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Ecoregion Description

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Species Richness

# of Endemic Species


139: West Texas Gulf

Major Habitat Type:

tropical and subtropical coastal rivers


Text modified from Abell et al. 2000. Freshwater Ecoregions of North America: A Conservation Assessment. Island Press, Washington, DC, USA.


United States


This ecoregion is primarily defined by the watersheds of the San Antonio and Nueces rivers and is contained entirely within southern Texas.

Drainages flowing into:

The drainages of this ecoregion flow into the Gulf of Mexico.

Main rivers or other water bodies:

In addition to the San Antonio and Nueces rivers, watersheds of this ecoregion include the Guadalupe River, Mission River, the Aransas River, and Petronila Creek.


The Nueces River rises in the Edwards Plateau and flows in a southeasterly direction into the Coastal Plain. The Edwards Plateau exhibits little relief except for stream valleys. The San Antonio rises at around 200 m and flows southeasterly towards its confluence with the Guadalupe River. The Coastal Plain is characterized by lowlands and scarps in the inner zone and deltaic and alluvial plains toward the coast (Connor & Suttkus 1986).


The eastern part of the ecoregion lies within the humid subtropical climate zone, where summers are hot and humid and winters are generally mild. The western part of the ecoregion grades into semiarid steppe, with the coldest month having an average temperature above 0 °C (Köppen 1936).

Freshwater habitats:

The Nueces, San Marcos, Frio and Sabinal rivers, among others, originate from seeps and springs in the Edwards Plateau. The Edwards Plateau lies in the northern portion of this ecoregion, as well as the East Texas Gulf [140] and the Lower Rio Grande/Río Bravo del Norte [135] ecoregions. This is a karst area characterized by the Edwards Aquifer, a body of groundwater that has a distinct biota associated with its caverns and springs.

Terrestrial Habitats:

The middle and lower portions of the Nueces and San Antonio drainages are dominated by Tamaulipan mezitiqual, also known as “brush country”. The coastal portion is dominated by coastal grasslands. The Edwards Plateau lies in the northern border, and is characterized by grasslands (Connor & Suttkus 1986).

Fish Fauna:

The fish fauna of this ecoregion is relatively depauperate compared to the other ecoregions of the western Gulf Slope [140, 141]. It is the northernmost limit for one Neotropical species, the Mexican tetra (Astyanax mexicanus); it represents the extreme southwesternmost limit for many eastern lowland and Mississippi Valley species, including golden shiner (Notemigonus crysoleucas), pugnose minnow (Opsopoeodus emiliae), weed shiner (N. texanus), mimic shiner (N. volucellus) and yellow bullhead (Ameiurus natalis). Although the San Antonio drainage harbors fewer native species than drainages to the east, it is recognized by its relatively high endemism and its position as the southwestern limit for disjunct populations of Mississippi and eastern lowland species (Connor & Suttkos 1986).

Description of endemic fishes:

The West Texas Gulf ecoregion contains few endemic fish species. The Edwards Plateau is noted for its subterranean habitats and highly endemic fauna, including the Edwards Plateau shiner (Cyprinella lipid). One particularly important river running through the West Texas Gulf ecoregion is the Guadalupe River. The upper part of the watercourse cuts through the karst bedrock of the Edwards Plateau, which houses the widemouth blindcat (Satan eurystomus) and toothless blindcat (Trogloglanis pattersoni). Other endemics include the fountain darter (Etheostoma fonticola) in the Guadalupe River system and San Marcos gambusia (Gambusia georgei), a spring isolate.

Other noteworthy fishes:

A number of species of this ecoregion are euryhaline or diadromous, occurring at times in coastal marine environments (Connor & Suttkus 1986).

Other noteworthy aquatic biotic elements:

The West Texas Gulf ecoregion contains a few endemic species of crayfish and no known endemic mussels or aquatic herpetofauna. The arid coastal plains region that forms the West Texas Gulf ecoregion shares many faunal assemblages with the ecoregions further south, on Mexico’s relatively dry Gulf Coastal Plain. Examples of these shared species are the black spotted newt (Notophthalmus meridionalis) and the Mexican burrowing toad (Rhinophrynus dorsalis). The lower reach of the Guadalupe River harbors one of the region’s unique herpetofauna, Cagle’s map turtle (Graptemys caglei) (Master et al. 1998).

Evolutionary phenomena:

The West Texas Gulf ecoregion is noted for its higher taxonomic endemism in fish (Abell et al. 2000). The region contains two endemic fish genera (Satan and Trogloglanis).

Justification for delineation:

Ecoregion boundaries are modified from Abell et al. (2000), which based its units on subregions defined by Maxwell et al. (1995).  Modifications to this ecoregion were made following recommendations from the Endangered Species Committee of the American Fisheries Society. This ecoregion was considered unique because of distinct faunal breaks that result from differences in drainage size and physiographic complexity. There are marked differences in richness as one moves east to west across the Gulf Slope region, as well as distinct species clusters at the 80% level among the Nueces-San Antonio drainages (in [139]), Colorado-Brazos drainages (in [140]), and Galveston-Sabine-Calcasieu drainages (in [141]).  


Abell, R. A., Olson, D. M., et al. (2000). "Freshwater Ecoregions of North America: A Conservation Assessment" Washington, DC, USA: Island Press.

Conner, J.V. and R.D. Suttkus (1986)"Zoogeography of freshwater fishes of the western Gulf slope" In Hocutt, C.H.a.E.O.W. (Ed.). The zoogeography of North American freshwater fishes. (pp. 413-456) New York, NY: John Wiley & Sons.

Maxwell, J. R., Edwards, C. J., et al. (1995) "A hierarchical framework of aquatic ecological units in North America (Nearctic Zone)". St. Paul, MN. North Central Forest Experiment Station, USDA Forest Service.

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