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This species has been observed on Reunion, Mauritius, Mayotte and Seychelles Islands

Unassigned Heterobranchia Superfamily : Acteonoidea Family : Aplustridae Distribution : Found circum-globally in tropical waters both in the Indo-West Pacific and the Atlantic. Maximal size : Shell 45 mm Abundance : Seldom on the rocky coast Species characteristics : The shell has numerous and unequal spiral brown or black lines The animal is quite variable in its colour shading. It can range from this very dark (pink or red) form to a pale pinkish with an opaque white or blue marginal line around the foot. The front of the head shield is developed into a pair of large and lobe-like tentacles on each side.
	Showing species characteristics...	Photo Philibert Bidgrain Réunion, Etang salé on the rocky coast, less 1 m, 2 December 2010, shell size : 20-22 mm

See more about : Sightening and mating periods

Remarks :

Identification confirmed by Nathalie Yonow
    Synonymous : (according Worms)
          - Aplustrum virgatum Mörch, 1852                                      - Bulla quoyana d'orbigny, 1845
          - Bulla atrolineata Schröter, 1804                                       - Bulla staminea Menke, 1853
          - Bulla physis Linnaeus, 1758                                             - Hydatina filosa Schumacher, 1817

Bibliographic data :

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.
    It feeds on Cirratulinae, a subfamily of the Cirratulinidae polychaete worms..
The cirratulids have no head appendages but have long thread-like tentacles along most of the body but especially around the head. They live in crevices, empty burrows or under rocks partially embedded in the sand or mud. They live surrounded by a "coccoon'" of debris, and they extend their tentacles out like the head tentacles of terebellid worms, to collect food particles.
    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.
    Often found in shallow waters, usually not deeper than 5 m, crawling over or through sandy substrate. It inhabits sands flats and rock pools

Material examined by Nathalie Yonow ( see reference/publications).

Two specimens collected from Flic en Flac, 15 October 1985, shell size : 23x18 and 22x17 mm. And one from Grand Baie, 2 November 1985, shell size : 33x23 mm

References :

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

Publications :

Rudman, W.B., 1972. The anatomy of the opisthobranch genus Hydatina and the functioning of the mantle cavity and alimentary canal. Zoological Journal of the Linnean Society, 51: 121-139.
    Yonow, N. and Hayward , P. J. (1991). Opistobranches de l'île Maurice, avec la description de deux espèces nouvelles (Mollusca : Opistobranchia) Revue française d'aquariologie herpétologie, 18 (1), 1-30

Other photos of Hydatina physis :

Christophe Cadet Reunion, Etang salé les bains on the rocky coast, less 1 m,18 December 2015, size : 25 mm The mantle cavity with the gills (a) faces the right side of the animal

Christophe Cadet Reunion, Etang salé les bains on the rocky coast, less 1 m, 26 January 2016, size : 25 mm During the night.   With its spawn... 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.

Christophe Cadet Reunion, Etang salé les bains on the rocky coast, less 1 m, 28 October 2008, during the night. A mating behavior...

Christophe Cadet Reunion, Etang salé les bains on the rocky coast, less 1 m, 12 December 2009, size : 20 mm. 4 specimens observed on this day...

Maurice Jay   Reunion, In this specimen, you can observe filamentous green algae on the shell. We have found specimens with a similar crop on its shell in two other species in Reunion Island. In Haminoea cymbalum and in Micromelo undata

Hugues Flodrops Reunion, Etang salé les bains on the rocky coast, less 1 m, 9 December 2007, size : 15 mm.

Hugues Flodrops Reunion, "Grande Anse pool", less 1m, 11 January 2008, during the night, shell size : 15 mm

Mauritius, drawing of a specimen of Hydatina physis, from Grand Baie, by Nathalie Yonow

Mayotte, Hydatina physis, at Petite Terre, by Norbert Verneau