Tracking hypoxia in flounder: do two parapatric flounder species differ in hypoxia exposure?

The occurrence of low-oxygen and oxygen-free zones in coastal and oceanic regions around the world has been increasing worldwide since the 1960s and low-oxygen events are observed with increasing frequencies and severities. The Baltic Sea is considered as the largest hypoxic zone in the world with a total area of over 60,000 km² in recent years. The restricted intrusion of oxygen-rich, saline waters combined with a permanent density stratification limits oxygen exchange between bottom and surface waters, especially in the central basins. Flounder are one of the most common fish that has adapted to the brackish conditions in the Baltic Sea. Flounder in the Baltic Sea have been considered to belong to a single species, the European flounder (Platichthys flesus), that had two distinct ecotypes in the Baltic. The two ecotypes were distinguished based on their spawning strategies, with a shallow coastal spawning ecotype that produces small demersal eggs in the central and northern part of the Baltic Sea and an off-shore spawning ecotype that produces larger pelagic eggs in the deep basins in the southern and central part of the sea. Recently, genome-wide genetic data of the two ecotypes revealed that the coastal spawning ecotype can be distinguished as a separate species, Platichthys solemdali sp. nov. Individuals of the two species may be differentially exposed to hypoxia as the deep waters in the central basins have a much higher prevalence for low oxygen conditions than the coastal waters. Previous studies have indicated that otolith microchemistry analyses using the redox-sensitive element manganese (Mn) can provide individual exposure histories of flounder to hypoxia. We compare differences between the two species in the duration and frequency of hypoxia exposure using otolith chemistry.