Embletonia gracilis
Risbec, 1928

Order: NUDIBRANCHIA
Suborder: AEOLIDINA
Family: Embletoniidae

PHOTO

UPPER: AM C127466, 14 February 1981, under rock off manager's house, Christmas Is., Indian Ocean. Photo: John Hicks. LOWER: Radular ribbon. [see further photo] Photo: Geoff Avern.

Embletonia gracilis has a wide Indo-West Pacific distribution and although published accounts suggest a small animal less than 8mm long, it can grow to 20-30mm. This translucent white nudibranch is characterised by the single row of cerata down each side of the body, the tip of each ceras usually being divided into four, and containing nematocysts. Oral tentacles are replaced by flattened oral lobes flanking the mouth [they are often described as a bilobed oral veil]. Embletoniids are small and often very elongate.

Generally the family has been considered to be aeolidinan, probably closely related to the Tergipedidae. Most authors accept a single genus, Embletonia. Miller & Willan (1991) consider this small group of three, possibly four, species to be dendronotinans, giving four reasons for considering Embletonia to be dendronotinan: presence of dendronotinan oral veil; lack of oral tentacles; three-lobed oral gland; and lack of cnidosacs. Of these characters, the first two are really aspects of one feature. Either the head region is a dendronotinan oral veil, but lacking the characteristic ventral groove along each side, or it is a tergipedid head lacking oral tentacles. Miller & Willan fail to explain the lack of sheathed rhinophores which are characteristic of all other dendronotinans.

The three-lobed oral gland closely resembles the elaborate oral gland developed in the tergipedids, especially the coral-feeding genus Phestilla. Similarly the nature of the cnidosac is open to interpretation. Although Marcus & Marcus (1958) deny the presence of nematocysts in Embletonia, both Baba & Hamatani (1963) and W.B. Rudman (personal observation) have observed sacs containing nematocysts at the ceratal tips. In the European Embletonia pulchra, the tips of the cerata are rounded and, although a cnidosac is lacking, there are up to four terminal pads of nematocysts on each ceras. In the Indo-West Pacific species, the end of each ceras is divided twice, forming four knobs, each have a terminal sac containing nematocysts.

In all species the anus opens between the second and third cerata on the right and has been variously described as pleuroproct or acleioproct. The rhinophores are simple. Oral glands are present and the single median tooth in the uniscriate radula has a central pointed cusp flanked by a few denticles. The cutting edge of the jaw plates is denticulate. Most reports suggest they feed on hydroids.

References:
• Baba, K., 1959b. A new record of an interesting species, Embletonia gracile Risbec, from Japan (Nudibranchia -Eolidacea). Publications of the Seto Marine Biological Laboratory, 7(3): 335-336, Pl. 29.
• Baba, K. & Hamatani, I., 1963. Anatomy of Embletonia gracilis paucipapillata n. ssp. from Osaka Bay, Japan (Nudibranchia - Eolidoidea). Publications of the Seto Marine Biological Laboratory, 11(2): 399-402, Pl. 17.
• Gosliner, T.M., 1979. The systematics of the Aeolidiacea (Nudibranchia: Mollusca) of the Hawaiian Islands, with descriptions of two new species. Pacific Science, 33(1): 37-77.
• Gosliner, T.M. & Griffiths, R.J., 1981. Description and Revision of some South African aeolidacean Nudibranchia (Mollusca, Gastropoda). Annals of the South African Museum, 84(2): 105-150.
• Miller, M.C. & Willan, R.C., 1991. Redescription of Embletonia gracile Risbec, 1928 (Nudibranchia: Embletoniidae): Relocation to Suborder Dendronotacea with taxonomic and phylogenetic implications. Journal of Molluscan Studies, 58: 1-12, Figs 1-8.
• Risbec, J., 1928a. Contribution à l'étude des nudibranches Néo-Calédoniens. Faune des Colonies Françaises, 2: 1-328.
• Risbec, J., 1953. Mollusques nudibranches de la Nouvelle-Calédonie. Faune Union Française, 15 : 1-189.
• Rudman, W.B., 1998. Suborder Aeolidina. Pp 1011-1017. In: Beesley, P.L., Ross, G.J.B. & Wells, A. (eds). Mollusca: The Southern Synthesis. Fauna of Australia, Vol 5. CSIRO Publishing: Melbourne. Part B, 565-1234.

Authorship details
Rudman, W.B., 2002 (September 11) Embletonia gracilis Risbec, 1928. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/emblgrac

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