39 



and Groves (1959, p. 18) it can be seen that in the month of April, surface 

 water temperatures are at about 15°, while bottom temperatures at below 

 1,000 meters are still as high as 12°C. During the warmest month of August, 

 surface waters may be as high as 28°, but at 200 meters, temperatures are 

 down to 15°, and at 1,000 meters no lower than 12°C. These warm temper- 

 atures, found at such great depths, have a marked effect on the com- 

 position of the fauna in the bottom and on the sides of these channels and 

 in Tiburon basin. Vertical stratification of assemblages, as it occurs in the 

 central and southern Gulf corresponding to changing depths, disappears, 

 and many species usually found in 150 to 200 meters may also occur at 

 the bottom in depths of 1500 meters. This offers some proof that temper- 

 ature is at least one of the more important controlling factors in deter- 

 mining faunal distribution on the bottom, and that pressure alone is 

 apparently not too limiting a factor. 



The inshore or shelf water temperatures have been discussed in detail 

 by Roden and Groves, but a brief description can be given here. A pro- 

 nounced seasonal variation has been observed in all portions of the Gulf, 

 but the variation in the northern region above the large islands is the most 

 extreme. In the vicinity of San Felipe and Puerto Pefiasco, the annual 

 range of water temperature in shallow water is about 16°C., from about 

 14° in the winter to over 3rC. in August. This extreme temperature range 

 possibly excludes many benthic animals, and in fact the south side of 

 Tiburon Island is the extreme northern end of the range for a large number 

 of invertebrate species (fig. 12). The summer temperatures are warm 

 enough to permit the existence of many Panamic or tropical forms, but 

 the cold temperatures are limiting to many species with a presumed low 

 tolerance to extreme change. 



At Guaymas and in the shallow waters south of Tiburon Island, the 

 annual range is from nearly 18° to 31.5°C., or a range of 13.8°, still limiting 

 to many tropical forms. However, at Mazatlan water temperatures range 

 from only about 20° to 30°C., a difference of about 9° to 10°. The Mazatlan 

 area also seems to be the northern limit for a number of strictly tropical 

 species, as well as the extreme southern limit for many of the endemic 

 Gulf of California species. Fig. 12 illustrates the various geographical 

 limits of a number of the common, shallow-water Gulf of California 

 species as compared to annual range of surface water temperatures. Of the 

 253 species of invertebrates chosen to illustrate this point (those with at 

 least 5 living station occurrences), 8 species are found no further south 

 than the vicinity of Tiburon Island on the east side of the Gulf. Also, 

 34 more species are not found north of the Hermosillo-Tiburon region. 



