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Now up to information of the book Fish Behaviour, if a group of Hamlets are only females, one of them will convert into a male to mate with the remaining females, or viceverse, if there are only males one of them will convert into a female to preserve the subspecie; now if they found themselves alone and no other of it's own subspecie is around. but a member of another subspecie is around, they will mate with it in order to preserve the specie.