Some fish can change their sex all the time
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Bluebanded goby fish, about 2 to 2.5 inches long, are able to change their sex when it suits their position in social hierarchies.
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“We can detect within minutes, via changes in behavior, when a fish begins the transition, and it generally takes one to two weeks to complete the transition from female to male,” Grober explained.
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For sex change in either direction, there seems to be one guiding social principle: If subordinate, be female. Because each group had only one dominant individual, each group ended up with only a single male.
Sex-changing fish: Secret lives of bluebanded gobies – Light Years - CNN.com Blogs
