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This species has been observed on Reunion, Mauritius and Madagascar 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 : 30 mm Abundance : Frequently on the rocky coast Species characteristics : Shell with undulating brown lines crossed with two or three other spiral lines. Bluish green animal with opaque white spots and yellow or pale yellow border with sometimes a blue submarginal band.
	Showing species characteristics...	Photo Philibert Bidgrain Reunion, Etang salé les bains rocky coast (8 November 2005) Size : 10-15 mm

See more about : Sightening and mating periods
   See more about : Micromelo undatus variability in Southwest Indian ocean
   See more about : Micromelo undatus captures and eats its prey : a worm ...
   See more about : Micromelon undatus with a strange black excrescence

Remark :

Identification confirmed by Bill Rudman
    There is some debate about Micromelo species. For some people, the genus Micromelo has only one biological species which is widespread in the tropical Pacific and Atlantic Oceans. For other people, the genus Micromelo has two species. M. guamensis has been considered an Indo-West Pacific species while M. undatus has been a Caribbean species, but externally there seems to be no difference between the Atlantic M. undatus and the Indo-West Pacific M. guamensis
    Synonymous : (according Worms)
          - Bulla elegans Menke, 1830                                      - Bulla undata Bruguière, 1792
          - Bulla nitidula Dillwyn, 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. The protective operculum has disappeared.
    Like H. physis and H. amplustre, this species feeds upon cirratulid 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.
    It is common in rocky pools in the lower intertidal zone

References :

Bill Rudman Seaslug site : Sea Slug Forum : Micromelo undatus
   Nudipixel Micromelo undatus

Publications :

Rudman, W.B., 1972. Studies on the primitive opisthobranch genera Bullina Ferussac and Micromelo Pilsbry. Zoological Journal of the Linnean Society, 51 : 105-119.
    Willan, R.C. 1984. The Guam Bubble Shell Micromelo undatus (Bruguiere, 1792) in Australia. Shells and Sea Life 16(5): 49-50.

Other photos of Micromelo undatus :

Philibert Bidgrain   Reunion, Etang salé les bains rocky coast (9 November 2005), Size : 10-15 mm In this specimen, you can observe a pale yellow border with a pale blue submarginal band. Shell with undulating brown lines crossed with three other spiral lines.

Philibert Bidgrain A collective spawning behavior Reunion, Etang salé les Bains rocky coast, 30 October 2006, 17h. I observed on a rock about 30 animals in 1 m2. In small anfractuosity's there are approximatively 10 animals burrowing among sediments and spawns... You can observe, some small yellowish spawns

A yellowish (in natural light) helicoidal spawn, observed with magnifying-glass (X20). Spawn size : 6 x 4 mm   Fragment of the gelatinous spawn ( X200). Six days after the "spawning day" There is only one egg in each capsule.

Christophe Cadet Reunion, Etang salé les bains, on the rocky coast, less 1 m, 2 September 2009. On the right side , under the shell, you cann observed the reproductive system.

Philibert Bidgrain Reunion, Etang salé les bains, on the rocky coast, less 1 m, 27 January 2010, size : 20 mm. A specimen with numerous undulating brown lines (a)

and its spawn (b)...

Christophe Cadet Reunion, Etang salé les bains, on the rocky coast, less 1 m, 12 February 2010, size : 15 mm. The shell, covered with algae (a) ...

Alain-Benoît Rassat Madagascar, Nosy Bé, Sakatia, 15m, 19 December 2015, size : 20 mm   Without yellow marginal band...

See more about : Micromelo undatus variability in Southwest Indian ocean

See more about : Micromelo undatus captures and eats its prey : a worm

See more about : Micromelon undatus with a strange black excrescence

Réunion, Micromelo undatus, at Etang salé, by Philibert Bidgrain

Reunion, posterior view of Micromelo undatus, at Etang salé, by Christophe Cadet 

Mauritius, Micromelo undatus, at Mahebourg, by Yann von Arnim