From ecology to genetics and back: the tale of two flounder species in the Baltic Sea

Recent years have brought the realization that evolutionary changes driven by selection can occur in ecological time scales. However, recent evolutionary events can be hard to detect and may easily go unnoticed. For harvested species, such cryptic diversity may lead to suboptimal management. These points are illustrated by the two flounder species in the Baltic Sea. Although early ecological studies identified two ecotypes of European flounder (Platichthys flesus) based on spawning differences, genomic studies only recently demonstrated that they were reproductively isolated species, P. flesus and P. solemdali, separated through rapid ecologically driven speciation. These morphologically indistinguishable species are harvested within a mixed-stock fishery. In the northern Baltic Sea flounder landings have declined since the mid1980s, with a drop in the Gulf of Finland (GoF) being particularly dramatic (similar to 90%). Genetic analyses of historical otolith samples from GoF catches have revealed that back in 1983 the fishery unknowingly targeted primarily P. flesus, whereas thereafter almost exclusively P. solemdali. Hence, the case of two flounder species illustrates (i) how ecological studies stimulated genetic investigations leading to discovery of ecological speciation and (ii) how cryptic species turnover discovered with genetic tools in turn improved ecological understanding with benefits to management and conservation.