The power of combined molecular and morphological analyses for the genus <i>Botrylloides</i>: identification of a potentially global invasive ascidian and description of a new species

Botryllids are colonial styelid ascidians for which morphological characters distinguishing genera and species are often difficult to identify and frequently ambiguous. <i>Botrylloides perspicuus</i>, <i>Botrylloides giganteus</i>, and <i>Botrylloides pizoni</i> have similar colony and zooid appearances and over recent years have been found in dispersed localities around the world. In order to determine how many valid <i>Botrylloides</i> species there are, and how they are geographically distributed, we performed both molecular and morphological analyses on fresh and archival museum specimens of these <i>Botrylloides</i> coming from six distant localities: southern Italy, southern California, Brazil, Galapagos Islands, New Zealand, and Australia. Moreover, we re-analysed the morphology of a number of formalin-preserved museum specimens. We concluded that both <i>B. perspicuus</i> and <i>B. giganteus</i> are indeed valid species and report morphological and molecular characters that distinguish these species; <i>B. pizoni</i> is concluded to be a junior synonym of <i>B. giganteus</i>. The geographic distribution, collection dates, and growth substrates of <i>B. giganteus</i> indicate that this species is rapidly spreading worldwide and has traits that may enhance invasiveness (i.e., presence in intertidal regions; high tolerance to air exposure, temperature and salinity variation). Therefore, <i>B. giganteus</i> should be a focus of both researchers and managers concerned with marine biosecurity. Interestingly, the morphological re-analyses of archival museum samples guided by molecular results, led to the taxonomic reassignment of several botryllid specimens as well as to the identification of a new species, <i>Botrylloides conchyliatus</i> sp. nov., which is here described. http://www.zoobank.org/urn:lsid:zoobank.org:pub:0D896E42-F460-45E2-B74C-576A011D9214