According to the results of our phylogenetic analysis, three genera Liopsetta, Pleuronectes and Pseudopleuronectes are monophyletic (Fig. 1) and a close relationship between these genera as proposed earlier by Cooper and Chapleau (1998) is not supported. The present study shows that Liopsetta is more closely related to Platichthys than to Pleuronectes. It corroborates the hypothesis of pleuronectid genera relationships based on analyses of cytochrome b sequence data (Kartavtsev et al., 2007; Sharina and Kartavtsev, 2010) and DNA sequences from nuclear and mitochondrial genes (Roje, 2010).

Pseudopleuronectes appears to be the sister group of other genera studied here. A synapomorphy of this genus is the interrupted coronal commissure, resulting in two pores instead of one behind upper eye, that are noticeable in unstained specimens (Fig. 3B). This character state is rare for Pleuronectiformes and is evident only in Pseudopleuronectes and Hippoglossoides (Voronina, 2009; Voronina, unpubl. data).

The genera Pleuronectes, Platichthys and Liopsetta share one synapomorphy: the length of pyloric caeca is equal to or less than the diameter of the pyloric part of stomach (Norman, 1934; De Groot, 1971; Nikol’skaya and Verigina, 1974; Voronina and Evseenko, 2001). Pyloric caeca of Verasper, Limanda and Pseudopleuronectes are longer than twice the pyloric stomach diameter. These character states corroborate Storer’s remark in his comparative description of L. putnami and P. americanus: “the difference in form of the caecal appendices, are very striking” (Storer, 1863: 245). The length of pyloric caeca compared to the diameter of the pyloric part of stomach is used in the present analysis instead of the number of pyloric caeca, traditionally used in the fish taxonomy. The number of pyloric caeca exhibits overlapping ranges of individual variation for the species studied (Norman, 1934), and thus it is difficult to use in analysis. Pseudopleuronectes obscurus shares characters, including an interrupted commissure, that we found in the genus Pseudopleuronectes, and it lacks synapomorphies of Liopsetta, confirming its appropriate placement within Pseudopleuronectes.

Pleuronectes is the sister group of the genera Liopsetta, Platichthys and Limanda. Two synapomorphies support the monophyly of this genus. The first synapomorphy, the radii completely surrounding the focus of scale, is revealed in species of Pleuronectes examined in the present study (Fig. 5). A photograph of a P. platessa scale at the ARKive web-site (http://www.arkive.org/european-plaice/pleuronectes-platessa/image-A23066.html) also shows this structure. It does not corroborate the coding of this character in the analysis of Cooper and Chapleau (1998), who referred to Batts (1964) as the source of their scale data. However Batts (1964) did not include Pleuronectes, Liopsetta and Pseudopleuronectes in his study of flatfish scales. The second synapomorphy is the absence of scleral ossicles (Fig. 7). Extensive investigation into their distribution in recent teleosts shows a consistent ossicle number in teleost families and the presence of a single anterior scleral ossicle in derived orders only (Franz-Odendaal, 2008). Scleral ossicle number varies from zero to two in Pleuronectiformes (Franz-Odendaal, 2008). The synapomorphies of Pleuronectes and Liopsetta support the hypothesis that these genera are distinct and with the states of six other characters readily distinguish these two genera.

Distribution and relationships among species of Liopsetta

The range of L. glacialis is Pacific high-boreal-Arctic (White Sea and Western coast of North America) according to modern range classifications (Chernova, 2011). Distribution of this species includes northern part of Sea of Ochotsk, Shantar Islands and Kamchatka Peninsula (Lindberg and Dulkeit, 1929; Sheiko and Fedorov, 2000; Fedorov et al., 2003). Distribution of L. pinnifasciata is Western Pacific low-boreal and includes Tartar Strait, Sea of Japan, Amur Bay, mouth of Amur River and off Sakhalin Island (Schmidt, 1904; Hubbs, 1915; Soldatov and Lindberg, 1930). The type location of this species – De Kastri Bay (Steindachner and Kner, 1870: 425). Jordan and Goss (1889: 290), referring to the original description, pointed out wrong distribution of L. pinnifasciata in ‘Sea of Kamchatka, Decastris Bay’. Since that time Kamchatka has been mistakenly included into this species distribution in consequent reviews of local ichthyofauna (Jordan and Starks 1907; Sakamoto in Masuda et al. 1984; Amaoka et al. 1995; Nakabo, 2013). However, there is no evidence that L. pinnifasciata occurs in northern part of the Sea of Okhotsk (Sheiko and Fedorov, 2000; Fedorov et al., 2003) and reports of such occurrences have been questioned (Lindberg and Fedorov, 1993). The distribution of L. putnami is Western Atlantic high-boreal.

The morphometric variability of L. glacialis (Esipov, 1939; Probatov, 1940) is too low over the wide distribution range of this species to distinguish subspecies (Voronina and Evseenko, 2001; Yusupov and Basov, 2005). Liopsetta pinnifasciata differs from L. glacialis in having more pronounced bars on the median fins and in gill raker number, while the differences between these two species in the depth of body, morphology postocular ridge and lower pharyngeals (Hubbs, 1915; Norman, 1934) were not confirmed (Voronina and Evseenko, 2001). Both species are polymorphic in colour. In particular, we reidentified specimens of L. pinnifasciata from Kamchatka (ZIN 49985; Tokranov and Maksimenkov, 1995) as L. glacialis. There are one or two specimens with pronounced bars among many typically coloured specimens in trawl catches of L. glacialis off Kamchatka (Sheiko B.A., pers. comm.).

We found no differences in the external morphology between L. putnami and L. glacialis. However the differences in the type of egg and early development (Laroche, 1981), some meristics (Voronina and Evseenko, 2001) and allopatric distributions among these species keep them separate. There is no evidence that L. glacialis, L. putnami and L. pinnifasciata are conspecific, but the relationships of the three species of Liopsetta were incompletely resolved and require further investigations with genetic analysis.