The genus Melanthalia (Gracilariales, Rhodophyta): new insights from New Caledonia and New Zealand

The Southern Hemisphere red algal genus Melanthalia (Gracilariales) was investigated as part of a study comparing shared elements in the floras of northern New Zealand and southern New Caledonia. Molecular sequence data were obtained from specimens that were collected from throughout New Caledonia and New Zealand and that encompassed the range of morphological variation. Before this study two species were recognised from New Caledonia and a single species from New Zealand. Our results supported the recognition of a single well-delimited monophyletic species that exhibited extreme phenotypic plasticity in New Caledonia, whereas in samples from New Zealand three lineages were identified, with two monophyletic and a third lineage, currently known from a single specimen. Between the New Zealand lineages there was only modest morphological variation but there was genetic differentiation between northern and southern populations. A single species, M. abscissa, was recognised in New Zealand pending further research to clarify species boundaries and enable description of new taxa. On the basis of sequence data, Melanthalia in New Zealand was distinct from Australian species, with a closer relationship shown between New Zealand and New Caledonia than between either of these and Australia. The two New Caledonian species were synonymised, with M. vieillardii having priority.