https://phys.org/news/2018-07-endemic-baltic-sea-fish-species.html

Currently the two species can be distinguished only with genetic methods, or by studying their eggs and sperm. The species also differ in their interaction with the environment: the newly described Baltic flounder lays sinking eggs on the sea floor in coastal areas while the European flounder spawns buoyant eggs in deep areas out in the open sea. The new species is more abundant in the Gulf of Finland while the distribution of the European flounder is centered to the central and southern Baltic Sea.

Previous research by the same research group uncovered the ecological speciation process that drove the evolution of these flounders, which is rarely witnessed in the marine environment and had occurred at record speed in evolutionary time scales. However, the two flounders can officially be considered separate species only now after the new species has been formally described.