Aim  To test whether distributional patterns of Neotropical freshwater taxa fit the generalized tracks already postulated for terrestrial groups occurring in the Mexican Transition Zone. Location  The study units comprised 17 hydrological basins located along the Pacific coast of the Americas from Mexico to Panama, and in the Gulf of Mexico from the Papaloapan to the Grijalva–Usumacinta basin. Methods  Distributional data for 22 fish species, 34 crab species of the tribe Pseudothelphusini, and 22 strictly freshwater species of angiosperms were analysed. Parsimony analysis of endemicity is based on presence/absence data of these taxa and uses the computer programs Winclada and NONA. Results  Three generalized tracks were obtained: (1) Mexican North Pacific, (2) Mexican Central Pacific, and (3) Southern Mexico–Guatemala. A node resulted at the intersection of the first two tracks, coinciding with the Neovolcanic Axis in central Mexico. Main conclusions  Freshwater generalized tracks with an altitudinal distribution below 1000 m, mainly including fishes and angiosperms, are close to the Tropical Mesoamerican generalized track. Generalized tracks above 1000 m, including freshwater crabs, have a stronger affinity with the Mountain Mesoamerican track. The Isthmus of Tehuantepec represents a node for the Neotropical freshwater and terrestrial biota. These results seem to indicate that common geobiotic processes have induced these patterns.

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Mexico has two main types of coasts, an Amero-trailing-edge coast along the Gulf of Mexico and a collision coast along the Pacific, each of which determine the characteristics of their coastal lagoons. The lagoons along the Gulf of Mexico are extensive, tropical and rich in seagrass beds and mangroves, and among these Laguna Madre has 84 species of fishes. In contrast, the lagoons along the Pacific are small, closed, mostly oligohaline and usually lack seagrass beds, and among these Laguna Salinas del Padre has 19 species of fishes. This marked difference in species richness, in turn, has a direct effect on the size and value of the fisheries, as these are regional in the first case and local and seasonal in the second.

Background. Laguna de Tamiahua is ecologically and economically important as a nursery area that favours the recruitment of species that sustain traditional fisheries. It has been studied previously, though not throughout its whole area, and considering the variety of habitats that sustain these fisheries, as well as an increase in population growth that impacts the system. The objectives of this study were to present an updated list of fish species, data on special status, new records, commercial importance, dominance, density, ecotic position, and the spatial and temporal distribution of species in the lagoon, together with a comparison of Tamiahua with 14 other Gulf of Mexico lagoons. Materials and methods. Fish were collected in August and December 1996 with a Renfro beam net and an otter trawl from different habitats throughout the lagoon. The species were identified, classified in relation to special status, new records, commercial importance, density, dominance, ecotic position, and spatial distribution patterns. Results. The 45 species collected by the authors plus an additional 125 species previously reported provided an updated list of 170 species. Families with the highest number of species were Sciaenidae (16 spp.) and Gobiidae (12 spp.). Poecilia latipunctata Meek, 1904 is endemic to Mexico and in danger of extinction and Hippocampus zosterae Jordan et Gilbert, 1882 is under special protection. New records are Ariopsis assimilis (Günther, 1864),