The systematics of the amphidromous shrimp Macrobrachium tenellum (Smith, 1871) (Decapoda: Caridea: Palaemonidae) from the Mexican Pacific slope

We had previously revised the systematics of four species of Macrobrachium from the Pacific coast of Mexico, including the Baja California Peninsula (BCP): M. americanum, M. digueti (), M. hobbsi Nates & Villalobos in , and M. occidentale Holthuis, 1950. Genetic analyses of the mitochondrial markers 16S ribosomal RNA (16S) and cytochrome oxidase I (COI) of individuals of these morphological species from BCP have shown that, even facing the Gulf of California (Sea of Cortes) as an apparent marine ecological barrier, they are conspecific to their respective genetic lineages (species) found from the Pacific slope of mainland Mexico. We discuss the systematics of the fifth species of Macrobrachium from the Mexican Pacific slope, M. tenellum () to test the hypothesis that individuals of the putative M. tenellum from BCP belong to the same species from the Pacific slope of mainland Mexico. We revised the morphological characters of taxonomic importance using material obtained in the field and voucher material deposited in Mexican scientific collections, and performed molecular genetic analyses with newly generated fragments of the mitochondrial markers 16S and COI. We conclude, as in our previous studies on Macrobrachium species of the region, that the examined specimens of the putative M. tenellum from BCP belong to the same species found on the Pacific slope of mainland Mexico, and that oceanic dispersal is responsible for the distribution of the species of Macrobrachium in the peninsula rather than by vicariant events.