The known distribution of Mesocyclops pehpeiensis Hu, 1943 includes South and Southeast Asia (India, Cambodge, Laos, Vietnam, Sri Lanka) and stretches eastwards to China and Japan. It has been recorded in the east coast of the United States as an introduced form. This species has not been found south of the United States or in tropical areas of the Americas. The analysis of samples collected in two ponds of the state of Chiapas, on the Pacific coast of Mexico, yielded the identification of male and female individuals of this species. The Mexican female individuals, like those found in the United States, tended to be slightly smaller than those of the type locality in China (1.2 mm vs 1.5 mm); the morphology of the Mexican specimens is identical in most characters to that described or depicted in previous works, including a recent redescription. Some of the morphologic and morphometric details of the caudal rami setae, the fourth leg endopod, and male and female antennules were found to differ among the different populations compared (Mexico, United States, China). This variability seems to be related to a speciation process in the group of Asian species of Mesocyclops to which M. pehpeiensis belongs. This is the first report of this species in Mexico and it marks a new meridional limit of its distribution in the New World. This finding represents the fourth effective record of an introduced freshwater copepod species in Mexico and the second in the Mexican Pacific coast. It is speculated that the events by which this species invaded the Atlantic coast of the United States and now the tropical Pacific coast of Mexico are attributable to human agency and not inter-related.
Crustacea, Copepoda, Cyclopidae, faunistic, zooplankton, freshwater, introduced species, Mexico.