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This species has been observed on Reunion and Mayotte Islands

Unassigned Heterobranchia Superfamily : Acteonoidea Family : Aplustridae Distribution : Tropical Indo-West Pacific Maximal size : Shell 40 mm Abundance : Seldom on the reef flat zone Species characteristics : The shell colour pattern consists of thin, rather irregular brown lines running across the translucent white shell, essentially parallel to the outer lip of the shell. There are spiral bands around the base and apex of the shell where the brown lines are absent, and sometimes another spiral band somewhere between the base and the apex. These colourless bands are sometimes bordered with a thin brown line.
	Showing species characteristics...	Photo Norbert Verneau Mayotte, Petit Moya, less 1 m, 25 June 2013, size : shell 15 mm

See more about : Sightening and mating periods

Remarks :

Identification confirmed by Nathalie Yonow
    Synonymous : (according Worms)
          - Bulla cinctoria Perry, 1811                                      - Hydatina circulata (Martyn, 1789)
          - Bulla velum Gmelin, 1791                                        - Hydatina infalata Dunker, 1877
          - Bulla zonata Lightfoot, 1786                                     - Hydatina velum (Gmelin, 1791)
          - Hydatina cinctoria (Perry, 1811)                               - Hydatina vexillum Sowerby, 1867

Bibliographic data :

Like H. physis, the animal varies in colour from translucent white with an opaque white margin to a deep brownish pink.
    Shell identical in shape to that of Hydatina physis.
    The Hydatinidae are the most modified of a group of cephalaspids which show the gradual diminution in the importance of the shell as a protective organ .In Hydatina and Micromelo the shell has become thin and the animal large and brightly coloured.
   The animal, can retract back into their shell but they seldom do. Most other mollusks have a solid shell and a central heavily calcified columella around which the shell, and animal, spirals. There is a huge columella muscle which attaches the animal to the shell and can quickly pull the animal back inside when necessary. In Hydatina, in which the shell is becoming reduced in importance, the columella has become very reduced, as has the associated columellar muscle, so their poor ability to retract back into their shell
   They rely on distasteful acid glands in their skin and burrowing in the sand to protect them.
   The protective operculum has disappeared.
    The egg ribbon of Hydatina is highly convoluted and is attached to the sand by a mucus thread.
It seems that anchoring the ribbon is the last stage in the process and only occurs after all the ribbon is completely exuded from the genital opening. A quite different process than that of nudibranchs and other opisthobranchs which gradually attach the ribbon as it is being exuded.

References :

Bill Rudman Seaslug site : Sea Slug Forum : Hydatina zonata
   Nudipixel Hydatina zonata

Publications :

Lightfoot, J., (1786). A Catalogue of the Portland Museum lately the property of the Dowager Duchess of Portland, Deceased: which will be sold by Auction . 194pp, Plate 1. London.

Other photos of Hydatina zonata :

Norbert Verneau   Mayotte, Petit Moya, less 1 m, 25 June 2013, size : shell 15 mm This specimen burrowing into the sand...

The egg ribbon of Hydatina is highly convoluted and is attached to the sand by a mucus thread (a).

Norbert Verneau Mayotte The shell colour pattern consists of thin, rather irregular brown lines running across the translucent white shell, essentially parallel to the outer lip of the shell. There are spiral bands around the base and apex of the shell where the brown lines are absent, and sometimes another spiral band somewhere between the base and the apex. These colourless bands are sometimes bordered with a thin brown line.

Reunion, Hydatina zonata, by Jean Drivas