Does the Seagrass Posidonia Really Occur in Madagascar?

Phycologia (2008) Volume 47 (4), 435–436 Published 3 July 2008

COMMENTARY

Does the seagrass Posidonia really occur in Madagascar?

CHARLES F. BOUDOURESQUE AND MARC VERLAQUE Center of Oceanology of Marseilles, University of the Mediterranean, CNRS, UMR6540, Campus of Luminy, case 901, 13288 Marseilles cedex 9, France

C.F. BOUDOURESQUE AND M. VERLAQUE. 2008. Does the seagrass Posidonia really occur in Madagascar? Phycologia 77: 435–436. DOI: 10.2216/08-38.1

In a recent (March) issue of Phycologia (47: 203–212), West named it ‘oceanica’ (i.e. from the Atlantic Ocean). Further et al. (2008) described a new species of Rhodophyta, records from the Bay of Biscay (Atlantic, France) (e.g. Rhodachlya madagascarensis J.A. West, J.L. Scott, K.A. Sauvageau 1890) actually concern large-leaved forms of West, U. Karsten, S.L. Clayden & G.W. Saunders, that also Zostera marina Linnaeus. More surprising was the constituted a new genus, family and order. According to inclusion of Posidonia oceanica within the Texas (Gulf of West et al. (2008), Rhodachlya madagascarensis is a small Mexico, USA) flora by Correll & Johnston (1970: 95), epiphyte growing on ‘Posidonia blades’ near Ifaty, Mada- subsequently illustrated by Correll & Correll (1972: 129– gascar (Indian Ocean) (Abstract: p. 203; Material and 130) in their authoritative flora of aquatic and wetland methods, p. 204; Latin diagnosis, p. 205 and Acknowledge- plants of southwestern United States [as Posidonia oceania ments, p. 210). Although they do not mention the (sic)]. McMillan et al. (1975) established that it was a gross authorship of Posidonia, it is clear that they are referring misidentification, the specimens in fact belonging to to the plant Posidonia Ko¨nig in Ko¨nig & Sims (1805), not Thalassia testudinum Banks ex Ko¨nig in Ko¨nig & Sims. the mollusc Posidonia Bronn 1828, the legitimate name of a Finally, some records of P. oceanica, which concern the Red genus of bivalves under the International Code of Zoolog- Sea and the northern Indian Ocean (e.g. Makkaveeva 1968) ical Nomenclature, that is widespread in the Carboniferous should actually refer to Thalassia hemprichii (Ehrenberg) (e.g. Amler & Winkler Prins 1999). Ascherson in Petermann, a seagrass widely distributed Posidonia Ko¨nig in Ko¨nig & Sims (Magnoliophyta, throughout the Indian Ocean, including Madagascar, and Posidoniaceae) is a genus of seagrasses with a clearcut often associated with mangrove habitats (Hartog 1970). antitropical pattern of distribution, which encompasses nine According to the present day knowledge, the genus species: Posidonia oceanica (Linnaeus) Delile, which is Posidonia is restricted to temperate seas. The occurrence of endemic to the Mediterranean Sea, and eight species a species of this genus in the mangroves around Ifaty, endemic to Australia: P. angustifolia Cambridge & Kuo, Madagascar, in the intertropical zone, is, therefore, highly P. australis J.D. Hooker, P. coriacea Cambridge & Kuo, P. unlikely, though not impossible. If this record is confirmed, denhartogii Kuo & Cambridge, P. kirkmanii Kuo & Cam- it would constitute a new and disjointed distributional bridge, P. ostenfeldii den Hartog, P. robertsoniae Kuo & record for the genus. We suspect, based on gross morphological similarities, that the host was more likely Cambridge and P. sinuosa Cambridge & Kuo (Hartog 1970; to be Thalassia hemprichii or Enhalus acoroides in the Kuo et al. 1990; Kuo & Hartog 2001; Hartog & Kuo 2006). Hydrocharitaceae (see Spalding et al. 2003). 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 REFERENCES additionally found in South Australia, and P. australis occurs in New South Wales, South Australia, Tasmania, AMLER M.R.W. & WINKLER PRINS C.F. 1999. Carboniferous Victoria and Western Australia (van Keulen 2008). All these bivalves of the Cantabrian Mountains. Scripta Geologica 120: 1–45. species are probably descended from the Tethyan fossil BRONN H.G. 1828. Posidonia becheri, eine neue fossile Muschel der species P. cretacea Hosius and von der Mark, and/or P. Uebergangs-Periode. Zeitschrift oder Taschenbuch fu¨r Miner- perforata de Saporta & Marion and P. parisiensis (Brong- alogie von Leonhard 1: 262–269. niart) Fritel (Larkum & Hartog 1989; Hartog & Kuo 2006). CAMPEY M.L., WAYCOTT M. & KENDRICK G.A. 2000. Re- evaluating species boundaries among members of the Posidonia Although P. oceanica is restricted to the Mediterranean ostenfeldii species complex (Posidoniaceae) – morphological and Sea (Hartog & Kuo 2006), some overseas records can be genetic variation. Aquatic Botany 66: 41–56. found in the literature. All of them have proved erroneous. CORRELL D.S. & CORREL H.B. 1972. Aquatic and wetland plants of The earliest and most amusing is the specific name of the southwestern United States. Stanford University Press, Stanford, CA, USA. 1777 pp. species; Linnaeus, who first named it (as Zostera oceanica), CORRELL D.S. & JOHNSTON M.C. 1970. Manual of the vascular probably inverted two parcels received from the Mediter- plants of Texas. Texas Research Foundation publications, ranean and the Atlantic Ocean, respectively, and therefore Renner, TX, USA. 1881 pp.

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