Baltic Flounder

The Baltic flounder (Platichthys solemdali) is a species of flatfish endemic to the Baltic Sea, where it is the only known endemic fish species.

It is sympatric with the closely related European flounder (P. flesus), which it looks identical to and can only be distinguished from via genetic analysis. However, both have very different spawning behaviors and habitat requirements; the strongly migratory European flounder requires a minimum level of salinity and spawns in the pelagic zone where its eggs are carried by the waves, whereas the largely resident Baltic flounder is more tolerant of salinity decreases and spawns in shallow neritic areas where its eggs sink to the seabed. Due to these different spawning behaviors, both species display a major degree of reproductive isolation, preventing them from intermixing and establishing them as different species.