their host anemone tentacles, especially at night when photosynthesis does not occur.
In a group of clownfish living within an anemone, the largest is female, and all others are male. The next largest colony member is the dominant male, breeding with the female 'leader'. All other smaller males actively stall their growth in order to remain subordinate and thus not present a threat to the dominant male (in case they are ejected from the anemone). A clear hierarchy exists, with each subsequent male member of the colony being smaller than the one above it. Some colonies can be 5, 6 or more members. The female will defend the anemone from invasion.
If the female clownfish leaves the colony, or dies for example, the dominant male will undergo a preiod of rapid growth and change sex to replace the previously dominant female colony leader. The next lowest male in the colony hierarchy will then also rapidly increase in size to become the dominant male and create a new breeding pair. This process of rapid and controlled growth is repeated for all juvenile males so that their ranking positions within the colony are preserved.