5. Conclusions
This work provides new insights to understand the evolution of molecules and morphology at the same time. The evidence presented here allowed to delimitate two species within C. verrucosa : two monophyletic groups were obtained; congruent mitochondrial, nuclear species delimitation was demonstrated. The magnitude of difference between the two distinguished species was similar to those among undisputed species pairs, and both species were distributed in sympatry. Morphological analysis suggests that the presence of a basal disc could be a morphological feature distinguishing the species. Nominal C. verrucosa sensu lato are distributed all around Antarctica and in the south of South America. However, the true extent of the species distribution area is obscured by the existence of two, possibly more species that have up until now been mistaken as a single species. Incongruent mitochondrial-nuclear phylogenies might be explained by different hypotheses as introgression/hybridization or ILS. Boundaries between sympatric species are maintained by barriers to gene flow, these restrictions may not be uniform in space, time or across the genome. In fact, it has been proposed that these barriers are semipermeable and speciation under gene flow is possible (Nosil, 2008). Further analysis employing several nuclear loci and an extended geographical sampling, would help to elucidate the evolutionary story of this broadly distributed Antarctic ascidian species. Patterns regarding genetic and morphological differentiation that are being underestimated or not registered systematically, can lead to important misunderstanding of species distribution patterns related to adaptation, habitat preference, competition and response to climate change. Our results once again emphasize that species identities, even for highly abundant and well-studied species on small local scales, must be assessed rather than assumed.
6. List of abbreviations
ABGD: Automatic barcode gap
APF: Antarctic Polar Front
ILS: Incomplete lineage sorting
ACC: Antarctic circumpolar current
MYA: Million years ago
7. Acknowledgements
The authors want to particularly thank the members of the Carlini (former Jubany) - Dallmann staff and Andrea Eschbach for kind assistance in laboratory work. Logistic and financial support was provided by Instituto Antártico Argentino (IAA), Alfred Wegener Institute (AWI), CONICET, IMCONet [FP7 IRSES (International Research Staff Exchange Scheme), action no. 319718], Short Term Grant 2019 Scholarship of Deutscher Akademischer Austauschdienst [German Academic Exchange Service (DAAD) Reference number: 91701637], and Lerner-Gray Memorial Fund of the American Museum of Natural History.  The authors are grateful to the crew of the R/V “Puerto Deseado” and Gisela Morán for help in sampling. This work was partially supported by the Área Marina Protegida Namuncurá (Ley 26.875). Particular thank to Marcos Tatián who provided scientific support and guidance in the morphological analysis, and Cristian Lagger who provided the underwater photos.