n.2 RELATIVE FOOD QUALITY 



Vascular plants are generally 

 recognized as having lower food value than 

 algae (Crisp 1975). The percent protein 

 content is higher in algae where there is 

 less accumulation of non-proteinaceous 

 structural compounds. The vascular 

 tissue, thick cell walls, and lignified 

 structural components make up a large 

 proportion of the organic matter of marsh 

 flowering plants. In addition, the 

 structural components of vascular plants 

 make them difficult for herbivores to 

 digest. Caswell et al . (1973) suggested 

 that plants with the C^ mode of 

 photosynthesis (their first products have 

 4-carbon molecules instead of the usual 3) 

 are particularly problematic for 

 consumers, since much of their 

 protein-rich food is contained within 

 thick- walled bundle sheath cells. 

 Decomposition appears to be a prerequisite 

 for use of vascular plants as food sources 

 for many marsh consumers. 



Are vascular plants utilized by 

 southern California marsh animals? 

 Williams (1981) examined the food value of 

 two halophytes from Tijuana Estuary by 

 preparing detritus and feeding it to young 

 mussels (Mytilus edulis ) under controlled 

 laboratory conditions. Cordgrass 

 ( Spartina foliosa . a C plant) and 

 pickleweed ( Salicornia virginica , a C 

 species were chosen to test the 

 hypothesis that C, species provide better 

 food for marsh coTisumers. Both types of 

 detritus were poor food sources, however, 

 even when detrital particles had been 

 partially decomposed following a four-day 

 aging process. Mussels lost weight when 

 fed detritus alone. In contrast, mussels 

 placed in channels of the Tijuana Estuary 

 grew rapidly, indicating that other 

 sources of organic matter were important 

 in their nutrition. 



A second factor which may make 

 vascular plants less palatable for marsh 

 grazers is high salt content. Winfield 

 (1980) determined the carbon content of 

 seven succulent and four non-succulent 

 marsh plants to be from 19? to 35X of 



their dry weight, whereas Spartina 

 alterniflora along the East Coast measures 

 38 to 47% carbon (Keefe 1972). Higher 

 internal salt content would explain the 

 lower carbon content for southern 

 California marsh plants. In addition, the 

 salt-excreting species ( Spartina foliosa , 

 Monanthochloe littoralis , Distichlis 

 spicata , Limonium calif ornicum , and 

 Frankenia grandifolia ) are all usually 

 coated with salt crystals, which should 

 make them less desirable to herbivores. 

 In our outdoor laboratory at Mission Bay, 

 we have witnessed more insect herbivory on 

 cordgrass plants grown in fresh water than 

 on plants grown in sea water (Boland, 

 unpub. data) , and it is tempting to 

 hypothesize that salty plant tissues act 

 to discourage herbivory. 



Most of the marsh algae are single 

 celled species or filaments of one cell 

 thickness. Fibrous tissues are lacking. 

 This suggests, but does not guarantee, 

 that algae are palatable and digestible. 

 Planktonic bluegreen algae are notorious 

 for their unpalatability (Porter and 

 Orcutt 1980). In part, this is attributed 

 to their filamentous nature, which makes 

 them difficult for zooplankton to ingest. 

 In addition, many secrete toxic compounds 

 which at least some zooplankton appear to 

 be able to detect and thus avoid. Other 

 consumers may be poisoned if toxins build 

 up to high concentrations. 



But the bluegreen algae of salt marsh 

 muds apparently do not inhibit 

 invertebrate feeding as do their 

 planktonic relatives. On the contrary, 

 the work of Brenner et al. (1976) shows 

 that marsh algal mats dominated by 

 bluegreen algae provide a high quality 

 food source which is both palatable and 

 digestible. Assimilation efficiencies 

 (amount digested as a percent of amount 

 ingested) of up to 67% were measured for 

 salt - marsh amphipods in their 

 Massachusetts marsh. 



Diatoms are also palatable to marsh 

 consumers. Whitlatch and Obrebski (1980) 

 examined the feeding habits of two 

 gastropods, Batillaria attramentaria 



51 



