Many members of the coral reef fish family Pseudochromidae may be able to undergo bi-directional sex changes.
According to experimental evidence from Wittenrich and Munday, which has just been published in the journal Zoological Science, the ability for males to become female and females to become male could be widespread in pseudochromids.
Pseudochromids, or dottybacks as they are more commonly known, have long been known to have the ability to change sex when there is a lack of partners of the opposite sex around to reproduce with.
Wittenrich and Munday undertook an experiment using fish in aquariums to determine whether there was any link between the colouration of fish and their sex changing behaviour.
The study looked at three pseudochromid species: P. flavivertex, P. albabraensis and P. cyanotaenia.
Of these, males and females of flavivertex and aldabraensis are different colours (sexually dichromatic), while males and females of cyanotaenia are indistinguishable from each based on their colours (sexually monochromatic).
When they placed two males of each species together, one of them changed sex to become a functional female and developed female colouration, and when two females were kept together one of them changed sex to become male, and developed male colouration, in those species which were sexually dichromatic.
Changing sex from male to female took longer at 52-93 days, compared to just 18-56 days to change sex from female to male.
Munday and Wittenrich think that bi-directional sex changes are probably widespread in the family and say that it can't be predicted by the presence of secondary sexual characteristics.
For more information see the paper: Wittenrich, ML and PL Munday (2005) - Bi-directional sex change in coral reef fishes from the family Pseudochromidae: an experimental evaluation. Zoological Science, 2005, July, 22 (7): 797-803.