Introduction: Of the vertebrates in the animal kingdom, sex determination is usually 
a fixed characteristic in terms of life history. Interestingly, there are a few organisms 
for whom sex is a plastic condition, often determined by a combination of internal
and external signals. One such group of organisms which follows this trend are 
the tropical teleosts: the conspicuous coloful fish inhabiting coral reefs.

The majority of reef fish change sex at some point throughout their life. In fact, 
reef fish that remain as the same sex for their life span (gonochoristic) are in the 

There are many different patterns for sex-change. Some species will begin life as 
males and switch to females (protandry), and others switch from female to male 
(protogyny). Further still, some will change sex in both directions, and others will
be both sexes at the same time.

Sex-change therefore becomes quite fascinating from several different perspectives.
From the behavioral standpoint, how does a fully functional female behaviorally 
become a male in a matter of hours, followed by a physiological and anatomical 
change to functionally become the opposite sex. The endocrine system is most likely 
responsible for this changing ability, but the hormones have yet to be identified. 
Also yet to be identified are the chromosomes and genetic sequences responsible 
for allowing this sexual plasticity.

Heniochus monoceros, Northern Great Barrier Reef, Australia. Photo by C.L.Rice.©1999

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This page was created as an assignment for the Animal Physiology Class, Department of Biology,

at Davidson College