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This species has been observed on Reunion and Mayotte Islands

Order : Nudibranchia Suborder : Euctenidiacea Superfamily : Doridoidea Family : Actinocyclidae Distribution : Widespread Indo-West Pacific Maximal size : 20 mm Abundance : Seldom if ever on the reef flat zone Species characteristics : This nudibranch has a charcoal grey to translucent body with low rounded black tubercles scattered over the mantle The lamellate rhinophores of this nudibranch have a translucent grey stalk and a black clavus The 11 to 14, small gills, are arranged in a tight, upright circle around the anal papillae. They are uniformly dark grey to black
	Showing species characteristics...	Photo Mathiass Deuss Mayotte, Moutsoumbatsou, less 1 m, 7 August 2009, size : 15 mm

See more about : Sightening and mating periods

Remarks :

Identification confirmed by Nathalie Yonow

Bibliographic data :

The intensity of the background colour of the mantle apparently varies with the age of the individual; juveniles are pale greyish because they posses relatively few dark pigment granules.whereas adults are black because they posses numerous pigment granules.
    The spawn is a white ribbon
    This nudibranch is another species of Hallaxa which feeds on a species of the "slime sponge" Halisarca. The sponges are so named because they are often form a thin smooth slimy film over the rock on which they are attached.

References :

Bill Rudman Seaslug site : Sea Slug Forum : Hallaxa fuscescens
    Nudipixel Hallaxa fuscescens

Publications :

Pease, W.H. (1871) Descriptions of new species of Nudibranchiate Mollusca inhabiting Polynesia. No.2. American Journal of Conchology . 7 (1): 11-19
    Pruvot-Fol, A. (1930) Diagnose provisoires (incomplètes) des espèces nouvelles et liste provisoire des mollusques nudibranches recueillis par Mme. A. Pruvot-fol en nouvelle Calédonie (Ile des Pins). Bulletin du Museum National d'Histoire Naturelle Paris, 2: 229-232
    Risbec,  J. (1928) Contribution a l'étude des nudibranches Néo-Calédoniens. Faune des Colonies Francaises, 2(1): 328, pls.1-12
    Gosliner, T.M. & Johnson, S. (1994) Review of the genus Hallaxa (Nudibranchia: Actinocyclidae) with descriptions of nine new species. The Veliger, 37(2): 155-191

Other photos of Hallaxa fuscescens :

Mathiass Deuss Mayotte, Moutsoumbatsou, less 1 m, 7 August 2009, size : 15 mm The lamellate rhinophores have a translucent grey stalk (a) and a black clavus (b) The 11 to 14, small gills, are arranged in a tight, upright circle (c) around the anal papillae. These are uniformly dark grey to black

Philibert Bidgrain Mayotte, Mtsanga M'Titi Sada, less 1 m, 28 Jully 2010, size : 10 mm The 11 to 14, small gills, are arranged in a tight, upright circle around the anal papillae. These are uniformly dark grey to black

Eva Fontaine Mayotte, "Gombé Ndroumé", less 1 m, 21 December 2011, size : 15-18 mm     with its spawn. The spawn is a white ribbon

Reunion, Hallaxa fuscescens, at Etang Salé, by Seb Vasquez