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

This article focuses on the systematics and biogeography of the freshwater shrimp species *Macrobrachium tenellum* along the Pacific coast of Mexico, including the Baja California Peninsula (BCP). Through morphological revision and molecular genetic analyses of mitochondrial markers 16S ribosomal RNA and cytochrome oxidase I (COI), the study demonstrates that *M. tenellum* populations from BCP are conspecific with those on the Pacific slope of mainland Mexico. Genetic data reveal shared haplotypes and low differentiation between these geographically separated populations, indicating oceanic dispersal rather than vicariant events as the primary mechanism for their distribution. Phylogenetic analyses place Mexican *M. tenellum* in a monophyletic clade closely related to *M. tenellum* from Costa Rica and its sibling species *M. acanthurus* from the Atlantic coast, supporting previous morphological and genetic findings.