The large school of reef fish in this photograph is made up of anthias. A variety of brightly colored anthias are abundant in the world's tropical coral reefs. They are also popular aquarium fish.

Anthias have an unusual adaptation. They are sequential hermaphrodites. This means they are born one sex, but can change to another.

There are two types of sequential hermaphrodites. Protandrous hermaphrodites are born male and can change to female. Protogynous hermaphrodites are born female and can change to male.

Anthias are protogynous hermaphrodites. They are all born female. When a male dies, one of the larger female anthias changes into a male. Most anthias remain female; even a large school of anthias like this one has only a few males.

The change from female to male takes about two weeks. It can include not only a new reproductive system, but a change in size, shape, and color. If a community of anthias forms with too many males, male anthias may change their sex again—and return to being female.

Noun

a modification of an organism or its parts that makes it more fit for existence. An adaptation is passed from generation to generation.

aquarium
Noun

a container or tank where aquatic plants and animals are kept, or an institution that keeps such containers.

coral reef
Noun

rocky ocean features made up of millions of coral skeletons.

protandrous hermaphrodite
Noun

organism that starts off as male but is capable of changing to female.

protogynous hermaphrodite
Noun

organism that starts off female, but is capable of changing to male.

reproductive system
Noun

organs involved in an organism's reproduction.

sequential hermaphrodite
Noun

organism that changes its sex at some point in its life.

tropical
Adjective

existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.