Boundaries

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.

Topography

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.

Freshwater habitats

Freshwater habitats are highly diverse, and include mountain creeks, big rivers along the coastal plain, meanders, side lagoons, and coastal lagoons with brackish waters.

Terrestrial habitats

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.

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.

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 [171], Coatzacoalcos, and Usumacinta [173]), 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.