nature today | A new harlequin snail discovered in the North Sea

Every now and then, targeted research into DNA and anatomy reveals that within common common species there are still other species that have never been recognized separately. The species are so similar in appearance that no further research has ever been done, but it was simply assumed to be a variable species. This multiplicity (polymorphism) of sea slugs can pose problems in the context of hidden diversity. This is also the case within the genre Polycer in the slug family Polyceridae, as the genus Polycer is involved in all kinds of ecological research around the world. Not only as it relates to environmental monitoring and changing colors and warning patterns, but also as an indicator of global warming.

Variant research

The original descriptions of slug species long known to science are in most cases based on external characteristics. As early as 2013, researchers found that between the two Polycerspecies present along the Dutch coast, namely the harlequin snail (Polycera quadrilineata) and the harlequin snail Breedkop (Polycera faeroensis), some individuals had outward appearances that differed and instead seemed to fall between the two species. A study has been launched to investigate the genetic identity of these “variants”. It was assumed that either hybridization had occurred between the two species or that one or more different or even new species were involved. In 2014, molecular and morphological analyzes, the relationship between the previously described species and the observed specimens was deepened. The study involved material from the following regions: Denmark (Lillebælt), Germany (Sylt), Netherlands (Doggersbank and Oosterschelde), Portugal (Setubal), Sweden (Skagerrak) and United Kingdom (Falmouth).

In 2014, Polycera capitata like a bloodline hidden inside Polycera quadrilineata discover. But this discovery has not yet been officially published. Because this species was also present in the material examined from the Dogger Bank, this was the first Dutch report as a new species for our part of the North Sea. The variants collected in the Eastern Scheldt turned out to be “ordinary” harlequin snails (Polycera quadrilineata).

A whole new species

In a follow-up study, relevant species within the genus Polycer extensively studied again. This research has also largely focused on the general problem of “cryptic species”, where incorrect identification can hamper ecological, evolutionary and other research related to biodiversity. The final conclusion was that there were four distinct species in the studied species complex. Two of them were already known to our waters (Polycera quadrilineata to Polycera faeroensis). In addition to the third species discovered, Polycer inhabitant, this follow-up study showed that there was clearly a fourth species hidden in the genus Polycer. This is closely related to Polycera faeroensis, but had not previously been recognized as a separate species and therefore still confused with the harlequin broad-headed snail. This brand new species for science was recently (September 2021) featured in an article in Nature described as Polycera kernowensis by Korshunova, Driessen, Picton and Martynov.

“Cornwall Arlequin” and “Petit Arlequin”

Floor Driessen, one of the authors of the post and co-descriptor of the new species, explains how the species name “kernowensis” came about. It seems to be derived from “Kernow”, an ancient Celtic (Cornish) name for Cornwall, the far southwest of Great Britain. In this area comes Polycera kernowensis generally for and from this area, the 5.6-millimeter-wide holotype has also been described (the calibration specimen on the basis of which the new species was described), as well as several paratype (Porthkerris location, Cornwall, Great Britain). Based on the above, ‘Cornish harlequin snail’ is therefore proposed as a suitable Dutch name for this entirely new species. For the third species described for science as early as 1854 Polycera capitata the name ‘Little Harlequin Snail’ is obvious, as it is the smallest of the four species mentioned.

Recognizing the Cornish Harlequin Snail

For in-depth comparisons between the four species, reference is made to the original English language. elements and see you soon in Spirula – the magazine of the Dutch Malacology Association – Dutch article on this subject to be published. Of the most similar broad-headed harlequin snail (Polycera faeroensis) Give in Polycera kernowensis sp. Nov. among others by the simple postbranchial lobes, never compound, thin, without additional tubercles and the fact that in Polycera kernowensis almost always less than nine projections on the front are present. Both Polycera quadrilineata like Polycera capitata differs Polycera kernowensis in addition, significantly due to a greater number of lamellae on the rhinophores (mean 14 to 15 versus 11 to 12 in Polycera quadrilineata and only 8 to Polycera capitata). Only to Polycera faeroensis the number of slats can be approximately the same. The number of gills on the back with Polycera kernowensis is always lower than that of other species. Bee Polycera quadrilineaata to Polycera capitata the number of colors and color patterns can be much more diverse than with the new species, of which, like many Polycera faeroensis mainly semi-transparent pale white with a yellow color pattern is known. Other differences can be found at the level of the teeth and genitals of the radula and at the phylogenetic molecular level.

From two to four

With this brand new species for science and the little harlequin snail already found in 2014 – also on the Dogger Bank – the number of known harlequin snails in Dutch waters will drop from two to four at one time.

Report sightings

All reports of these and other species of molluscs, as well as those of marine organisms, are welcome to Anemone Foundation and platforms like Waarneming.nl

Text: Floor Driessen and Rykel de Bruyne, both Anemone Foundation
Photos: Martijn Spierenburg (main photo: ‘Cornish Harlequin Snail‘ (Polycera kernowensis). 6.7 millimeters, Porthkerris, Cornwall, UK; 15 meters deep; 2013) and Floor Driessen, both from the ANEMOON Foundation