From ecology to genetics and back: the tale of two flounder species in the Baltic Sea

Abstract

Recent years have brought the realization that evolutionary changes driven by selection can occur in ecological time scales. However, recent evolutionary events can be hard to detect and may easily go unnoticed. For harvested species, such cryptic diversity may lead to suboptimal management. These points are illustrated by the two flounder species in the Baltic Sea. Although early ecological studies identified two ecotypes of European flounder (Platichthys flesus) based on spawning differences, genomic studies only recently demonstrated that they were reproductively isolated species, P. flesus and P. solemdali, separated through rapid ecologically driven speciation. These morphologically indistinguishable species are harvested within a mixed-stock fishery. In the northern Baltic Sea flounder landings have declined since the mid-1980s, with a drop in the Gulf of Finland (GoF) being particularly dramatic (∼90%). Genetic analyses of historical otolith samples from GoF catches have revealed that back in 1983 the fishery unknowingly targeted primarily P. flesus, whereas thereafter almost exclusively P. solemdali. Hence, the case of two flounder species illustrates (i) how ecological studies stimulated genetic investigations leading to discovery of ecological speciation and (ii) how cryptic species turnover discovered with genetic tools in turn improved ecological understanding with benefits to management and conservation.

Biases in demographic modelling affect our understanding of recent divergence

“…The evolutionary origin of some specific taxa, such as the Baltic Sea populations of Pleuronectiformes, remains controversial ( Momigliano et al. 2017 , 2018 ; Jokinen et al. 2019 ; Le Moan et al.…”

Biases in demographic modelling affect our understanding of recent divergence

“…Its marine fauna has probably more than one origin, with evidence of populations and species in the Baltic Sea being the result of both primary and secondary divergence (reviewed in Johannesson et al ., 2020). The evolutionary origin of some specific taxa, such as the Baltic Sea populations of Pleuronectiformes, remains controversial (Jokinen et al ., 2019; Le Moan et al ., 2019; Momigliano et al ., 2017, 2018). In our empirical study our model fit suggests that S. maximus from the Baltic Sea originated from a very recent invasion (< 6 kya) from the North Sea and diverged with continuous gene flow.…”

Ecological connectivity of the marine protected area network in the Baltic Sea, Kattegat and Skagerrak: Current knowledge and management needs

“…Furthermore, European flounder and cod need areas with high salinity and oxygen levels for egg and larval survival and are therefore limited to spawn in isolated deep water basins with suitable conditions, such as the Bornholm Basin, the Gdansk Deep, and the Gotland Basin. Limited spawning due to unfavourable oxygen conditions in the Gotland basin may explain the large declines in catches of European flounder close to its geographical boarder in the Gulf of Finland (GoF) since the mid-1980s (Jokinen et al 2019 ). Similarly, the spatial contraction of the eastern Baltic cod population to the Bornholm basin has been suggested to be a consequence of a loss of suitable spawning habitat in the Gotland Basin (Bartolino et al 2017 ).…”