Nembrotha purpureolineata

Nembrotha purpureolineata is a species of colourful sea slug, a dorid nudibranch, a marine gastropod mollusc in the family Polyceridae. Nembrotha rutilans, classified as a separate species until 2008, has now been reclassified as Nembrotha purpureolineata.^[3][4]

Nembrotha purpureolineata was first described from a preserved specimen collected from Abrolhos Islands off Western Australia by O'Donoghue in 1924. The short colour description given is probably enough to identify it to be the same as Nembrotha rutilans (Pruvot-Fol, 1931).^[2] The description of this species was done purely from a painting of marine life on the Great Barrier Reef in a book by William Saville-Kent, (1893)^[5] and so the earlier name has been given precedence.^[3][4]

This nudibranch occurs in the west of the tropical Indo-West Pacific Ocean.^[6]

Nembrotha purpureolineata is a large pale-bodied dorid that may reach a total length of 120 mm.^[6] Individual specimens vary extremely in terms of colour and patterning, which can make it very difficult to differentiate from both Nembrotha aurea and Nembrotha chamberlaini.

The base colour of the body is always a creamy white, and there are always brown markings. The brown markings may present as extremely thick longitudinal lines, and may converge to form an enormous brown patch that covers the entire notum.

On other occasions, the brown lines are more distinct and separated, and may be accompanied by patches of pale diffuse yellow. These forms are extremely difficult to differentiate from Nembrotha aurea. The only reliable difference is that N. aurea will always have two patches of bright orange on its back, one between the rhinophores and gills, and one behind the gills. N. purpureolineata will have either no orange patches, or only one, between the rhinophores and gills.

Other variants have no brown lines at all, and instead have two large brown patches on its notum, one that sits between the rhinophores and gills, and the other behind the gills. These are the forms that are constantly confused with Nembrotha chamberlaini. The most reliable method of differentiation is to observe the gills and surrounding area. In N. chamberlaini, the gills and gill stalks are always uniformly pigmented with no variation and end in patch of the same colour on the back. However, in this specific variant of N. purpureolineata, the pigmentation of the gills fades into white as it travels down the stalks, and below that will be a well-defined band of yellow-brown where the stalks join the body.

Gill and rhinophore colouration varies wildly, with some correlation observed between variants. Rhinophore clubs range from bright red to a brownish-orange, the stalks are often white or pink. The rhinophore sheaths may be a bright purple or blue-purple fading into the white head, or the upper part of the sheath may be purple with a yellow to brownish-yellow band below it. Gills can be bright red with blue or purple stalks, often with diffuse yellow where the stalks join the body. Or, the gills and gill stalks could be anywhere from brownish-red to deep purple, the colour the most pigmented at the gill fronds and fading into white down the stalk. In these cases there will be a well-defined band of yellow-brown where the gills join the body.

• Nembrotha purpureolineata.

This species feeds on ascidians and tunicates. It has been seen feeding on the yellow-lined ascidian, Clavelina meridionalis.^[6]

• Debelius H. (2001). Nudibranchs and Sea Snails. Frankfurt, Germany: IKAN - Unterwasserarchiv.

• Photos of Nembrotha purpureolineata in the Sealife Collection