Old Taxonomy Masks the Phenotypic Diversity of Pleurobrachia Fleming (Ctenophora: Tentaculata): An Analysis of the Morphological Variation in the Genus from Mexican Coasts

Pleurobrachia pileus and P. bachei are among the most frequently recorded ctenophores worldwide. Despite their long-standing descriptions dating back nearly 250 years, their taxonomic boundaries remain ambiguous due to the lack of definitive diagnostic features. In Mexican waters, records of both species are primarily based on geographic distribution, with limited morphological evidence supporting their separation. This study aimed to evaluate the morphological diversity of Pleurobrachia spp. along the Mexican coast through a comprehensive morphological approach. Specimens from four sites representing two marine regions (the Gulf of Mexico and the Southern Mexican Pacific coast) were examined for phenotypic variation. A standardized methodology was developed for specimen handling, observation, and digital reconstruction of body plans, enabling the extraction of 38 discrete and continuous morphological traits. Our analysis revealed two distinct and internally consistent phenotypic groups, each corresponding to one of the two regions, with notable differences in tentacular sheath structure, tentacular canals, gastrovascular cavity pigmentation pattern, and pharyngeal morphology. A clinal pattern in body size was observed, with smaller specimens in the north and larger ones in the south. Importantly, neither phenotype matched the diagnostic traits of P. pileus or P. bachei, suggesting that the Mexican populations represent separate, previously unrecognized species. These findings highlight the necessity to re-evaluate the taxonomy of Pleurobrachia in these regions and contribute valuable morphological data toward resolving longstanding questions about species boundaries within this genus.