Molecular data suggest the worldwide introduction of the bryozoan Amathia verticillata (Ctenostomata, Vesiculariidae)

The arborescent bryozoan Amathia verticillata (Delle Chiaje, 1822) can be found in both tropical and temperate waters but little is known on how natural dispersal and human-mediated transport mediates its current widespread distribution. The species can rapidly cover and dominate large areas of natural and artificial substrates, and infests port areas, ships, pipelines, and industrial marine installations. In addition, A. verticillata could be a complex of cryptic species, similar to many other bryozoan species with presumed cosmopolitan distributions. In this study, we tested whether A. verticillata represents a complex of cryptic species or it is a single species, and assessed whether natural and/or anthropogenic dispersal can explain its cosmopolitan distribution. We performed phylogenetic and population genetic analyses based on two mitochondrial genes (16S and COI) from samples collected in the United States, Brazil, Spain, Australia, and Mexico. Our results show a low genetic structure between Atlantic and Pacific oceans, with one haplotype of both genes widely distributed, while the other haplotypes (all with very low nucleotides diversity) were specific to each region. This suggests that A. verticillata is a single species, whose broad distribution seems to have been mediated by humans; the biology of the species in addition to its common occurrence near port areas also indicate this fact. However, to elucidate its origin and dispersal routes, a more extensive sampling effort and the usage of the genome-wide molecular markers should be explored. Until these studies are carried out, we recommend that the species should still be considered as cryptogenic.