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This species has been observed on Reunion, Mayotte and Madagascar

Order : Nudibranchia Suborder : Euctenidiacea Superfamily : Doridoidea Family : Chromodorididae Distribution : Tropical Indo-West Pacific Maximal size : 120 mm Abundance : Species characteristics : The posterior end of the mantle occurs about midway along the body forming a prominent, curved "horn" just behind the gills. It can be characterised by the three mantle lobes on each side of the body. There is a large anterior lobe on either side of the head and a large lobe on each side by the gills. Between these is a smaller lobe. The edge of the lobes and the ridge joining the posterior lobes is usually outlined with a broken purple line. There is usually no broken purple line between the head lobe and the posterior lobes
	Showing species characteristics...	Photo Sophie Darnis Reunion, Saint Paul, "La Barge", 25 m, 5 December 2010, size : 15 mm

See more about : Sightening and mating periods

Remarks :

Identification confirmed by Nathalie Yonow and Hsini Lin
    Synonymous (according worms) :
          - Ceratosoma bicorne, Bergh, 1905
          - Ceratosoma francoisi, Rochebrune, 1894
          - Ceratosoma jousseaumi, Rochebrune, 1894
          - Ceratosoma ornatum, Bergh, 1890
          - Ceratosoma rhopalicum, Rochebrune, 1894

Bibliographic data :

The rhinophores stalks are translucent white, and the lamellate portion vivid orange with a violet tip
    The smaller intermediate lobe is joined by a ridge to the gill lobe, but not the head lobe.
    It has a long recurved dorsal "horn'" which acts as a defensive lure attracting potential predators to the part of the animal which contains most of the distasteful chemicals stored from their food.
    The colour pattern is complicated and very variable :
            - It ranges from yellow-orange to deep maroon blotched with cream.
            - Presence of bright orange or yellow spots in some specimen
    The egg masses is a pink coiled ruban
    In species of Ceratosoma the oral tube is brightly coloured rather than colourless as in species of Chromodoris.
    Mature animals are diurnally active and feed on a blue sponge in the genus Dysidea. The mantle glands, which contain chemicals obtained from sponges and distasteful to fish, are concentrated in the large "horn." A fish nipping off the protuberance may "expect" the entire animal to taste this way and leave the rest of it intact. Animals with damaged or missing "horns" and intact bodies support this hypothesis. The defense is not foolproof, however, as some C. tenue have been found in the stomachs of large fish that swallow their prey whole (Rudman, 1988).
    It is one of the dorids that occasionally hosts the commensal shrimp Periclimenes imperator (Hoover, 1998)
    The two common tropical species of Ceratosoma , C. tenue and C. trilobatum, are variable in colour, and as both can have very similar colour patterns, considerable confusion has occurred in the past over the identity of specimens.
        But these two similar species have some differences.

	Ceratosoma trilobatum	Ceratosoma tenue
Mantle lobes	It is characterised by having only one large lateral lobe on each side of the body, just anterior to the gills	It is characterised by the three mantle lobes on each side of the body
Purple border	The purple border to the mantle and foot is usually an unbroken line	The purple border to the mantle and foot is always a dotted line
Gills and rhinophores pockets	The gills and rhinophores pockets bear purple around its margin	The gills and rhinophores pockets do not bear purple around its margin

References :

Bill Rudman Seaslug site : Sea Slug Forum : Ceratosoma tenue
   Nudipixel Ceratosoma tenue

Publications :

Rudman, W.B. (1984) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: a review of the genera. Zoological Journal of the Linnean Society 81 : 115-273
    Rudman, W.B. (1988) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: the genus Ceratosoma J.E. Gray. Zoological Journal of the Linnean Society, 93(2) : 133-185

Other photos of Ceratosoma tenue :

Hugues Flodrops Madagascar, Nosy Tanikely, 20 October 2010, size : 80 mm

Vincent Dinhut Mayotte, Passamainty (12° 48.356'S / 45° 12.995'E), 6 m, 13 June 2012, size : 100 mm Downstream of a small fringing reef broken up, in a sandy-muddy area with small bryozoan colonies and burrows The rhinophores stalks are translucent white, and the lamellate portion vivid orange with a violet tip

Alain-Benoît Rassat Madagascar, Olaf, Nosy bé, 18 m, 11 March 2013, size : 100 mm A specimen with a yellow pattern,bbut always the characteristic edge outlined with a broken purple line

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