Body
Up to 12mm long. Elliptical outline [image1], more discoid at rest [5]. Low dome in profile. Ample white or pale brown mantle covers whole body. Translucent, showing long obliquely aligned spicules subdorsally and dark viscera dorsally [1]. Small brown freckles on mantle and on some apices of abundant small round-tipped tubercles [3]. Viewed from directly above, pale tubercles form light halos around their apical spots [2]. Larger black flakes frequent, but missing from Alder & Hancock’s original illustration [7]. Tubercles give a rough appearance to mantle surface [1].
Rhinophores
Basal third smooth, coloured as mantle. Distal two thirds with about ten (up to fourteen) closely arranged yellow lamellae [9], often with a few flecks of brown pigment [1]. Small white apex protrudes only slightly above lamellae [5].
Gills
About ten small translucent gills, coloured as mantle, around anus. Sometimes, brown pigment distally on gill plumes. Tubercular processes within the gill circle, often dark brown or black [5].
Head
Large semicircular oral veil with large depression leading to mouth [5].
Foot
Anterior of foot truncated [3],sometimes slightly curved back from mouth [4]. Usually completely concealed dorsally by mantle, but posterior tip may protrude a little when moving. Sole white [4] or yellow [3].Translucent showing central purple-black liver which is pear-shape, widest anteriorly, when foot extended [3] & [8].
Key identification features
- Pear-shape purple-black liver visible through sole. [3].
- Abundant small round-tipped tubercles on mantle (not swollen or club-shaped) [3].
- Often, dark tubercular processes within gill circle. [5].
- Scattered random brown freckles, and often black flakes, on mantle. [1].
Similar species
- Flat topped club-shape tubercles on mantle.
- Usually, no dark pigment marks.
- Club-shape tubercles with obtusely pointed apices.
- Usually, no dark pigment marks.
Onchidoris oblonga
- Orange-brown liver visible through sole.
- Randomly arranged brown pigment marks on mantle.
- Brown pigment marks arranged in longitudinal lines on mantle.
- Asymmetric orange-brown liver extends forwards on left of animal.
- Very depressed profile.
- Long soft non-spiculate tubercles on mantle.
- Orange-brown liver visible through sole.
- Mantle predominately brown, peripherally dissected into blocks by wide, light, unpigmented lines.
- Around rhinophore base; pit, large tubercles and rim are distinctly lighter or darker than adjacent mantle.
- Lamellae on rhinophores thick and distantly spaced.
- Often, dark ring around base of tubercles.
Ecology and behaviour
On lower shore and sublittorally. Feeds on Cellepora pumicosa (EML), Porella concinna and, possibly, Cellaria sinuosa (EML). Spawn deposited on or near prey in a tightly wound, thin, circa 7mm diameter spiral line of about eight turns [6] in March (Northumberland, 1848 [10]) and May (Orkney, 1975).
Distribution ans status
Orkney, S. Sweden, Northumberland, Yorkshire, Pembroke, Devon, Normandy and Brittany. (GBIF map). Most records are from Britain, but even there it is very rarely recorded.
References and links
Alder, J. & Hancock, A. 1845-1855. A monograph of the British nudibranchiate mollusca. London, Ray Society.
Rudman, W.B. Sea Slug Forum Onchidoris inconspicua fact sheet (Same images shown as on this page, but colours less true on forum versions.)
Thompson, T.E. & Brown, G.H. 1984. Biology of opisthobranch molluscs 2. London, Ray Society.
Current taxonomy: World Register of Marine Species (WoRMS)
Irish distribution map and prey organisms:
Picton, B.E. & Morrow, C.C., 2010. Encyclopedia of Marine Life of Britain and Ireland (EML).