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

Order : Sacoglossa Superfamily : Plakobranchoidea Family : Plakobranchidae Distribution : Indo West Pacific Maximal size : 25 mm Abundance : (May) Frequently on the reef flat zone Species characteristics : The animal is dark brown to black with eight to ten white or cream lines along the outer side of the parapodia and on the head The inner margins of the parapodia, the front of the foot and the tips of the rhinophores are bright orange
	Showing species characteristics...	Photo Eva Fontaine Mtsanga guini, Mayotte, on the reef flat zone, 1 m, 29 July 2008, size 18-20 mm

See more about : Sightening and mating periods
   See more about : Thuridilla phylogeny by Gosliner 1995
   See more about : Thuridilla gracilis variability in Southwest Indian ocean

Remarks :

Some authors, (Brodie and Brodie, 1990; Gosliner, 1995) considered T. bayeri "blue form" and T. ratna "ratna form" as synonymous. Others (Marcus, 1965; Carlson and Hoff, 1978; Jensen, 1992) have considered them to be distinct species.
    According Gosliner (1995) about these two form : bayeri and ratna
         - No consistent difference in pattern could be discerned
         - The fact that some specimen are intermediate in coloration between the description for the two species
         - No consistent anatomical differences could be found in branching of the dorsal vessel, morphology of the buccal mass, radula or reproductive system that could be correlated with differences in coloration
- All of these data strongly suggest that the described differences between T. bayeri and T.ratna are simply variation within a single species that is extremely variable in its coloration
- Both species were described in the same paper (Marcus, 1965) but T. bayeri appears first in the text and is, therfore, regarded as the senior synonym
    Some authors considered that the earliest name for this species must be T. gracilis ( Risbec, 1928 ), other considered that the original description of Risbec's under the name Elysia gracilis is insufficient to allow an accurate identification, so they used Marcus 's name Thuridilla ratna and or Thuridilla bayeri...
    Synonymous : (according Worms):
               - Elysia bayeri Er. Marcus, 1965                                - Thuridilla bayeri (Er. Marcus, 1965)
               - Elysia gracilis Risbec, 1928                                     - Thuridilla ratna (Er. Marcus, 1965)
               - Elysia ratna Er. Marcus, 1965              

Bibliographic data :

The spawn mass is tightly coiled and orange. The coils are laid flat against the substrate. Within the spawn are strings of capsules (120-190 µm in diameter), each containing a single ova ( 80-85 µm), around a continious core of yolk
    The behavior of periodically "nodding" the head ( raising and lowering the anterior section of its foot off the substrate) as it crawls is typical of this species.
    The description below corresponded to the "ratna form" of T. gracilis
    In specimen with coloration typical of T. ratna, the animal is dark brown with eight to ten white or cream lines along the outer side of the parapodia and on the head.
    Basally, the rhinophores are the same color as the remainder of the body. In most specimens, some red or orange red pigment may be present on the rhinophores. This may be present as a series of red longitidinal lines or a distinct transverse band. An apical transverse black band may also be present on the rhinophores. A white band under the orange one may also be present on the rhinophores.
    The margin of the anterior end of the foot is red or orange. The posterior junction of the parapodia may contain some orange pigment
    The inside of the parapodia is usually brownish with scattered turquoise blue pigment. A thin, white marginal band is present and a broader, orange submarginal band may be prominent or abscent.
    In some specimens, the white or cream lines widen distally and form a transverse band
The description below corresponded to the "bayeri form" of T. gracilis
    This form have many similarities with "ratna form" by there are some differences...
                The edge of the parapodia may be lined with an orange to red orange line
                Six to seven bright blue, ovoid spots are found submarginally along the length of each parapodia. These spots can be coalescing in some specimens.
                Basally, the rhinophores are the same color as the general body, with cream longitudinal lines. These lines expand and unite into a large transverse white band. This band may continue to the apex of the rhinophores or may be followed by a red band of streaks
                The head may also have a mid dorsal blue mark or paired spots between the rhinophores
                In specimen with heavy blue spotting, the inside of the parapodia has a narrow white marginal band, a broad orange band, and a broader black area. Inside the black band is an area of bright blue pigment
    T. bayeri "blue form" is similar to T. splendens. But T. splendens have yellow spots on the surface of the parapodia.

References :

Bill Rudman Seaslug site : Sea Slug Forum : Thuridilla bayeri
   Nudipixel Thuridilla gracilis

Publications :

Risbec, J. (1928) Contribution a l'etude des nudibranches Neo-Caledoniens . Faune de Colonies Francaises: Paris. 1-328
    Boucher, L.M.(1983). Extra-capsular yolk bodies in the egg masses of some tropical Opistobranchia. Journal of Molluscan Studies 49: 232-241
    Jensen, K.R. (2007). Biogeography of the Sacoglossa (Mollusca, Opisthobranchia). Bonner Zoologische Beiträge. 55: 255–281
    Johnson, S. & Boucher, L.M.(1983). Notes of some Opistobranchia ( Mollusca: Gastropoda) from the Marchall Ilands, including 57new records. Pacific Science . 37(3): 251-291.
    Gosliner, T.M. (1995) The genus Thuridilla (Opisthobranchia: Elysiidae) from the tropical Indo-Pacific, with a revision of ther phylogeny and systematics of the Elysiidae. Proceedings of the California Academy of Sciences, 49(1) : 1-54.
    Marcus, Er. (1965). Some opistobranchian from Micronesia. Malacologia 3(2) : 263-286

Other photos of Thuridilla gracilis :

Different pattern of the rhinophores observed in this species :
16 specimens	14 specimens	6 specimens	3 specimens

Eva Fontaine Mtsanga guini, Mayotte, on the reef flat zone, 1 m, 29 July 2008, size 18-20 mm for the larger specimen. A "ratna" form of T. gracilis In this specimen, some orange red pigment is present on the rhinophores and form a distinct orange-red transverse band. An apical transverse black band is also present on the rhinophores.

Marina Poddubetskaia "Récif de Peïtro", Koungou, Mayotte, 14 m, 15 November 2003, size : 13 mm   In this specimen an apical transverse black band and an extra- white band under the orange one is also present on the rhinophores.

Matthias Deuss Mayotte, Passe en S, buoy n° 12, 10 m, 29 December 2009, size : 15-20 mm T. bayeri "blue form"

Philibert Bidgrain Mayotte, Mtsanga Mtiti Sada, on the reef flat zone, less 1 m,17 July 2010, size : 15 mm About ten specimens crawling on a massive colonie of Porites In this specimen an apical transverse black band and an veru thin extra- white band under the orange one is also present on the rhinophores. Inside of the parapodia : A thin, white marginal band (a) is present and a broader, orange submarginal band (b)

Christophe Mason-Parker Seychelles, Lilot, Mahe, 12 m, 3 September 2010 No orange marking on the rhinophore of this specimen, but with some blue marking like in"bayeri" form Probably an intermediate between "bayeri" and "ratna" form of T.gracilis

Philibert Bidgrain Mayotte, M'Titi Sada, less 1 m, 14 October 2012 A specimen without orange coloration on the rhinophores.

Philibert Bidgrain Mayotte, Tanaraki, less 1 m, 8 August 2012, size : 15-8 mm   A specimen with abnormality on the left parapodia...

Frédéric Lecomte Reunion, Cap la Houssaye, 12 m

See more about : Thuridilla phylogeny by Gosliner 1995

See more about : Thuridilla gracilis variability in Southwest Indian ocean

Mayotte, foot orange corner of Thuridilla bayeri/gracilis, at Sakouli, by Philibert Bidgrain

Mayotte, Thuridilla bayeri/gracilis, at mboueanatsa, by Eva Fontaine 

Mayotte, Thuridilla bayeri/gracilis, by Sylvain Le Bris

Mayotte, important population of Thuridilla bayeri/gracilis, at Tanaraki, by Philibert Bidgrain  

Mayotte, Thuridilla bayeri/gracilis "blue form" with important blue coloration, by Sylvain Le Bris  

Madagascar, important population of Thuridilla bayeri/gracilis, at Nosy Be, by Sully Bachel  

Madagascar, Thuridilla bayeri "blue form", at Nosy Bé, by Alain-Benoît Rassat