101 



known concerning the biology and life history of the common inhabitants 

 of this environment. It was impossible to determine from the literature, 

 the type of feeding or development from the Corbiculids, Mytilopsis or 

 Rangia, although it is suspected that all may be suspension feeders, 

 specialized to select lighter organic material from the more dense inorganic 

 particles by some sort of ciliary mechanisms. Of the 18 genera listed for 

 this environment in the Gulf of California, 55 "/o are probably suspension 

 feeders. However, about half of these are adapted to high turbidity waters, 

 such as oysters and Rangia, which can function in high concentrations of 

 inorganic material, and are in reality feeding upon the deposits. Of the 

 remaining, 227o are herbivores, 14% are predators or scavengers, and 

 Melampus can be considered a terrestial herbivore. No estimate can be 

 made of the usual mode of larval development, except that virtually all of 

 the large lamellibranchs have planktonic larvae, which are thought to 

 settle rather rapidly in response to tidal and discharge changes in the 

 estuaries. Less is known for the gastropods, although Melampus is known 

 to take up an aquatic existance only to reproduce, the juveniles soon 

 creeping out of the water to live under damp vegetation along the shore. 



The environmental factors limiting this assemblage have been discussed 

 at length by Parker (1956, 1959 and 1960) for its counterpart in the Gulf 

 of Mexico. Salinity certainly has a limiting effect in this environment, 

 since many species die or fail to reproduce whenever salinities rise above 

 10 or 15%. The extreme range of water and air temperatures encountered 

 in these shallow waters may serve to exclude many other invertebrate 

 species which might compete with the lagoonal species which are adapted 

 to changes of up to 30"^ C. between minimum and maximum. Sediment is 

 limiting, only when it is too soft to support the populations of oysters and 

 possibly Anadara. It is known that pH or hydrogen-ion concentrations are 

 lower in the lagoons and estuaries than in the open sea, but it is not known 

 whether these low pHs have any effect on estuarine faunas. 



This assemblage is relatively uncommon in the fossil deposits surrounding 

 the Gulf of California, but it comprises a large percentage of shell middens 

 of the prehistoric peoples in the southern part of the Gulf. The species 

 typical of low-salinity lagoons are valuable in paleoecological inter- 

 pretations, since they are excellent indicators of climatic change (dry to 

 wet or vice-versa) and as in the previous assemblage, good shoreline 

 indicators. Since they thrive only in waters of low to medium salinity, it 

 is unlikely that they will be found living in open sounds, gulfs or oceans. 

 In order for low salinity conditions to be maintained in areas where these 

 mollusks live, there should be little connection with the open sea. There- 



