Sidedness Frequencies in the Flounder Platichthys Flesus ( Pleuronectiformes ) along a Biogeographical Cline

Flounders ( Platichthys flesus ) are either left or right sided, depending on which side the migrating eye will be expressed after metamorphosis. We examined a total of 4191 individuals, both fry and adults, from six locations along the Swedish coast, with the objective of exploring the frequency of sidedness in relation to biogeographical variations. In our total sample of adult flounders (3791), 23.1% were left sided. The frequency of left sidedness was higher on the west coast (27.5%), while gradually declining down the Swedish coast towards the south (22.4%), and then north in the Baltic Sea (20.1%). Our sample of flounder fry (400) showed that the frequency of left sidedness was higher among fry (34%) than among adults. Also, left-sided fry were longer than right-sided fry, but the adults did not differ in size. We propose that left-sided fry may be favoured by less competition with plaice (a right-sided species), but suffer a higher mortality because of ontogenetic problems. This may explain why both left- and right-sided flounder exist, and why the frequencies differ geographically.