Morphology, genetics, and historical records support the synonymy of two ascidian species and suggest their spread throughout areas of the Southern Hemisphere

Abstract

Taxonomic uncertainties and the lack of ecological knowledge can hinder the correct identification and the assignment of biogeographic status of marine species. The ascidian Asterocarpa humilis (Heller, 1878), originally described from New Zealand, has a broad distribution in shallow temperate areas of the Southern Hemisphere, having recently colonised areas of the Northern Hemisphere. A closely related species, Cnemidocarpa robinsoni Hartmeyer, 1916, has been reported in the South-Eastern Pacific and the South-Western Atlantic, and several authors considered it a junior synonym of A. humilis. We gathered for the first time morphological and genetic data from specimens from distant areas. We studied the morphology of specimens collected at seven locations of South America. We also re-examined specimens from museum collections and revised the available literature on these species. Genetic data were obtained from specimens from Argentina and compared with available sequences of A. humilis from Chile, New Zealand, England and France. Morphological and genetic analyses showed that all compared specimens were conspecific. Furthermore, specimens from different continents shared haplotypes and exhibited low genetic distance among them. These results, the biological characteristics of this ascidian, and its longstanding presence in different habitats from disjoint areas, allow us to question its native range. We support the idea that A. humilis is a cryptogenic and neocosmopolitan species that has been transported by maritime traffic through the Southern Hemisphere, revealing frequent processes of exchange through this wide area for more than a century, with presumably associated alterations in the marine biota.