The family Zosteraceae in Australia and New Zealand currently comprises the monotypic Heterozostera (H. tasmanica) and four Zostera species (Z. capricorni, Z. muelleri, Z. mucronata, Z. novazealandica) all in subgenus Zosterella. Species recognition in Zostera has always been difficult in this region due to the use of taxonomic characters that exhibit extensive intraspecific variability. Recent anatomical, developmental and molecular evidence raises further questions on generic delimitation (notably the distinctness of Heterozostera) and subgeneric limits. To clarify the taxonomy of Zosteraceae, we have undertaken an interspecific phylogenetic investigation that focusus on Australian species of subgenus Zosterella. We have examined material (including multiple accessions of many taxa) comprising all Zosteraceae genera (Heterozostera, Phyllospadix, Zostera), 6/7 species of Zostera subgenus Zosterella (including all Australian/New Zealand species), and one of four species of Zostera subgenus Zostera. Analyses of morphological characters and DNA sequences from nuclear (ITS) and plastid (trnK) genomes were used to construct phylogenetic trees of the Zosteraceae, from which taxonomic relationships might better be ascertained. Our results conclusively show 1) the nesting of "Heterozostera" within the genus Zostera; 2) the lack of molecular divergence among Australian/New Zealand Zostera collections formerly segregated as four distinct species; and 3) the lack of reliable morphological characters to separate these four species. We recommend the abandonment of the genus Heterozostera (while reinstating Zostera tasmanica), and the taxonomic merger of remaining Australian/New Zealand Zostera within a single species Z. capricorni which has priority of publication.

Key words: Heterozostera tasmanica, Zostera capricorni, Zostera mucronata, Zostera muelleri, Zostera novazealandica, Zosteraceae