Version française...

This species has been observed on Reunion, Mauritius, Madagascar, Seychelles

Order : Nudibranchia Suborder : Euctenidiacea Superfamily : Polyceroidea Family : Polyceridae Distribution : Tropical West Pacific Maximal size : 20-25 mm Abundance : Seldom on the rocky coast and the reef flat zone. Species characteristics : Body elongate and limaciform with a long and pointed posterior end of the foot. The ground colour of the body is blue-black with yellow-orange longitudinal lines, which can be continuous or discontinuous. The proportionally large rhinophores and gills are a very characteristic feature of this species. They can be deep blue, blue-grey or purple depending on the specimens
	Showing species characteristics...	Photo Guillaume Boeye Reunion, Saint Leu lagoon, 50 cm, 3 February 2007, size 18 mm

See more about : Sightening and mating periods

Remarks :

Identification confirmed by Nathalie Yonow
    In all specimens observed in Reunion Island, there are only two lateral ones extending along each side from behind the rhinophores to almost the end of the tail, one or two short orange bands between the rhinophores, and one or two extending from behind the rhinophores to the gill sheath.
    Synonymous : (according Worms)
               - Nembrotha gracilis Bergh, 1877

Bibliographic data :

Usually the longitudinal lines or spots are arranged in the following pattern: 2 stripes in the median part of the back between the head and the front of the gills, 2 in the lateral parts of the back between the rhinophores and the rear of the gills, 1 around the margin of the mantle and 3–4 on the sides of the body. If the lines are discontinuous they give the appearance of yellow-orange spots.
    Usually the yellow lines are narrower than the blue-black background. All these lines consist of a series of glandular pits, which give them a corrugated aspect. It has been reported that when the animals are disturbed, the yellow-orange spots along the body open, releasing an acidic yellow fluid. Once the chemical has been released, they close the spots again. Presumably, this is a defence mechanism but it appears to be quite noxious to other sea slugs
    The oral tentacles are smooth, cylindrical, project in front of the head, very well developed and dorsolaterally grooved. They are found in all members of Roboastra and are probably important in tracking down their prey, perhaps by following its mucus trail.
    The perfoliate rhinophores are very large, with 15–25 packed lamellae.
    There are from three to six very large and nonretractile unipinnate gills. The gills form a semicircle surrounding the anal papilla, which is not elevated
    The genital pore opens on the right side, slightly closer to the rhinophores than to the gills.
    Habitat : Coral or sandy substrates. Generally found at the reef crest beneath dead coral slabs or crawling in shallow pools
    There is a similar sibling species, Roboastra luteolineata. But, R. luteolineata is characterized by its thinner mid-dorsal lines which extend up onto the gills, and the gills themselves which are highly branched (tripinnate) rather than simply pinnate in R. gracilis .

References :

Bill Rudman Seaslug site : Sea Slug Forum : Roboastra gracilis
   Nudipixel Roboastra gracilis

Publications :

Bergh, L.S.R. (1877). Malacologische Untersuchungen. In: C.G. Semper, Reisen im Archipel der Philippinen, Wissenschaftliche Resultate. Band 2, Heft 11 : 429-494, Pls. 54-57.
    Gofas, S.; Le Renard, J.; Bouchet, P. (2001). Mollusca. in: Costello, M.J. et al. (eds), European Register of Marine Species: a check-list of the marine species in Europe and a bibliography of guides to their identification. Patrimoines Naturels. 50: 180-213.
    Pola, M.., Cervera, J.L. & Gosliner, T.M. (2005) Review of the systematics of the genus Roboastra Bergh, 1877 (Nudibranchia, Polyceridae, Nembrothinae) with the description of a new species from the Galápagos Islands . Zoological Journal of the Linnean Society ,144, 167–189. With 12 figures

Other photos of Roboastra gracilis :

Emilie Delarue Réunion, Saint Pierre, less 1 m, December 2017,	Mating Behavior

Guillaume Boeye Reunion, Saint Leu lagoon, 50 cm, 3 February 2007, size 18 mm It has been reported that when the animals are disturbed, the yellow-orange spots along the body open, releasing an acidic yellow fluid. There are is this specimen five very large and nonretractile unipinnate gills. The gills form a semicircle surrounding the anal papilla, which is not elevated

Philibert Bidgrain Reunion, Etang salé on a rocky coast, less 1 m, 5 January 2007, size 10 mm There are is this specimen three very large and nonretractile unipinnate gills The perfoliate rhinophores are very large, with 15–25 packed lamellae. The oral tentacles are smooth, cylindrical, project in front of the head, very well developed and dorsolaterally grooved.

Christophe Cadet Réunion, Etang Salé, moins de 1 m, 10 December 2012, size : 11 mm In this specimen there is only one line (a) extending from behind the rhinophores to the gill sheath. It has been reported that when the animals are disturbed, the yellow-orange spots along the body open, releasing an acidic yellow fluid.

Alain-Benoît Rassat Madagascar, Sakatia, Nosy B è, 6m, 13 December 2014, size : 25 mm The ground colour of the body is blue-black with yellow-orange longitudinal lines, which can be continuous or discontinuous.

Madagascar, Roboastra gracilis, at Nosy Be, by Sully Bachel

Mauritius,Roboastra gracilis, at Tamarin, by Hilary Jones