Is the Mediterranean nudibranch Cratena peregrina (Gmelin, 1791) present on the Brazilian coast?: integrative species delimitation and description of Cratena minor n. sp.

Abstract

One of the main difficulties in the taxonomy of heterobranch sea slugs is the interpretation of small morphological and body colour differences in a group of specimens, sympatric or allopatric, as variation of a single species or indicative of similar, but different, species. The aeolid Cratena peregrina is one of the most common and typical nudibranchs from the Mediterranean Sea and was recently informally recorded from Senegal, South Africa, India and in the western Atlantic. In the present work, we investigate the potential presence of C. peregrina on the coast of Brazil. Brazilian and Mediterranean specimens are compared through multiple approaches, including (1) a molecular phylogenetic analysis based on a mitochondrial and a nuclear marker (cytochrome c oxidase subunit I and H3, respectively); (2) performing population analyses such as haplotype networks via TCS and Birky’s coalescence-based K/u ratio; (3) automatic barcode gap discovery and (4) comparative morphological study. As a result of our integrative species delimitation approach, we conclude that the morphological and body colour differences observed between Mediterranean and Brazilian specimens are not due to intraspecific variation in C. peregrina and that C. peregrina is not present in Brazil. Instead, Brazilian specimens belong to a new species, C. minor n. sp., which is described herein. We use this case study to discuss currently available methods of species delimitation and their integrative application to heterobranch sea slugs.