About misinterpretations of colour patterns

Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021) have explicitly stated that whereas Sørensen et al. (Reference Sørensen, Rauch, Pola and Malaquias2020) did not investigate the numerous synonyms of P. quadrilineata but only referred to the list of names available in MolluscaBase (2020), they have ‘carefully investigated all existing available synonyms of P. quadrilineata (Figures 1, 2, 3, 4, 5, 6, 7, Table 1, Supplementary information Figure S1)’. This is a misleading statement because it suggests that all the various nominal species names currently considered to be synonyms of P. quadrilineata are discussed by Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021) when in fact their figures and tables illustrate only the new material studied by the authors. There are no comparative figures or discussion or reference to original descriptions and illustrations of the various synonyms, which the authors seem to equally assume are the ones listed in MolluscaBase (2020), but this is not explicitly stated.

The only synonyms of P. quadrilineata discussed by Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021) out of the previous 11 available (10 names after Korshunova et al., Reference Korshunova, Driessen, Picton and Martynov2021) (see MolluscaBase, 2021) are Polycera ornata d'Orbigny, 1837 and Thecacera capitata.

Regarding the name Polycera ornata, Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021) based on the original description by d'Orbigny (1837: 9–12, pl. 107), have argued that it may agree with the lined morphs known for P. quadrilineata (Sørensen et al., Reference Sørensen, Rauch, Pola and Malaquias2020; Korshunova et al., Reference Korshunova, Driessen, Picton and Martynov2021), a conclusion with which we agree. Moreover, the lack of available type material of P. ornata for comparative analyses would make any other decision equivocal.

When it comes to Thecacera capitata, the reasoning given by Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021) to regard it as the valid name for the recently described species Polycera norvegica seems surprisingly different. Thecacera capitata was based on a vague description that included no illustrations of the living animal and was reassigned by Thompson & Brown (Reference Thompson and Brown1984) to the genus Polycera. Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021) have stressed the fact that in the original description of T. capitata by Alder & Hancock (Reference Alder and Hancock1854) ‘the colouration was solely indicated as freckled with brownish greenish that immediately allows exclusion of any morphs with evident black stripes, that are present only in true P. quadrilineata’. What Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021) have not stated was that the specimens studied by Alder & Hancock (Reference Alder and Hancock1854: 103) were presented to them already preserved in ‘spirits’. In fact, in a following paper, Alder & Hancock (Reference Alder and Hancock1855: appendix iv) went further and specified that among the two specimens given to them ‘The larger specimen was nearly colourless when it reached us (in spirits), but had the orange tubercles very distinct … The smaller specimen was darker and more distinctly freckled, but the orange colour of the processes was gone. In this specimen only two tubercles were observed on each side of the veil.’ Hence, all that Alder & Hancock had available to study were two whitish specimens the colouration of which was already largely lost, and quite distinct colour patterns between them, certainly because of the effect of preservation. It is well known that sea slugs become colourless or nearly colourless after preservation, not retaining any of the distinctive patterns that living animals exhibit, and, thus, descriptions of colour patterns based on fixed specimens are not reliable or, at the most, can yield only a biased assessment.

The freckled pattern mentioned by Alder & Hancock (Reference Alder and Hancock1854, Reference Alder and Hancock1855) for T. capitata could be an effect of the liquid fixative, leading to the disappearance of the dashed or continuous dark lines that characterize morphs of P. quadrilineata. But, of course, this remains and will always remain speculative. Overall, the limitations regarding the full details of the pattern of T. capitata are well documented in the descriptions provided by Alder & Hancock (Reference Alder and Hancock1854, Reference Alder and Hancock1855) and in their decision to not illustrate the species. Any attempt to reconstruct the colour pattern of this species from the original description can only be regarded as a highly speculative exercise.

Nevertheless, Thompson & Brown (Reference Thompson and Brown1984: 69, pl. 18c) surprisingly suggested that a discrete and colourful morph – and we quote directly from them here – ‘matches well a recognizable variety of Polycera quadrilineata’ was probably the same as T. capitata. They included a drawing of this morph without further explanations; this drawing is in notable contradiction with the original descriptions of T. capitata by Alder & Hancock (Reference Alder and Hancock1854, Reference Alder and Hancock1855).

The uncritical approach by Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021) towards these historical inconsistencies has led the authors to assume that the colour pattern of Thecacera/Polycera capitata corresponds to the one depicted by Thompson & Brown (Reference Thompson and Brown1984: pl. 18c). However, as we explain above, it is impossible to ascertain the extent of the colour pattern of the two specimens described by Alder & Hancock (Reference Alder and Hancock1854, Reference Alder and Hancock1855) as T. capitata. Additionally, there is one striking feature in the illustration by Thompson & Brown (Reference Thompson and Brown1984: pl. 18c), which is distinctive of morphs of P. quadrilineata (absent in P. norvegica), namely the dark colour of the anterior side of the stems of the rhinophores (Sørensen et al., Reference Sørensen, Rauch, Pola and Malaquias2020: Figures 3, 4, 7, 8; Korshunova et al., Reference Korshunova, Driessen, Picton and Martynov2021: Figure 1).

Thecacera capitata or Polycera capitata: to have or not to have rhinophoral sheaths

In addition, and in contrast to Korshunova et al.'s (Reference Korshunova, Driessen, Picton and Martynov2021) stated view, uncertainty about the presence of rhinophoral sheaths in T. capitata cannot be ignored. Alder & Hancock (Reference Alder and Hancock1854) referred to ‘tentacles retractile within cavities’ (in reference to the rhinophoral tentacles) and the same authors referred to the presence of sheaths and used it in fact as the main argument to place the species in the genus Thecacera (Alder & Hancock, Reference Alder and Hancock1855). Thompson & Brown (Reference Thompson and Brown1984) after examining the type material of T. capitata claimed that they ‘can find nothing to distinguish it from Polycera quadrilineata’ and reassigned the species to the genus Polycera, considering it to be a ‘probable synonym’ of P. quadrilineata. This can be interpreted as a sign of the uncertainty of these authors about the taxonomic status and affiliation of Thecacera capitata.

Yet, despite all these vague statements and lack of clarity and the fact that retractable rhinophores and rhinophoral sheaths are conspicuous features, Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021) have assumed that the ‘presence of rhinophoral sheaths for P. capitata was thus indicated mistakenly’ by Alder & Hancock (Reference Alder and Hancock1854) and that the species name Thecacera capitata is undoubtedly a Polycera. We must state though, that we also believe that most likely the two specimens studied by Alder & Hancock (Reference Alder and Hancock1854, Reference Alder and Hancock1855) do not belong to the genus Thecacera, because of the reference to a ‘veil with two to four orange tubercles on each side’ (Alder & Hancock, Reference Alder and Hancock1854, Reference Alder and Hancock1855), a feature absent in the single species of the genus present in European waters (Thecacera pennigera). Yet, the discrepancies in the descriptions of Thecacera/Polycera capitata by Alder & Hancock (Reference Alder and Hancock1854, Reference Alder and Hancock1855) and Thompson & Brown (Reference Thompson and Brown1984) warrant caution.

About misinterpretations of the internal anatomy

Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021: supplementary data) have stressed that there are ‘considerable errors’ in the diagnostic characters used to discriminate between P. quadrilineata and P. capitata (=P. norvegica) by Sørensen et al. (Reference Sørensen, Rauch, Pola and Malaquias2020). They first focus on incongruences in the radula description since Sørensen et al. (Reference Sørensen, Rauch, Pola and Malaquias2020) mentioned the presence of four outer lateral teeth in P. norvegica and five outer lateral teeth in P. quadrilineata, whereas Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021) have shown that P. norvegica (as P. capitata) can also have five or even six teeth. Although, interestingly in Table 1 (diagnostic characters of species) they stress that four is the common number of outer lateral teeth in P. norvegica (as P. capitata) and five is the common number in P. quadrilineata. In addition, a detailed analysis of Korshunova et al.'s (Reference Korshunova, Driessen, Picton and Martynov2021) Figure 7, in which the authors depict details of the radulae of several specimens of P. norvegica (as P. capitata), reveals considerable intraspecific variability, with the number of outer lateral teeth varying between two and five; but, most interestingly, differences also occur within the same specimen with the number of outer lateral teeth varying between radular rows (e.g. Figure 7C [spc. ZMMU Op-766] and Figure 7V [spc. ZMMU Op-770]). These details have apparently escaped the attention of Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021). Despite recognizing that the number of teeth in P. norvegica (as P. capitata) is variable and overlaps with that in P. quadrilineata, and is therefore a non-diagnostic feature, these authors still have used the presence of four outer lateral teeth, as illustrated in the original figure of the radula of T. capitata supplied by Alder & Hancock (Reference Alder and Hancock1855: pl. 46, Figure 19, supplementary), to support their argument that T. capitata and P. norvegica are conspecific (Korshunova et al., Reference Korshunova, Driessen, Picton and Martynov2021: Discussion, paragraph 5).

In our paper (Sørensen et al., Reference Sørensen, Rauch, Pola and Malaquias2020), we stated that P. norvegica only had four outer lateral teeth and P. quadrilineata always five – this is because the material examined by us revealed exactly that. On the basis of a larger number of specimens from across a broader geographic span, Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021) have, however, demonstrated otherwise, showing that this is not a reliable character for separating these two species.

The same is true of Korshunova et al.'s (Reference Korshunova, Driessen, Picton and Martynov2021) interpretation of the labial cuticle (referred to by them as ‘jaws’) where they have confirmed that both P. quadrilineata and P. norvegica (as P. capitata) have strong labial cuticles.

These differences between our results and those by Korshunova et al. (Reference Korshunova, Driessen, Picton and Martynov2021) are not, to use their phrase, ‘considerable errors’ but simply the way that science works. New studies provide new evidence that leads to new conclusions and to the reinterpretation of current knowledge.