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# of Endemic Species
Major Habitat Type:
tropical and subtropical coastal rivers
Salvador Contreras Balderas, Universidad Autónoma de Nuevo León. Additional text was modified from Abell et al. 2000. Freshwater Ecoregions of North America: A Conservation Assessment
The extent of this ecoregion is defined by the watershed of the Río Coatzacoalcos. The ecoregion lies at the northern end of the Isthmus of Tehuantepec – the shortest distance between the Gulf of Mexico and the Pacific Ocean – in northeastern Oaxaca and eastern Veracruz.
Drainages flowing into:
Gulf of Mexico
Main rivers or other water bodies:
The primary river of the ecoregion is the Río Coatzacoalcos. Major tributaries of the Río Coatzacoalcos are the Jaltepec and Uspanapa rivers. Together these rivers drain an area north of the Continental Divide to the Gulf of Mexico. Other water bodies include the Laguna Sontecomapan and some small basins between it and the Río Tonalá (= Sanabria).
The topography of the ecoregion is mostly lowlands of the coastal plain. Elevations range from 2 to 2160 m, with the highest peaks at the southern end of the rugged Sierra Madre de Oaxaca.
This ecoregion has a tropical wet and dry climate, as well as a tropical monsoon climate. The area experiences a distinct dry season, with the driest month receiving less than 60 mm of precipitation. The mean annual precipitation, however, is nearly 2400 mm. The mean annual temperature is 25 °C, mean maximum temperature is 16 °C, and mean maximum temperature is 33 °C.
Freshwater habitats are highly diverse, and include mountain creeks, big rivers along the coastal plain, meanders, side lagoons, and coastal lagoons with brackish waters.
The Petén-Veracruz moist forests cover the low-lying coastal plain, whereas Sierra Madre de Oaxaca pine-oak forests, dry forests and Oaxacan montane forests occur at higher elevations in the southwest. Pockets of Chimalapas montane forests and Southern Pacific dry forests also line the southern edge of the ecoregion. A large area of Mesoamerican Gulf-Caribbean mangroves occurs at the mouth of the Río Coatzacoalcos.
The montane headwaters of this ecoregion are particularly rich in species within the families Characidae (Astyanax sp.), Heptapteridae (Rhamdia sp.), Belontiidae (Strogylura sp.), Poeciliidae (Poeciliopsis gracilis), Mugilidae (Agonostomus sp.), Eleotridae (Gobiomorus sp.), and Cichlidae.
Description of endemic fishes:
The Coatzacoalcos ecoregion exhibits relatively high endemism among its fish, with roughly ten percent of its native fish species strictly endemic to the ecoregion, as well as a number of other near-endemics. These fish are mostly cichlids and livebearers (poeciliids), and include the yellow swordtail (Xiphophorus clemenciae), Mixe swordtail (X. mixei), southern mountain swordtail (X. monticolus), and Coatzacoalcos gambusia (Heterophallus rachovii) (Poeciliidae); Sarabia cichlid (Paraneetroplus bulleri) and Almoloya cichlid (Vieja regaini) (Cichlidae). La Palma silverside (Atherinella ammophila) is also endemic.
Other noteworthy aquatic biotic elements:
The area does not support the same numbers of aquatic herpetofauna as the montane Pacific ecoregion of Chiapas-Fonseca  to the south. In fact, only two of its native herpetofauna are endemic to the region. This ecoregion has no endemic crayfish.
The Río Coatzacoalcos ecoregion shows faunal exchanges with neighboring basins. This has resulted in closely related forms, although without sympatry.
This is one of the areas where two forms of Astyanax exist in basically distinct habitats, usually headstreams (A. mexicanus) and low coastal areas (A. aeneus).
Justification for delineation:
Ecoregion delineations were based on qualitative similarity/dissimilarity assessments of major basins, using the standard administrative hydrographical regions of the Mexican federal government.The Coatzacoalcos has relatively high endemism. Although the Papaloapan, Coatzacoalcos, and Usumacinta rivers share genera or lower level species complexes, the species are endemic and distinctive for each basin. For example, each of the genera Priapella, Atherinella, Theraps, and Thorichthys have one endemic species in each of the basins (Papaloapan , Coatzacoalcos, and Usumacinta ), justifying the distinction of these basins as ecoregions.
Level of taxonomic exploration:
Good except for small basins, and the middle reaches of the Río Coatzacoalcos.
Abell, R. A., Olson, D. M., et al. (2000). "Freshwater Ecoregions of North America: A Conservation Assessment" Washington, DC, USA: Island Press.
Chernoff, B. (1986). "Systematics of American atherinid fishes of the genus Atherinella. I. The subgenus Atherinella" Proc. Acad. Nat. Sci. Phila. 138(1) 86 - 188.
Kallman, K. D., Walter, R. B., et al. (2004). "Two new species of Xiphophorus from the Isthmus of Tehuantepec, Oaxaca, México, with a discusión of the distribution of the X. clemenciae clade" Amer. Mus. Novit. 3441 1-34.
Miller, R. R.,Hubbs, C. L. (1974). "Rivulus robustus, a new cyprinodontid fish from southeastern Mexico" Copeia 1974(4) 865-869.