Ample sandy-brown mantle covers whole body. Profile is raised convex, but varies with degree of extension; anterior part lower than posterior. This colour form resembles that described by Alder & Hancock (1845). No small opaque white glandular structures are visible near periphery of the mantle and there was no mention of them by Alder & Hancock or Thompson & Brown (1984), though they are frequent on other forms. This may be just intra-specific variation, but might indicate segregates within J. tomentosa

marinvert.senckenberg.science/image-browse/
Key identification features
Differences in profile, colour, markings and spawn-form suggest that what is regarded as J. tomentosa in British and Irish waters may, with future sequencing of DNA, prove to be a complex of species. The situation in Iberia and the Mediterranean is probably even more complex.
Jorunna tomentosa
1: Up to 60mm long, often similar size to Doris pseudoargus.
2: Dorsally, sandy-brown 01 Jorunna tomentosa. Length 60 mm. Intertidal. Orkney. April 1975. , grey, grey-cream, yellow 02 Jorunna tomentosa. , yellow cream 03 Jorunna tomentosa. Length 55mm. Menai Strait, Wales. June 2013. , buff-orange, white 04 Jorunna tomentosa. West Anglesey, Wales. March 2014. or bluish white. Small opaque white glandular structures near periphery of mantle on some 03 Jorunna tomentosa. Length 55mm. Menai Strait, Wales. June 2013. but not all 01 Jorunna tomentosa. Length 60 mm. Intertidal. Orkney. April 1975. . Often, a few dark marks in two or three longitudinal rows 05 Jorunna tomentosa. Split, Croatia. November 2012. © J. Prkić . .
3: Mantle has crowded, match-like, retractile tubercles with skirt of sharp projecting spicules (caryophyllidia) 37 Jorunna tomentosa album. Comparison image of Jorunna onubensis. . Body has velvety appearance and soft feel.
4: Large pale gills around brown anal papilla 02 Jorunna tomentosa. & 04 Jorunna tomentosa. West Anglesey, Wales. March 2014. .
5: Base of extended gills surrounded by distinct collar-like sheath 09 Jorunna tomentosa, gills and gill sheath. Lengths of animals 55mm and 43mm. Anglesey, Wales. which closes over the gills when they are retracted into a pit 12 Jorunna tomentosa. Length of animal 43mm. Menai Strait, Wales. February 2014. .
6: Slender tapered oral tentacles 15 Jorunna tomentosa. Animal 55mm long. Menai Strait, Wales. June, 2013. .
7: Convex body with raised profile 07 Jorunna tomentosa. West Anglesey, Wales. March 2014. .

Similar species

Rostanga rubra
1: Up to 15mm long (20mm in Mediterranean).
2: Mantle bright scarlet 21 Jorunna tomentosa album; comparative image of Rostanga rubra. to orange 22 Jorunna tomentosa album; comparative images of Rostanga rubra. , with sparse scattered black or reddish-brown spots . White stripe from rhinophore to rhinophore like spectacles 21 Jorunna tomentosa album; comparative image of Rostanga rubra. , sometimes indistinct 22 Jorunna tomentosa album; comparative images of Rostanga rubra. . Many opaque white glandular structures near periphery of mantle.
3: Mantle covered with densely packed caryophyllidia which are thicker than those on J. tomentosa with shorter projecting spicules 23 Jorunna tomentosa album; comparative image of Rostanga rubra rhinophores.
4: Translucent unipinnate gills, tinted as mantle, usually held erect to form cylinder around brown tubular anus 24 Jorunna tomentosa album; comparative image of Rostanga rubra .
5: Base of extended gills surrounded by slightly raised rim bearing caryophyllidia, not a distinct collar-like sheath 24 Jorunna tomentosa album; comparative image of Rostanga rubra .
6: Slender tapered oral tentacles 25 Jorunna tomentosa album; comparative images of Rostanga rubra. .
7: Convex body with raised profile 22 Jorunna tomentosa album; comparative images of Rostanga rubra. .

Doris pseudoargus Rapp, 1827.
1: Up to 120mm long, but often similar size to Jorunna tomentosa.
2: Usually yellowish, often with large blotches of bright colours 26 Jorunna tomentosa album; comparative images of Doris pseudoargus. & 27 Jorunna tomentosa album; comparative image of Doris pseudoargus. .
3: Variously sized, rounded, spiculose tubercles 26 Jorunna tomentosa album; comparative images of Doris pseudoargus. give stiff unyielding feel to mantle.
4: Voluminous gills resembling curled kale. Often blotched with variable amount of lilac/purple . Anal papilla similar colour to surrounding gills, so may be brownish if gill colour thus 28 Jorunna tomentosa album; comparative images of Doris pseudoargus. .
5: Base of gills not surrounded by collar-like sheath 28 Jorunna tomentosa album; comparative images of Doris pseudoargus. .
6: Short, thick, oral tentacles 29 Jorunna tomentosa album; comparative images of Doris pseudoargus. .
7: Convex body with raised profile 26 Jorunna tomentosa album; comparative images of Doris pseudoargus. .

Acanthodoris pilosa (Abildgaard in Müller, 1789)
1: Usual maximum length 40mm.
2: Dorsally white or brown 30 Jorunna tomentosa album, comparison images of Acanthodoris pilosa. , occasionally, black or yellow (P.Lightfoot) flic.kr/p/oJLWhQ . Pale colours may be evenly freckled darker, but not with large patches of pigment.
3: Soft, tall, thin, conical tubercles on mantle 31 Jorunna tomentosa album, comparison image of Acanthodoris pilosa. , but sometimes contracted/ poorly developed and not so distinctive (J.Anderson) Acanthodoris pilosa. Body soft and yielding.
4: Voluminous tripinnate gills can contract but not retract into pocket; usually, opaque white along each side of midribs forms a star when gills expanded 32 Jorunna tomentosa album, comparison images of Acanthodoris pilosa. . Anal papilla not brown, except possibly on brown specimens.
5: Base of gills not surrounded by collar-like sheath.
6: Narrow oral veil extended laterally into broad, flat oral tentacles. Anterior of veil has two protrusions centrally 33 Jorunna tomentosa album, comparison image of Acanthodoris pilosa. .
7 Convex body with raised profile 30 Jorunna tomentosa album, comparison images of Acanthodoris pilosa. .

Cadlina laevis (Linnaeus, 1767)
(image J.Anderson) flic.kr/p/6e53ZS
1: Usual maximum length 32mm.
2: Shiny white with opaque yellow or white marks near the periphery and, sometimes, has an opaque yellow or white peripheral border.
3: Unobtrusive, very small, uncrowded, soft tubercles give very soft texture to mantle.
4: White gills not voluminous, hidden much of the time retracted into pocket.
5: Base of gills not surrounded by collar-like sheath.
6: Very small oral tentacles.
7: Body has a flattened low profile.

Similar species in the Mediterranean

Jorunna onubensis Cervera, García-Gómez & García, 1986,
is very similar to J. tomentosa; reliable identification of this species requires examination of dissected anatomical features and, ideally, D.N.A sequencing. The external features below are described in the original description of J. onubensis [based on four intertidal specimens from Huelva, southern Spain]. The features illustrated in this Flickr account are on a specimen from Catalonia of J. onubensis identified by Miquel Pontes and confirmed by Prof. J. Lucas Cervera. Many features are common to both J. onubensis and J. tomentosa; those in bold font may be the best features for distinguishing them, but confirmation by dissection and/or D.N.A, sequencing is desirable. Specimens of J. onubensis photographed by one of the authors of its first published description, J. C. García Gómez, can be seen at opistobranquis.info/en/guia/nudibranchia/euctenidiacea/do…
and other images by L. Sánchez-Tocino at
wpd.ugr.es/~litoraldegranada/el-litoral/el-litoral-sumerg…
It has been recorded from Spain, Portugal, Madeira and the Canary Islands.

1: 12mm to 18mm long (four specimens March & April 1984). Up to 55mm long (Sánchez-Tocino, accessed February 2018).
2: Light brown, often slightly pinkish 34 Jorunna tomentosa album. Comparison image of Jorunna onubensis [sometimes grey?]. Sometimes with dark, greyish or greyish-brown, blotches of varying size. Small opaque white marks near periphery of mantle 36 Jorunna tomentosa album. Comparative ventral view of Jorunna onubensis. . [Di questo colore sono anche alcune macchioline localizzate lungo il bordo posteriore del mantello.]
3: Very fine, crowded, linear tubercles (caryophyllidia), similar to J. tomentosa, some with opaque, bright-white apex 37 Jorunna tomentosa album. Comparison image of Jorunna onubensis. [Alcuni dei tubercoli dorsali persentavano il loro apice di colore bianco iridescente.]
4: The whitish gills have a fine brown punctuation, hardly distinguishable to the naked eye 38 Jorunna tomentosa (right) and comparison image of Jorunna onubensis. & 39 Jorunna tomentosa album. Comparison image of Jorunna onubensis. [Le branchie, biancastre, mostrano una fine punteggiatura marrone, difficilmente distinguibile ad occhio nudo.]. The end of the anal papilla is brown with whitish striations that subdivide it into unequal parts 38 Jorunna tomentosa (right) and comparison image of Jorunna onubensis. [L’estremità della papilla anale appare marrone, nonostante Possieda delle strie biancastre che la suddrvidòno in parti diseguali .]
5: Base of extended gills surrounded by collar-like sheath 39 Jorunna tomentosa album. Comparison image of Jorunna onubensis. which closes over the gills when they are retracted into a pit 35 Jorunna tomentosa album. Comparison image of Jorunna onubensis. .
6: Slender tapered oral tentacles, similar to J. tomentosa.
7: Convex body with raised profile, similar to J. tomentosa.

Tayuva lilacina (Gould, 1852)
Synonym: Discodoris maculosa Bergh, 1884.
Occurs in Mediterranean and most tropical seas; not recorded in NW Europe.
1: Up to 60mm long. Juveniles c. 10mm length may lack dark pigment on the sole, which is present on most (all?) adult T. lilacina, and are superficially very similar to Jorunna tomentosa.
2: Light brown to light olive grey with many slightly darker circular patches 40 Jorunna tomentosa album. Comparative image of Tayuva lilacina. . Often, two or three rows of the patches have black-brown marks on them 41 Jorunna tomentosa album. Comparison image of Tayuva lilacina. .
3: Crowded, low, conical tubercles (not linear caryophyllidia), often several apically capped with opaque white; body has hard texture 42 Jorunna tomentosa album. Comparative image of Tayuva lilacina . On older specimens many tubercles are white, and the mantle may be abundantly speckled white 43 Jorunna tomentosa album. Comparative image of adult Tayuva lilacina. .
4: Gills golden brown with white tips 40 Jorunna tomentosa album. Comparative image of Tayuva lilacina. , but often retracted out of sight.
5: Base of extended gills surrounded by large raised sheath (R. Fernández) flic.kr/p/qjC8nR which closes over the gills when they are retracted into a pit 41 Jorunna tomentosa album. Comparison image of Tayuva lilacina. .
6: Slender tapered oral tentacles 42 Jorunna tomentosa album. Comparative image of Tayuva lilacina .
7: Body has a flattened low profile 40 Jorunna tomentosa album. Comparative image of Tayuva lilacina. and (R. Fernández) flic.kr/p/qASRxx , but can ocasionally assume a more raised profile.