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are known (Keen, 1958), and possibly another 1,500 are yet to be de- 

 scribed. One explanation for the great variety of mollusks within the Gulf 

 of California may be found within its geological history. In middle Tertiary 

 times, the relatively deep marine connections between the Caribbean Sea 

 and Pacific Ocean permitted a mixing of the two faunas, the greater part 

 of the migration taking place from east to west (Ekman, 1953). A large 

 number of new species were added to an already abundant, partly endemic 

 eastern Pacific fauna from the Caribbean. There has also been a continual 

 renewal of species from the Indo-Pacific region. Many western and southern 

 Pacific species are transported first to islands such as the Galapagos, Cocos 

 and Clipperton Islands, and eventually to the mainland (Rosenblatt, 

 1959; and Walker, 1960). During the Pleistocene, it seems that there was 

 also a southern migration of some California province species. Aside from 

 this recruitment of species from outside, the evolution of new species has 

 also certainly occurred. With the high primary productivity of the Gulf 

 as a basis for high benthic production plus the great variety of habitats 

 available for moUusk species, virtually all species from these various 

 zoogeographic regions can be successfully maintained in various parts of 

 the Gulf. Since no very drastic seasonal environmental changes occur, and 

 salinities remain constant and optimum for marine animals, most species 

 may only be eliminated by predation or competition for food. Sparck 

 (1926) suggested that both water temperature and availability of food may 

 affect the diversity and abundance of marine populations, although this 

 was implied indirectly. Through a series of tank experiments, Sparck 

 showed that a lack of food would tend to eliminate a large number of 

 invertebrate species growing and reproducing in cold waters, especially 

 those with high metabolic requirements. When the food supply was 

 reduced, only those with low energy requirements would remain, and these 

 could then become the dominant organisms in the environment. On the 

 other hand, a lack of food in warm (20" C.) waters had little affect on most 

 of the species adapted to these waters, since their metabolism would be 

 lower, and thus would need less food to survive. The population would 

 then consist of many species with relatively few individuals of each species. 

 There is yet another theoretical explanation for lack of dominance in 

 most of the Gulf of California assemblages. The majority of mollusks, and 

 perhaps other invertebrates, may have a very short life span of one to two 

 years, leading to an alternation of species in any one spot. During the 

 course of a year, one group of invertebrate larvae may settle in a particular 

 part of an environment, live and reproduce there, while the larvae of this 

 generation may settle elsewhere (see Thorson, 1950). The adults may also 



