Traveling around the Mediterranean Sea: the bryozoan Amathia verticillata (Delle Chiaje, 1822) together with non-indigenous and endemic overlooked nudibranchs (Mollusca, Gastropoda)

The Mediterranean Sea is recognized as a hot spot of biodiversity, but the ever-increasing commercial and recreational marine traffic and global warming pose significant threats to its fauna by enabling and facilitating, respectively, the introduction of non-indigenous species (NIS). To mitigate negative effects in the native communities, greater efforts are being made to generate updated lists of NIS and to monitor their spread over space and time. In this regard, Amathia verticillata (delle Chiaje, 1822), a worldwide distributed arborescent bryozoan, can play an important role as a vector of NIS due to its ability to be a species ‘carrier of other species’. Despite its importance, a detailed study on the highly specialized Nudibranchia molluscs associated with A. verticillata is still lacking. Given that some nudibranchs serve as powerful bioindicators of changes in the Mediterranean Sea, and considering that A. verticillata is an invasive species capable to facilitating the establishment and spread of NIS, a broad bibliographic study coupled with field investigations in different anthropized areas of the Central Mediterranean Sea were conducted. The nudibranch fauna associated with A. verticillata in the Mediterranean Sea was assessed for the first time. Nine species were found associated with this bryozoan, five of which were directly observed and analysed using morphological and molecular techniques. A case of cryptic diversity was revealed, with Polycerella recondita Schmekel, 1965 resurrected as a valid species, and the neglected Tenellia granosa (Schmekel, 1966) reported for the fourth time in the Mediterranean Sea. Notably, only three out of the nine species are Indo-Pacific, while the remaining six taxa have an Atlantic/Mediterranean distribution. This finding prompted further phylogenetic and haplotype network analyses on the two nudibranch species directly associated with A. verticillata: Bermudella polycerelloides and P. recondita. These analyses revealed different and previously unknown ecological traits of the spread of these two species.