A newly-discovered flatfish species, the Baltic flounder (Platichthys solemdali), is the first endemic fish known from the Baltic Sea, according to new research published in the journal Frontiers in Marine Science.
The Baltic flounder (Platichthys solemdali). Image credit: Mats Westerbom.
The Baltic flounder belongs to the family Pleuronectidae, a group of fishes containing the soles, flounders, turbots, etc., remarkable for having the body greatly compressed laterally.
The fish is endemic to the Baltic Sea, where it has a wide distribution in coastal and bank areas across the region up to the Gulf of Finland and the southern Bothnian Sea.
It lives in brackish water of varying salinities in the coastal zone at 1-164 feet (0.5-50 m) depth on soft and hard bottoms.
Adult Baltic flounders measure between 8 and 10 inches (20-25 cm) in length. The right-eyed side form is predominant.
The eye side is dull to dark brown, gray and olive, with scattered reddish spots. The blind side is usually white, with or without scattered brown patches of varying size.
Coloration is highly variable among individuals and the species can change its coloration according to the substratum providing effective camouflage.
Adults reside in the archipelago and coastal areas throughout their lives undertaking only limited migrations between deeper wintering grounds and shallower spawning and feeding areas.
The Baltic flounder is a benthivore, feeding on various mussels and other benthic invertebrates from its juvenile stage onwards.
Spawning occurs in April-June in near-shore areas on a 16 to 65 feet (5-20 m) depth with a preferable water temperature of around 46 degrees Fahrenheit (8 degrees Celsius).
Photographic images of the holotype of the Baltic flounder (Platichthys solemdali), adult female, 21.3 cm long, right form specimen, from the western Gulf of Finland: (A) dorsal view before fixation; (B) ventral view before fixation; (C) dorsal view after fixation; (D) ventral view after fixation; (E) photograph under UV light showing bony plates, and (F) X-ray image. Image credit: Momigliano et al, doi: 10.3389/fmars.2018.00225.
“The fact that a new vertebrate species is found and described from European waters, and especially from the species-poor Baltic Sea still after more than a century of biological research in the area, makes this finding significant,” said study lead author Dr. Paolo Momigliano from the University of Helsinki and colleagues.
“The reason why the Baltic flounder has not been recognized before is that it appears to be near to identical to the European flounder (Platichthys flesus), a flatfish with a broad distribution in coastal and brackish European waters, from the Black Sea to the White Sea,” added co-author Professor Juha Merila, also from the University of Helsinki.
“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 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.”
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Paolo Momigliano et al. Platichthys solemdali sp. nov. (Actinopterygii, Pleuronectiformes): A New Flounder Species from the Baltic Sea. Front. Mar. Sci, published online July 11, 2018; doi: 10.3389/fmars.2018.00225
