Evidence of oceanic dispersal of a disjunctly distributed amphidromous shrimp in Western North America: first record of Macrobrachium occidentale from the Baja California Peninsula

Abstract

Six morphological species of the genus Macrobrachium occur in the Baja California Peninsula (M. americanum, M. digueti, M. hobbsi, M. michoacanus, M. olfersii, and M. tenellum). Their presence is an interesting topic for the systematics of the group, given that their distribution shows a distinct disjunct on the coastal plains of the northern part of the Gulf of California slope. Extensive collection of freshwater shrimp of the peninsula allowed us to discover individuals whose morphology corresponds to Macrobrachium occidentale Holthuis, 1950. This species, described in 1950 from the Pacific slope of Central America has received little atention. The type locality is in Guatemala, but it is also found in Mexico, El Salvador, Costa Rica, and Panama. In Mexico it has been reported in the states of Sinaloa, Nayarit, Guerrero, and Oaxaca. The aim of the present work is to contribute to the systematics of M. occidentale through two approaches. First, by reviewing the geographical distribution in Mexico, using taxonomic morphological revision of extensive field collections, as well as material deposited in three scientific collections. Second, through molecular genetic analysis of fragments of the mitochondrial genes 16S rRNA and cytochrome oxidase I, determine whether individuals from the peninsula belong to the same entity found on the mainland Pacific slope of Mexico or whether they form a distinct lineage. The results indicate the presence of the same genetic entity in both regions and show a disjunct distribution similar to other species of the genus in this region. We present a systematic account for M. occidentale, including a taxonomic treatment of the studied populations and a report of their genetic identity and relationships. Further, we suggest and discuss that the presence of this amphidromous species on the peninsula is explained by the oceanic dispersal hypothesis.