Up to 120mm long. Ample mantle covers whole body. Mantle covered with blunt spiculose tubercles , variable size, many large . Firm and stiff to touch. Ground colour most often buff-orange  or lemon yellow . Dorsally, very often camouflaged with disruptive blotches of brown, pink, white, green, purple or red. Variety flammea completely red or purple. Underside of mantle uniformly coloured with no markings, but strong dorsal colour may show slightly through periphery .
Conical. Smooth whitish stem, broadening rapidly at base. Distally, about twenty yellow lamellae, alternately large and small, and a very small truncate apex . Rhinophores stand in distinct pits.
About 9 large tripinnate gills, similar shade as , paler or darker  colour than mantle. In water: recurved, undulating, spreading, semi-prone , inclined backwards. Arranged in circle, open at rear, around anal papilla  which is coloured as mantle.
Small with vestigal oral tentacles , only visible from underside.
Sole and upper surface of foot yellow or orange . May protrude slightly when in motion .
- Up to 120mm long.
- Usually yellow or buff-orange, often with large blotches of bright colours .
- Variously sized spiculose tubercles give stiff unyielding feel to mantle .
- Gills often blotched with colour, anus similar colour to mantle .
- Gills tilt backwards, not upwards on a raised collar .
- Underside of mantle lacks dark markings  (may have slight staining at edge).
- Head small with vestigal oral tentacles.
- Up to 60mm long, often similar size to medium Archidoris pseudoargus.
- Sandy [1Jt], buff, whitish (Flickr) or bluish white (Flickr), any other marks small and dark.
- Very small tubercles; velvety appearance, soft feel.
- Large pale gills around contrasting dark brown anus [1Jt].
- Base of extended gills surrounded by raised collar [2Jt]
- Up to 65mm long.
- Dull yellow or orange-brown, generally less colourful and variegated than A. pseudoargus.
- Tubercles [3Gp] smaller than larger ones on A.pseudoargus, bigger than on Jorunna tomentosa. Mantle feels even stiffer than A. pseudoargus.
- Dark marks on underside of mantle [7Gp].
- Foot much smaller than mantle. [7Gp].
- Up to 12 pale vaguely stellate areas on dorsal surface of mantle [1Gp].
- Head has well developed linear oral tentacles [5Gp].
Middle and lower shore, and sublittorally to 300m. Well camouflaged on prey; Halichondria panicea (EML) and Hymeniacidon perleve (EML). Exudes defensive chemicals, but has been used for fish bait. Simultaneous hermaphrodite. Breeding observed in all months but usually spawns and dies in spring. Juveniles soon present, but often overlooked until late summer because of small size. Usual pattern on Menai strait, Wales: adults 60mm long in March, spawn and die; juveniles c.15mm April, c.30mm June; large enough for frequent casual observations by late summer; grow over winter. Spawn mass (EML) a large whitish spiral ribbon attached by its edge to substrate. Planktonic veliger larvae for about four weeks before metamorphosis.
Iceland and N. Norway to Portugal and into Mediterranean and Baltic as far in as Kiel (GBIF map). Widespread round Britain and Ireland, and frequently recorded because of large size ( UK interactive distribution map. NBN.) Records from Pacific and Gulf of Mexico are probably of other spp. of similar appearance.