Sequential hermaphroditism

Some species change sex during the life span. An example is Labroides , a small fish living in coral reefs. This fish lives in family groups containing 1 male and 6 to 8 females. If the male dies, or otherwise becomes ineffective, a female transforms into a male as a replacement. This is accomplished in about 2 weeks after experimental removal of a male from a group. If a family group grows too large by addition of younger fish, another female transforms into a male, and the group splits into two families.

The group structure is maintained by aggressive behavior, or what is sometimes called "pecking order". The male acts aggressively towards the largest female and dominates her, and this suppresses secretion of the hormones that cause transformation to a male. The largest female similarly dominates the next largest female, etc. -- so it will normally be the largest female that becomes male after removal of a male. This is a sensible solution to competition among males that are able to mate with many females. It is a better strategy for a small fish to remain female and be assured of successful reproduction rather than to try to be male and risk losing to a larger male competitor with no reproducion at all. (Ovaries of females contain inactive spermatogenic tissue. Males can retain spermatogenesis even if isolated.) (Robertson, Science 177:1007-9,1972)

There are other examples in coral reef fish.

Another classic example is the slipper limpet, Crepidula.

Revised from a page on Polymorphism.

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