Authors: Ian Smith & Malcolm Storey. Revised March 2013
Up to 65mm long, exceptionally up to 120mm in Netherlands (P.H. van Bragt). Ample mantle covers whole body and extends well beyond foot [image7]. Often, low profile similar to planarian worm, wide mantle skirt almost horizontal  even when body humped up . Mantle covered with tightly packed rounded spiculose tubercles . Various shades of, usually dull, yellow (SSF) & , brown , orange-brown , pink-brown  or purple-brown  & . Up to about 16 whitish acid producing papillae , usually arranged subdorsally along flanks . Acid-papillae often surrounded by a pale patch which may radiate a short distance . Underside of mantle usually has distinct dark marks of reddish  or purplish brown , but sometimes few  or no marks (SSF). Rhinophores
Stout stem mottled brown and purplish . 12 or 13 yellowish lamellae . Margins of pits slightly raised by tubercles. Gills
About 7 small pale tripinnate  gills in ring, partially separated into left and right groups . Often marked with brown ; retract into cavity . Head
Small and indistinct , apart from well developed linear oral tentacles. More distinct when mouth protruded . Foot
Rounded anterior bilaminate ; upper lamina cut centrally and extends forward of lower layer. Sole and upper surface of foot yellow  or orange . Upper surface of foot usually also has dark marks  similar to underside of mantle. May protrude slightly when in motion , otherwise much smaller than mantle .
Key identification features
Up to 65mm long (exceptionally 120mm in Netherlands).
Dull yellow or shade of brown, generally less colourful and variegated than A. pseudoargus.
Tubercles  smaller than largest ones of A.pseudoargus, bigger than on Jorunna tomentosa.
Dark marks on underside of mantle . (Occasionally absent SSF )
Base of extended gills surrounded by raised collar [2Jt].
Underside of mantle lacks dark markings.
Ecology and behaviour
LWS and sublittorally to 45m. Well camouflaged on stone by dull colour and lack of shadow from flat profile . Feeds on sponges such as Hemimycale columella (EML), Mycale rotalis (EML) and Mycale micracanthoxea (SSF). Exudes defensive acid from large whitish papillae on mantle. When disturbed, tends to remain flattish while it contorts, rather than contracting tightly like Archidoris pseudoargus. Simultaneous hermaphrodite. Little known about breeding season and biology. Spawn seen in September; yellowish ribbon attached by edge to substrate in a coil .
Distribution and status
Britain, Ireland, Netherlands (common in Oosterschelde SSF), Brittany, Spain and New England (USA) (GBIF map). Population explosion around year 2000 in Oosterschelde, Netherlands led to its Dutch name, "Millenium wratslak". Scattered finds on west and south coasts of Britain and Ireland, but few records despite large size; perhaps overlooked as Archidoris pseudoargus, but probably scarce in Britain, possibly increasing. ( UK interactive distribution map. NBN.)
References and links
Alder, J. & Hancock, A. 1845-1855. A monograph of the British nudibranchiate mollusca. London, Ray Society.