outcrops or reefs occur in some nearshore areas, particularly off 

 Louisiana. However, the best known of these natural nearshore reefs is, 

 perhaps, Seven and One-Half Fathom Reef which is located offshore from 

 Padre Island, Texas. As indicated by a comparison of Figures 1 and 15» 

 artificial reefs in the form of petroleum platforms and shipwrecks 

 provide most of the hard substrate habitat in nearshore waters, 

 particularly offshore from Louisiana. Assuming the submerged portion of 

 a typical petroleum platform consists of about 3>500 m 2 , and that there 

 are 3t3^2 of these structures in the gulf, these structures provide a 

 total of about 1 ,500 ha of artificial reef habitat. 



FAUNAL ASSEMBLAGES AND COMMUNITIES 



Zoogeographic treatments of the Gulf of Mexico based upon benthic 

 communities of the shelf of the northern gulf, have been treated by a 

 number of authors (see Defenbaugh 1976 and Chittenden and McEachran 1976 

 for reviews based upon macroinvertebrates and fishes, respectively). 

 Most of these studies are in general agreement that, although the faunal 

 assemblages of the northern gulf basically represent an extension of the 

 Carolinean Province of the Atlantic coast of the southern United States 

 resulting from a Pleistocene connection across peninsular Florida, the 

 eastern gulf fauna differs in a major way from that of the western gulf 

 in that there is a stronger representation of Caribbean fauna in the 

 eastern gulf. These differences appear largely attributable to (1) 

 substrate differences (carbonaceous sediments east of the Mississippi 

 River Delta and terrigeneous sediments west), and (2) circulation 

 patterns and related hydrographic conditions (e.g. temperature, 

 salinity, turbidity) . 



Considered as a whole, the Gulf of Mexico is often designated as a 

 subtropical sea, but the surface waters over the inner shelf (to 20 m) 

 of the northern gulf from just east of the Mississippi River Delta to 

 Matagorda Bay are cold enough in winter to be considered warm- temperate 

 (Parker 1960). Temperatures of the inner shelf waters change from warm, 

 subtropical in the eastern gulf to cooler temperate waters in the 

 central region, and back to warm subtropical on the southwestern coast 

 near the Mexican border. As described above, starting at the 

 Mississippi River Delta, salinity also increases on the shelf from the 

 east to the west and southwest. This increase is largely a function of 

 freshwater input which is, in turn, strongly correlated with climatic 

 conditions. Parker (1960) classified the region from the delta to near 

 Galveston, Texas as a Humid Zone, and from this point to Matagorda Bay 

 as a Wet-Humid Zone. The region from Matagorda Bay to about Corpus 

 Christi, Texas is labeled as Dry Sub-Humid changing southward of Corpus 

 Christi to Semi- Arid. 



Although the fauna of the Texas-Louisiana continental shelf is 

 basically Carolinean, the above climatic and hydrographic conditions 

 result in measurable faunal differences, leading some workers to 



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