Polycera quadrilineata

Polycera quadrilineata (O. F. Müller, 1776)

Polycera quadrilineata by Enric Madrenas

Taxonomical note: Polycera quadrilineata has recently revealed to be part a group of several cryptic species with similar morphologies. Until 2020, morphotype of Polycera norvegica Sørensen, Rauch, Pola & Malaquias, 2020 was included within most published descriptions of P. quadrilineata. Korshunova et al (2021) published their work on Polycera quadrilineata complex, where four species from the European Atlantic shores were recognized, including a new species for science, Polycera kernowensis, based on a multidisciplinary (molecular, morphological, phylogenetic, phylogeographical and ecological) approach to species determination. In this work, the authors have treated the nudibranch species Polycera norvegica as a junior synonym of Polycera capitata (original designation: Thecacera capitata), but Malaquias et al (2022) show that, in accordance with the ICZN, Thecacera capitata (and in consequence also Polycera capitata) should be considered a nomen dubium, so they reinstate Polycera norvegica as the valid name for that species.

Synonyms

• Doris quadrilineata O. F. Müller, 1776
• Doris cornuta Rathke, 1806
• Doris flava Montagu, 1804
• Polycera lineatus Risso, 1826
• Doris ornata (d’Orbigny, 1837)
• Polycera ornata d’Orbigny, 1837
• Polycera typica W. Thompson, 1840
• Polycera varians M. Sars, 1840
• Polycera mediterranea Bergh, 1879
• Polycera quadrilineata mediterranea Bergh, 1879
• Polycera nigropicta Ihering, 1885
• Polycera quadrilineata nigropicta Ihering, 1886
• Polycera nigrolineata Dautzenberg & Durouchoux, 1913
• Polycera quadrilineata nigrolineata Dautzenberg & Durouchoux, 1913
• Polycera salamandra Labbé, 1931

Description
The body reaches a maximum size of about 30 mm in length, with a reported maximum length of 45 mm (Sørensen et al., 2020), although in the Mediterranean they hardly exceed 20 mm (Pers.Obs.). Translucent white body, with orange or pink tones due to the digestive gland inside, visible by transparency. Mantle is smooth with a series of orange or yellow markings along the edge, from below the rhinophores to the large process on each side of the gills. The anterior part of the border often has a small triangular lateral projection that can sometimes be long enough to be confused with an oral veil process. The anterior border of the oral veil may have four, five, or up to seven, elongated yellow or orange processes. Below the veil the mouth is flanked by two ridges, which may be slightly yellow. Most specimens have yellow or orange-tipped body appendages, with a dorsal line of the same color on the upper part of the tail, and tuberculate markings (but not fine specks) of variable number and size aligned to form lines discontinuous (which may become continuous) along the body. There could be black markings, especially on the inner side of the base of the rhinophores. Some specimens have a lot of black pigment (which can be brownish or greenish) forming lines along the dorsum and body, which can merge and even mask the yellow line on the tail. The color pattern depends on the locality and the growth, since the darker color patterns develop with the age of the animal. The rhinophores have a smooth, transparent, unpigmented basal third, although it may be tinged with black, especially on the inner side. The upper two thirds of the rhinophores usually have between 11 and 17 lamellae, with yellow pigment mainly in the anterior part, the rear part does not have this pigment. The rhinophores are slightly curved backwards in their middle part. The gills are located behind the pericardium lump and are made of six or seven (sometimes up to 11) unipinnate leaves. In specimens with little or no dark pigment on the body, the gill blades are often translucent white with yellow or orange tips, but in specimens with some black body pigment, the gill is often spotted with these dark colors as well. The gills cannot retract into the body but can contract into a lump if the animal is disturbed. The genital opening is located on the right side of the animal, between the rhinophore and the gill. The anal papilla is located on the back, behind the base of the gills. The foot is translucent white, often with some small yellow markings. The sole is truncated and of translucent white color, it is wider towards the front and gradually tapers towards the tail. It has a pair of propodial tentacles with yellow tips. The tail has a dorsal midline, yellow or orange in color, which can also be seen through the sole by transparency.

Biology
P. quadrilineata lives at the infralittoral in rocky shores down to depths of 60 m. Largest specimens, easier to find, appear in February and March. Its diet consists of encrusting bryozoans such as Electra pilosa, Membranipora membranacea, Callopora dumerilii, Celleporella hyalina and Tegella unicornis. It is a simultaneous hermaphrodite. The spawn, shaped like a short, curved ribbon 2 mm high, is attached to the substratum by one of its rims, near or on top of the bryozoans on which it feeds. The spawn contains about 20,000 white eggs with a diameter of about 80 microns. Upon hatching, the veliger larvae (which still have a protective shell) live in plankton for some time before metamorphosing into tiny nudibranchs.

Etymology

• Polycera. From Greek “polys”, many + “keras”, horn.
• Quadrilineata. Four lined. From Latin “quadrus”, square, derived from “quattuor”, four + “līnea”, a string, linen thread, a fishing line, a plumbline, a bowstring, a line (geometry), an outline, sketch, a boundary line, and other similar meanings.

Distribution
P. quadrilineata is found from the Norwegian Arctic to the Mediterranean. It is common, and sometimes abundant, in certain places in early spring, mainly on rocky shores. In the Iberian Peninsula it has been cited in all coastal areas, also in Canary Islands, Azores and Madeira (Cervera et al., 2004). In the Catalan coast it has been found in several localities of the Costa Brava like Cala Sant Antoni, Es Caials, Cadaques, Roses, L’Escala, Llafranc, Aiguafreda, Aiguablava, Tossa de Mar, Blanes and Cala Sant Francesc, in the southern coast of Catalonia it has been found in the port of Vallcarca (Garraf), Sitges, Cubelles and Torredembarra.

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Bibliography

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Bibliography based on the works by Steve Long, 2006. Bibliography of Opisthobranchia 1554-2000 and Gary McDonald, 2009. Bibliographia Nudibranchia, with later updates from other resources.

Further reading

• AsturNatura
• Biodiversity Heritage Library
• BiologiaMarina.org
• CIB - Club Immersio Biologia
• DORIS
• El Litoral de Granada
• Encyclopedia of Marine Life of Britain and Ireland
• Estran 22 - Faune et flore de la zone de balancement des marées en Côtes d'Armor
• European Sea Slugs by Morddyn
• Flickr pictures
• GROC
• Interim Register of Marine and Nonmarine Genera
• NCBI GenBank
• OBIS - Search by Taxon
• Scottish Nudibranchs
• Sea Slug Forum
• The Conchological Society of Great Britain and Ireland
• The SlugSite
• The SlugSite
• World Register of Marine Species

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