Abstract
The European flounders Platichthys flesus (Linnaeus, 1758) displays two contrasting reproductive behaviors in the Baltic Sea: offshore spawning of pelagic eggs and coastal spawning of demersal eggs, a behavior observed exclusively in the Baltic Sea. Previous studies showed marked differences in behavioral, physiological and life-history traits of flounders with pelagic and demersal eggs. Furthermore, a recent study demonstrated that flounders with pelagic and demersal eggs represent two reproductively isolated, parapatric species arising from two distinct colonization events from the same ancestral population. Using morphological data we first established that the syntypes on which the original description of P. flesus was based belong the pelagic-spawning lineage. We then used a combination of morphological and physiological characters as well as genome-wide genetic data to describe flounders with demersal eggs as a new species: Platichthys solemdali sp. nov. The new species can be clearly distinguished from P. flesus based on egg morphology, egg and sperm physiology as well as via population genetic and phylogenetic analyses. While the two species do show some minor morphological differences in the number of anal and dorsal fin rays, no external morphological feature can be used to unambiguously identify individuals to species. Therefore we developed a simple molecular diagnostic test able to unambiguously distinguish P. solemdali from P. flesus with a single PCR reaction, a tool that should be useful to fishery scientists and managers, as well as to ecologist studying these species.
Highlights
The European flounder Platichthys flesus (Linnaeus, 1758) is a flatfish from the family Pleuronectidae (Rafinesque, 1815) with a broad distribution in coastal and brackish European waters, from the Black Sea to the White Sea (Whitehead et al, 1986)
These two lineages were found to co-occur in multiple locations of the central and northern Baltic Sea, where no intermediate genotypes have yet been found (Figures 1, 4; Momigliano et al, 2017a), confirming previous suggestions that spawning behavior and reproductive traits are likely acting as strong reproductive barriers (Momigliano et al, 2017b)
In contrary to Mikelsaar (1958), the demersal Baltic flounder might differ from the European flounders from North Sea, Skagerrak Strait and Atlantic Ocean off Portugal (Voronina, 1999) by a fewer dorsal and anal fin rays
Summary
The European flounder Platichthys flesus (Linnaeus, 1758) is a flatfish from the family Pleuronectidae (Rafinesque, 1815) with a broad distribution in coastal and brackish European waters, from the Black Sea to the White Sea (Whitehead et al, 1986). The European flounder is one of the few marine species that thrive in the low salinities of the Baltic Sea, a large body of brackish water that became connected to the North Sea at the end of the last glaciation, ∼8.5 kya (Gustafsson and Westman, 2002). The most common reproductive strategy in marine fishes, is no trivial affair in the low salinity waters of the Baltic Sea. As salinity decreases, so does the specific gravity of seawater, rendering the production of neutrally buoyant eggs a challenging task. The relatively few marine fish species that thrive in the Baltic Sea exhibit adaptations to the low salinity and temperature they have encountered (Nissling et al, 2002, 2006; Hemmer-Hansen et al, 2007; Florin and Höglund, 2008; Nissling and Dahlman, 2010; Guo et al, 2015). Numerous studies have investigated these two types of flounders, hereafter referred to as “demersal flounders” and “pelagic flounders.”
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