determine how the two crabs interact. 

 When each species was caged alone, 

 Willason found that Pachygrapsus crassipes 

 did not burrow; hence its common use of 

 burrows is dependent on excavations of 

 other species. Hemigrapsus oregonensis 

 was found to dig burrows readily. Further 

 study showed that Pachygrapsus could 

 expand Hemigrapsus burrows to accommodate 

 its larger carapace. Pachygrapsus ate 

 Hemigrapsus in the field, and laboratory 

 experiments confirmed that equal or 

 larger-sized Pachygrapsus killed 

 Hemigrapsus , but smaller Pachygrapsus did 

 not. Cannibalism occurred among 

 Pachygrapsus , but only after recent 

 molting of the victim. Field experiments 

 demonstrated substantial mortality of 

 Hemigrapsus when both species were placed 

 together in cages, but better survival of 

 Hemigrapsus when it was given time to 

 burrow before individuals of Pachygrapsus 

 were added to the cage. Thus, burrows 

 provide a partial refuge from predation, 

 but even burrowing Hemi grapsus can be 

 displaced by the aggressive Pachygrapsus . 

 How, then, do Hemigrapsus populations 

 persist? 



The greater recruitment of 

 Hemigrapsus in muddy habitats is one 

 possibility; however, differential 

 sensitivity to salinity may also play a 

 role. Experiments on salinity tolerance 

 suggest that Pachygrapsus crassipes 

 (especially small individuals) is less 

 tolerant of the low salinities which 

 occasionally occur following winter and 

 spring rainfall (Willason 1980). 



Insects 



By virtue of their high species and 

 trophic diversity, insects have many roles 

 in coastal salt marshes. They feed on 

 both vascular plants and algae; they feed 



This section draws heavily on unpublished 

 data, correspondence and references 

 provided by Chris Nagano, Research 

 Associate, Entomology Section, Natural 

 History Museum of Los Angeles County. 



on decaying plant and animal material; 

 some are carnivores; and many serve as a 

 food source for birds and other marsh 

 vertebrates. Marsh insects are also 

 important to the pollination of marsh 

 halophytes. While most of the marsh 

 plants lack the showy flowers that attract 

 insects, at least one may owe its 

 existence to insect pollinators. The salt 

 marsh bird's beak ( Cordylanthus maritimus 

 ssp. maritimus , previously mentioned as an 

 endangered species, is pollinated by bees. 

 Certainly its long closed tubular flowers 

 prevent wind pollination. Since this 

 species is an annual, it relies on seeds 

 to reproduce, and hence pollinators are 

 important both for fertilization and the 

 maintenance of genetic diversity through 

 cross-pollination. 



Some insect species are very 

 sensitive to human impacts and can assist 

 in indicating the general quality of 

 coastal habitats. Unfortunately, they 

 generally receive little attention because 

 of their small size, the great difficulty 

 in identifying most species, and the 

 incorrect assumption that they are 

 ecologically insignificant. Where they 

 have been investigated (e.g. Cameron 1972, 

 Davis and Gray 1966, Nagano et al. 1981), 

 salt marsh insect communities were highly 

 diverse both in composition and function. 



Foster and Treherne (1976) reviewed 

 the literature on salt marsh insects and 

 suggested a long list of potential 

 environmental problems which must be 

 overcome for insects to survive in 

 intertidal marsh habitats. First to be 

 discussed was the question of how oxygen 

 is obtained for respiration — both for 

 aquatic insects in anaerobic mud and water 

 where many eggs, larvae and pupae develop, 

 and for aerial larvae and adults inundated 

 by tides. The latter may tap oxygen from 

 air tissues in plants, but no evidence 

 supports this suggestion. Escape to the 

 upper plant canopy might allow such 

 insects to avoid inundation, but Cameron 

 (1976) found that San Francisco Bay 

 insects remain within the vegetation 

 during high tides. However, he also 

 indicated the possibility that insect 



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