Some fish can actively change sex in either direction. This presents a socially opportunisitic strategy to maximize genetic fitness under any environmental situation. Both-ways sex change deviates from the size-advantage model, and is advantageous because it allows the individual to change to the optimal sex under any conditions.
Since both-ways sex-change is assumed to be fairly derived, it is much more rare than other forms of sequential hermaphroditism. It has been observed in aquaria for many fish which are normally protogynous (see Nakashima et al., 1996).
In the field, many of the goby species have been observed to change sex, such as Gobiodon micropus and G. oculolineatus (Nakashima et al., 1996). While the female growth rate is faster, swithing to male is beneificial, but when moving to a different coralhead, being switching to female is advantageous.
As both-ways sex change would probably be the optimal gender allocation strategy, it is quite possible that many other species follow similar patterns
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