The Acrotylaceae (Gigartinales) revisited: molecular data indicate family-level differences in one of the most enigmatic red-algal families

Abstract. The Acrotylaceae (Gigartinales) has been an obscure, puzzling red-algal family composed of six genera (Acrotylus, Amphiplexia, Antrocentrum, Claviclonium, Hennedya and Ranavalona) that have virtually no habit and vegetative features remotely in common. Excepting Amphiplexia, with two species, all are monotypic and, save for Ranavalona, endemic to the southern half of Australia, Ranavalona being known only by a single collection from southern Madagascar. The most noteworthy unifying feature of the genera is the cystocarps, in which carposporangial filaments grow into a hollow cavity from inner hull layers. In almost all other respects, apart from zonate tetrasporangia, there are no anatomical features that appear uniformly throughout the complex. Molecular studies indicate that the family is composed of the Acrotylaceae sensu stricto (for Acrotylus, Hennedya and Ranavalona) and the newly proposed Clavicloniaceae (for Claviclonium, Amphiplexia and Antrocentrum). We recognise a new species of Acrotylus (A. cryptographicus) and transfer Rhabdonia mollis Harvey and R. hamata Zanardini to Antrocentrum. Closest affinities of both families lie with the Dicranemataceae, Mychodeaceae and Mychodeophyllaceae of the Gigartinales. The South African genus Reinboldia, which is based on a single 19th century collection, has been questionably included in the Acrotylaceae previously but should, in our opinion, be excluded.