New research has come up with some answers to why, like humans, some fish turn out to be left- or right-sided.
The presence of these two morphological types (technically known as "lefty" or "righty") are an example of antisymmetry or laterality. It's previously been thought that fish either inherit this, or that it's simply a 50-50 chance thing.
It's said that predators either pick out prey with the opposite laterality to their own - so a righty pike might choose to attack lefty roach - through a process called "cross predation", or they choose "parallel predation" in which predator and prey share the same laterality.
To investigate whether cross predation drives dimorphism in lefties or righties a scientist from the Ocean Research Institute, University of Tokyo used a mathematical population dynamic model incorporating one prey and one predator and three-trophic-level systems with omnivory.
The study showed that cross predation is more common than parallel predation, and that as time goes by, if only lefties or righties exist in a population, the other type can make a comeback. This suggests that dimorphism is maintained in all interacting species.
For more details read the full paper: Nakajima M. (2004) - Persistence and fluctuation of lateral dimorphism in fishes. Am Nat. 2004 May;163(5):692-8.