103 



the nearshore and intermediate shelf off Texas (Parker, 1960). A com- 

 parison of the common animals from these three regions shows an almost 

 identical generic composition, which substantiates Thorson's (1951) 

 concept of parallel communities. For instance, in the check list of common 

 invertebrates for this environment (2 to 24 meters) off the Mississippi 

 Delta, 8 of the 1 1 gastropod species have their exact counterparts (even 

 to subgenus) in the nearshore, 11 to 26 meter environment in the Gulf of 

 California. Fifteen of the 22 pelecypods, all of the echinoderms, and a large 

 number of crustaceans are also represented by closely related species. 

 Most of these twin species were found too in the same depths and bottom 

 type off Texas. There was no doubt as to similarity of the assemblages 

 between the Gulf of California and Gulf of Mexico, although it is surprising 

 that no exact generic counterparts have been described previously, since 

 this assemblage undoubtedly exists elsewhere in the world. Three other 

 regions have been studied in tropical latitudes, but the environmental 

 factors are much more extreme and variable than those in the Gulf of 

 California. These are Madras, India in the Bay of Bengal, the Persian 

 Gulf, and off Ghana, West Africa. None of these regions was characterized 

 by the diversity of animals found in the American Gulfs, nor was there 

 much similarity in the dominant groups. However, the fluctuation of envi- 

 ronmental parameters in these regions, undoubtedly contributes to less 

 diversity. 



Samuel (1944) published a few observations on benthic communities 

 in similar depths off Madras, India. She found that the primitive chordate, 

 Branchiostoma, was the most abundant animal on sand bottom between 

 18 and 30 meters. Two echinoids, including Lovenia (which also occurs 

 in the same environment in the Gulf of California) were the next most 

 abundant animals. Only two mollusks, a Marginella and Glycymeris were 

 common. No others were taken more than once. Except for the presence 

 of Lovenia, there seems to be little resemblance between the Madras 

 community and the one found at similar depths and on the same sediment 

 in the Gulfs of California and Mexico. A slightly different community 

 was found on clay bottom in these depths off Madras. An Anomuran crab 

 and several other decapod crustaceans were the dominant animals. The 

 only common mollusks were the gastropods, Oliva, Cantharus and Murex, 

 which were also found at similar depths in both Gulfs, but occupied no 

 position of importance within the community. 



Thorson (1957) mentioned several communities from the Persian Gulf, 

 in depths ranging from two to 60 meters on sand and shelly sand bottom. 

 Only the dominants were given, none of which corresponded to the 

 dominants of the American communities or assemblages at these depths. 



