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Does the seagrass Posidonia really occur in Madagascar?

In a recent (March) issue of Phycologia (47: 203–212), West et al. (2008) described a new species of Rhodophyta, Rhodachlya madagascarensis J.A. West, J.L. Scott, K.A. West, U. Karsten, S.L. Clayden & G.W. Saunders, that also constituted a new genus, family and order. According to West et al. (2008), Rhodachlya madagascarensis is a small epiphyte growing on ‘Posidonia blades’ near Ifaty, Madagascar (Indian Ocean) (Abstract: p. 203; Material and methods, p. 204; Latin diagnosis, p. 205 and Acknowledgements, p. 210). Although they do not mention the authorship of Posidonia, it is clear that they are referring to the plant Posidonia Konig in Konig & Sims (1805), not the mollusc Posidonia Bronn 1828, the legitimate name of a genus of bivalves under the International Code of Zoological Nomenclature, that is widespread in the Carboniferous (e.g. Amler & Winkler Prins 1999). Posidonia Konig in Konig & Sims (Magnoliophyta, Posidoniaceae) is a genus of seagrasses with a clearcut antitropical pattern of distribution, which encompasses nine species: Posidonia oceanica (Linnaeus) Delile, which is endemic to the Mediterranean Sea, and eight species endemic to Australia: P. angustifolia Cambridge & Kuo, P. australis J.D. Hooker, P. coriacea Cambridge & Kuo, P. denhartogii Kuo & Cambridge, P. kirkmanii Kuo & Cambridge, P. ostenfeldii den Hartog, P. robertsoniae Kuo & Cambridge and P. sinuosa Cambridge & Kuo (Hartog 1970; Kuo et al. 1990; Kuo & Hartog 2001; Hartog & Kuo 2006). According to Campey et al. (2000) and Spalding et al. (2003), P. robertsoniae may be a synonym of P. coriacea. The eight Australian species are all found in Western Australia. P. angustifolia, P. coriacea, P. denhartogii and P. sinuosa are additionally found in South Australia, and P. australis occurs in New South Wales, South Australia, Tasmania, Victoria and Western Australia (van Keulen 2008). All these species are probably descended from the Tethyan fossil species P. cretacea Hosius and von der Mark, and/or P. perforata de Saporta & Marion and P. parisiensis (Brongniart) Fritel (Larkum & Hartog 1989; Hartog & Kuo 2006). Although P. oceanica is restricted to the Mediterranean Sea (Hartog & Kuo 2006), some overseas records can be found in the literature. All of them have proved erroneous. The earliest and most amusing is the specific name of the species; Linnaeus, who first named it (as Zostera oceanica), probably inverted two parcels received from the Mediterranean and the Atlantic Ocean, respectively, and therefore named it ‘oceanica’ (i.e. from the Atlantic Ocean). Further records from the Bay of Biscay (Atlantic, France) (e.g. Sauvageau 1890) actually concern large-leaved forms of Zostera marina Linnaeus. More surprising was the inclusion of Posidonia oceanica within the Texas (Gulf of Mexico, USA) flora by Correll & Johnston (1970: 95), subsequently illustrated by Correll & Correll (1972: 129– 130) in their authoritative flora of aquatic and wetland plants of southwestern United States [as Posidonia oceania (sic)]. McMillan et al. (1975) established that it was a gross misidentification, the specimens in fact belonging to Thalassia testudinum Banks ex Konig in Konig & Sims. Finally, some records of P. oceanica, which concern the Red Sea and the northern Indian Ocean (e.g. Makkaveeva 1968) should actually refer to Thalassia hemprichii (Ehrenberg) Ascherson in Petermann, a seagrass widely distributed throughout the Indian Ocean, including Madagascar, and often associated with mangrove habitats (Hartog 1970). According to the present day knowledge, the genus Posidonia is restricted to temperate seas. The occurrence of a species of this genus in the mangroves around Ifaty, Madagascar, in the intertropical zone, is, therefore, highly unlikely, though not impossible. If this record is confirmed, it would constitute a new and disjointed distributional record for the genus. We suspect, based on gross morphological similarities, that the host was more likely to be Thalassia hemprichii or Enhalus acoroides in the Hydrocharitaceae (see Spalding et al. 2003).