marsh animal community, it may be 

 unrealistic to expect to explain their 

 occurrences without considering the other 

 invertebrates, and especially their 

 vertebrate predators. 



Southern California marsh molluscan 

 communities are consistently dominated by 

 Assiminea californica , Cerithidea 

 californica , and Melampus olivaceus 

 (Figure 38), all of which are epifaunal 

 surface feeders (Tables 10 and 11). These 

 snails are important grazers on the marsh 

 algal mats. Simple field experiments 

 which excluded hornsnails ( Cerithidea 

 californica ) from small mudflat areas at 

 Mission Bay resulted in local algal mat 

 blooms, while control cages had little 

 algal accumulation (SDSU Aquatic Ecology 

 class, unpub. data). Removal of such 

 grazers by shorebirds should create the 

 same effect, and part of the patchiness of 

 intertidal algal mats and invertebrates 

 may be due to patchiness in the feeding 

 effects of birds and other carnivores. 



Marsh tidal creeks usually appear to 

 be dominated by the hornsnail, Cerithidea 

 californica . However, Macdonald (1967) 

 also found abundant Acteocina sp. and 

 several bivalve molluscs (Table 10), all 

 of which are burrowing forms that are 

 rarely visible. McCloy (1979) studied 

 hornsnails in detail at Sweetwater Marsh 

 (San Diego Bay), and documented several 

 factors which influence their population 

 size. Marshes often support densities 

 over 1,000/m ; individuals under 0.5 cm 

 length are more common in the tidal 

 creeks, and large individuals are more 

 abundant on the marsh soil. McCloy 

 attributed this size zonation to 

 differential desiccation tolerance, since 

 individuals under 0.5 cm in length rarely 

 survived more than 12 hours in a 

 desiccator, while individuals over 1.5 cm 

 all survived for 48 hours (McCloy 1979). 

 Also characteristic of the species is a 

 non-random dispersion pattern. 

 Individuals usually appear to be evenly 

 spaced, suggesting some behavioral 

 mechanism which avoids crowding. McCloy 

 experimentally crowded the snails, first 

 in the open without restricting snail 



movements, and later within cages. The 

 response was then examined over several 

 months. In the open, enhanced and 

 depleted densities returned to normal 

 within a month, as snails moved away from 

 high density areas and into low density 

 areas. Behavior, then, is density 

 dependent and can regulate small-scale 

 dispersion patterns. Within cages, McCloy 

 tested to see if mortality and recruitment 

 also changed with crowding or depletion. 

 Again, the results showed density 

 dependence of the population behavior. 

 Growth rates declined under crowded 

 conditions, suggesting food limitation. 

 Some snails in the high-density cages 

 moved up the cage sides, while others 

 attached to the incoming tide water 

 surface, probably in an attempt to 

 disperse by flotation. Cages with fewer 

 hornsnails had higher larval settling 

 rates, so that densities converged to 

 normal. 



There were also effects on other 

 marsh creek species. Mortality of adult 

 hornsnails was higher under crowded 

 conditions, and the large number of empty 

 shells enhanced settling of the anemone 

 Haliplanella luciae . Anemone densities 

 became nearly four times higher than 

 normal. Some invertebrates are rare in 

 marshes because hard substrates are 

 unavailable for larval settling. This 

 result shows how events within one 

 population can have unexpected effects on 

 other species. The deposit feeder 

 Acteocina culticella was also influenced 

 by altered hornsnail densities. It was 

 eliminated in crowded cages and enhanced 

 by decreased densities of hornsnails, 

 suggesting that competitive interactions 

 occur between the two deposit feeders, and 

 perhaps food is a limiting factor (McCloy 

 1979). 



Other factors which McCloy determined 

 to affect hornsnail densities were 

 smothering by deposits of dense floating 

 algae (e.g. Diva ) , shorebird predation and 

 crab predation. Shorebirds appear to be 

 less damaging to tidal creek populations 

 of hornsnails than to snails on mudflats. 

 This suggests that creeks are important 



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