53 



IV. Nearshore, sand and sand-mud, 1 1-26 meter assemblage. 



This is by far the most prolific assemblage in species and numbers of 

 individuals of invertebrates in the Gulf of California, so far as the present 

 sampling is concerned. In accordance with Liebig's "Law of the Minimum", 

 as restated in Parker (1959), there is good reason for the great diversity 

 and size of the population, since the ecological conditions are optimum 

 for most marine species. Salinities are constant and at normal oceanic 

 values, and water temperatures have a much smaller annual range than in 

 the inshore environments. The water is also relatively quiet at these depths 

 which are below wave base for the rather small-sized waves of the Gulf 

 of California. Sediments therefore range from sand to sand-mud and 

 mud-sand, or in Shepard's (1954) terminology, sand, silty sands, clayey 

 sands, sandy silts and sand-silt-clay. These sediments are less well-sorted 

 than those in the previous environments, permitting greater amounts of 

 organic detritus to accumulate, thus making it possible to support a 

 larger population of deposit-feeding animals. These, then, are optimum 

 conditions for the existence of most marine invertebrates. A total of 258 

 species (living and dead) from all stations, and 172 living species from the 

 27 characteristic stations were identified from this environment. A com- 

 parison of the number of species taken at all depth ranges is given in fig. 15. 



The separation between the northern and southern Gulf is relatively 

 distinct in this environment. The contingency matrix provided the basis 

 for two separate assemblages, both on sand bottom and in from 1 1 to 

 26 meters depth, one in the Tiburon region and the other south of Mazatlan. 

 There is a possibility that separate groupings of animals resulted because 

 of different kinds of sampling gear. The majority of samples in the northern 

 region were taken by shell dredge (10 shell dredges, 2 Van Veens, 1/15 m^ 

 and 2 3-meter otter trawls), while in the southern region grab samples 

 outnumber dredges and trawls (4 Petersen grabs, 1/10 m^, 3 Van Veen, 

 1/20 m^, 2 orange peels, 1/15 m- grabs, 3 3-meter otter trawls, and 1 mini- 

 dredge) as shown on Table III. Regardless of the difference in balance of 

 collecting devices between the two areas, at least some dredging and grab 

 sampling was carried out in both places. Examination of the list of living 

 animals only from the northern and southern nearshore shelf region 

 (Table II) shows roughly the same size and kinds of animals from the two 

 regions, even though the species were often different within the same 

 subgenus. For instance, 25 species were taken in common between the two 

 areas, or slightly less than one quarter of the total of northern species and 

 slightly less than half of the total of southern species. There were two more 

 species of corals, almost twice as many species of prosobranchs and 



