Fifty shades of white: morphological and molecular diversity of the Cadlina laevis species complex (Gastropoda: Nudibranchia) in the North-West Pacific

Abstract

We provide a morphological examination and a barcoding study to investigate the species identity and variation limits within the Cadlina laevis species complex. Our molecular analysis based on the COI marker revealed seven new clades in the North-West Pacific Cadlina diversity. The distances between these clades are low in some cases (2.08-7.51% overall), and the species delimitation tests gave controversial results (1, 2, 13, 14 groups, depending on the analysis method). This does not allow to conclusively classify this diversity as interspecific or intraspecific. Morphological analysis showed a significant similarity of all examined groups, with minor differences found in the morphology of the central tooth of the radula and the reproductive system. However, these variations fit into the morphological variability of the North Atlantic species Cadlina laevis s.str. and cannot serve as evidence of the isolation of these identified groups. The discovered diversity may represent both a complex of at least 11 very close and cryptic species with not well-established species boundaries, or be a part of a single amphiboreal species Cadlina laevis s.l. This indicates an extremely complex evolutionary history of Cadlina laevis species complex, making this group is an interesting model object for studying speciation in boreal and Arctic communities.