The aquatic plant genus Ruppia (Ruppiaceae) comprises eight species mainly in coastal brackish areas of the world. While the known taxa of Ruppia thus far generally had either four- or eight-carpelled flowers, our recent Ruppia collection from Western Cape, South Africa showed flowers with only two carpels. This characteristic morphological evidence, together with elongated coiled peduncles, implied either: i) extensive morphological variation of the cosmopolitan R. cirrhosa; or ii) the occurrence of a new species in the genus. We tested these alternative hypotheses of the bicarpellate Ruppia taxon in a phylogenetic framework. Sequence data from four plastid DNA regions and nuclear phyB were analyzed using maximum parsimony, maximum likelihood, and Bayesian inference. We obtained moderately to highly resolved phytogenies with both data sets. The collection from Western Cape showed unique DNA sequences which were, in both plastid and nuclear phylogenetic trees, placed as sister to the rest of the genus and thus clearly rejected the first hypothesis. Given the distinctive phylogenetic position and the unique morphological evidence, here we describe Ruppia bicarpa, a new species from Western Cape, South Africa. The other two Ruppia populations from the region are classified or categorized into either a tetraploid or an apparent hexaploid entity of the R. maritima complex.
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