Boundaries

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

Topography

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).

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).

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).

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]).