AbstractNudibranchs of the genus Spurilla (Aeolidiidae) are currently represented worldwide by five species: S. braziliana, S. croisicensis, S. dupontae, S. neapolitana, and S. sargassicola. Two of these, S. neapolitana and S. braziliana, have been previously reported from the Argentine Sea. This study aimed to identify Spurilla specimens collected from intertidal zones at four sites in the San Matías Gulf. We conducted detailed morphological and anatomical examinations, including external traits (body coloration, rhinophores, and cerata) and internal structures (radula, jaws, nervous system, and reproductive system). These characteristics were compared with published descriptions of all five known species. To strengthen our findings, we incorporated a molecular approach based on mitochondrial gene sequences. The analysis revealed low genetic differentiation between populations from Argentina and Brazil, supporting the presence of a single, widespread species, S. braziliana, throughout the southwestern Atlantic. Genetic patterns also suggest that Brazil may represent the species' center of origin, with subsequent southward dispersal to Argentina and northward expansion toward Central America and the Caribbean. This revision confirms that S. braziliana is the only species of the genus present along the western coast of the San Matías Gulf, contributing to a clearer understanding of regional marine biodiversity and species distributions.
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