Monday, July 30, 2018

[Ichthyology • 2018] Platichthys solemdali • A New Flounder Species (Actinopterygii, Pleuronectiformes) From the Baltic Sea

Platichthys solemdali 
Momigliano, Denys,  Jokinen & Merilä, 2018
photo: Mats Westerbom 

The European flounder 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 to 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 ecologists studying these species.

Family Pleuronectidae Rafinesque 1815

Genus Platichthys Girard 1854

Platichthys solemdali sp. nov.
  Baltic flounder

Diagnosis: Platichthys solemdali sp. nov. is diagnosable from P. stellatus by the absence of stripes on the dorsal and anal fin rays [Figures 6A, 2B; vs. presence of stripes for P. stellatus (Morrow, 1980)]. It can be distinguished with more than 99.999% certainty from P. flesus using genotypes of at least three of the outlier loci which were genotyped in this study (Loci 886, 3599, and 1822) by comparison with publically available reference data deposited in the Dryad digital repository (Momigliano et al., 2017a). P. solemdali sp. nov. (N = 50) has 46–59 dorsal fin rays vs. 51–66 for P. flesus recorded in this study, in Voronina (1999) and in Galleguillos and Ward (1982), and 35–41 anal fin rays vs. 35–45 in P. flesus from this study, Voronina (1999) and Galleguillos and Ward (1982). Hence, none of these meristic characters provide unambiguous species diagnosis. However, reproductive traits (viz. egg morphology and buoyancy, as well as sperm physiology) are unambiguous diagnostic characters. Eggs of P. solemdali sp. nov. become neutrally buoyant at salinities between 16 and 21.5 psu and are 0.99 ± 0.05 mm in diameter (Table 6; Figure 7), whereas the eggs of P. flesus in the Baltic Sea are larger (1.3–1.5 mm) and reach neutral buoyancy between 11 and 18 psu (Table 6; Nissling et al., 2002). Spermatozoa of P. solemdali sp. nov. activate at minimum salinities between 2 and 4 psu, in contrast to a required salinity above 10 psu for P. flesus (Table 7).

Geographic distribution:   P. solemdali sp. nov. 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. Confirmed individuals of P. solemdali sp. nov. have been sampled as far south as Öland (SD 27) (species identity confirmed via genetic analyses, Figure 1) and Hanö Bay (SD 25) (based on egg morphology, see Wallin, 2016; Nissling et al., 2017). In a recent paper Orio et al. (2017) suggested that environmental conditions in the entire southern Baltic Sea are suitable for demersal spawning flounders, and already Mielck and Künne (1935) reported ripe female flounders with small eggs from shallow low-saline (6–7‰) areas in the southern Baltic Sea (Oder Bank, SD 24). However, the current occurrence of Psolemdali sp. nov. in the southern regions is poorly known and, hence, it is still unclear whether the species is found throughout the coastal Baltic Sea area.

Habitat: P. solemdali sp. nov. lives in brackish water of varying salinities in the coastal zone at 0.5–50 m depth on soft and hard bottoms.

Etymology: This species is dedicated to Per Solemdal (1941–2016) who was the first researcher to study the Baltic Sea flounder's eggs and sperm in connection to salinity and discovered that “the specific gravity of the eggs is a fixed population characteristic which is almost unchangeable” (Solemdal, 1973) laying the foundations on which many subsequent studies on local adaptation and speciation of Baltic Sea marine fishes were built.

Paolo Momigliano, Gaël P. J. Denys, Henri Jokinen and Juha Merilä. 2018. Platichthys solemdali sp. nov. (Actinopterygii, Pleuronectiformes): A New Flounder Species From the Baltic Sea.  Frontiers in Marine Science.  DOI: 10.3389/fmars.2018.00225