The Baltic flounder:
Native to namesake region.
Eggy difference.
A new species of flounder has been identified as separate from the European flounder by Momigliano et al (2018). The Baltic flounder (Platichthys solemdali sp.) is native only to the Baltic Sea – the first fish species to be identified as endemic to the area.
Its reproductive behaviour differs from the European flounder, spawning eggs that sink in coastal areas as opposed to buoyant eggs in open water. There are also differences between the species in egg morphology, egg and sperm physiology. Unfortunately, the small morphological differences mean that it is difficult to unambiguously distinguish the species and genetic methods or egg/sperm analyses are required.
Original research: http://dx.doi.org/10.3389/fmars.2018.00225