The Baltic flounder (''Platichthys solemdali'') is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

flatfish A flatfish is a member of the Ray-finned fish, ray-finned demersal fish order (biology), order Pleuronectiformes, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the ...

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...

to the

Baltic Sea The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain. The sea stretches from 53°N to 66°N latitude and from ...

, where it is the only known endemic fish species. It is

sympatric In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species sh ...

with the closely related

European flounder The European flounder (''Platichthys flesus'') is a flatfish of European coastal waters from the White Sea in the north to the Mediterranean and the Black Sea in the south. It has been introduced into the United States and Canada accidentally th ...

(''P. flesus''), which it looks identical to and can only be distinguished from via genetic analysis. However, both have very different

spawning Spawn is the eggs and sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of releasing the eggs and sperm, and the act of both sexes is called spawning. Most aquatic animals, except for aquati ...

behaviors and habitat requirements; the strongly migratory European flounder requires a minimum level of

salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

and spawns in the

pelagic zone The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...

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 The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illuminated ...

areas where its eggs sink to the seabed. Due to these different spawning behaviors, both species display a major degree of

reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offspring ...

, preventing them from intermixing and establishing them as different species. Analyses indicate that the Baltic flounder and the Baltic population of the widespread European flounder both derive from the same ancestral population, but invaded the Baltic Sea at different times, with the ancestors of the Baltic flounder invading the area and diverging from the European flounder shortly after the Baltic Sea had formed from

Ancylus Lake Ancylus Lake is a name given by geologists to a large freshwater lake that existed in northern Europe approximately from 9500 to 8000 years B.C being in effect one of various predecessors to the modern Baltic Sea. Origin, evolution and demise The ...

(circa 8,000 years ago), while the widespread European flounder invaded the area about 1,500 to 3,500 years later (or circa 5,000 - 6,500 years ago). The Baltic flounder has been steadily outcompeting the Baltic population of the European flounder over the past 5,000 years due to the declining level of salinity in the water; in areas such as the

Gulf of Finland The Gulf of Finland ( fi, Suomenlahti; et, Soome laht; rus, Фи́нский зали́в, r=Finskiy zaliv, p=ˈfʲinskʲɪj zɐˈlʲif; sv, Finska viken) is the easternmost arm of the Baltic Sea. It extends between Finland to the north and E ...

, the European flounder has almost completely disappeared due to

climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...

and

eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...

impacting its closest breeding grounds, leaving the Baltic flounder the predominant species in the area.

References

{{Taxonbar, from=Q55713149 Platichthys Fish of the Baltic Sea Fish described in 2018