Based on a reconnaissance study of the zoogeography and ecology of benthic invertebrates in the Gulf of California, 11 faunal assemblages have been established which characterize various environments--(I) intertidal rocky shores; (II) intertidal beaches and sand flats to 10 m; (III) low-salinity lagoons and mangrove mud flats; (IV) nearshore shelf, sand bottom, 11-26 m; (V) intermediate shelf, clayey sand and sandy bottom, 27-65 m; (VI) outer shelf, clay bottom, southern Gulf, 66-120 m; (VII) outer shelf, sand bottom, northern Gulf, 66-120 m; (VIII) northern Gulf basins and troughs, 230-1,500 m; (IX) upper slope, central and southern Gulf, 121-730 m; (X) middle slope, 731-1,799 m; (XI) abyssal southern borderland basins and lower slope, 1,800-4,122 m.

Compared with benthic communities elsewhere in the world, the diversity of shallow-water species in the Gulf of California is striking. No single species is dominant. The distribution of shell remains with depth provides indications for former low stands of sea level. Shells of shallow-water species at 110-115 m were dated by radiocarbon method at 17,000 to 19,000 years B.P. Shells belonging to the California shelf province are found in abundance in deposits of the northern Gulf basins and in deep water in the southern end of the Gulf. The occurrence of these cold-water species implies that during the Pleistocene, migration southward of more than 700 miles was possible.

Comparisons between the macro-invertebrate assemblages of the Gulf of California and those of the Gulf of Mexico and other parts of the world demonstrate that great similarities, generally at the subgeneric level, exist in similar environments throughout the subtropical and tropical regions of the world.