Nitrogen, phenolic acids, and other feeding cues for salt marsh detritivores

A number of feeding cues determine the palatability of detritus to detritivorous invertebrates. In salt marsh detritus the feeding cues include flavor provided by phenolics such as ferulic acid, pH, salinity, and nitrogen content. We examined the feeding responses to each cue by using experimental manipulations where we changed concentrations of these chemical cues. The palatability of detritus of the salt marsh grass Spartina alterniflora to the detritivorous snail Melampus bidentatus is reduced by increases in ferulic acid in the detritus. The acidity of the acid is partly responsible for inhibition of feeding, but other flavors of the ferulic acid are the major factor. Changes in salinity makes detritus more or less palatable to different species of detritivores. Available nitrogen confers greater palatability to detritus.In the field the feeding cues are all present simultaneously, and detritivores feed based on a hierarchy of cues. For M. bidentatus the presence of sufficient available nitrogen overwhelms the response to ferulic acid, the second most important cue. Salinity and pH, although used as discriminatory cues by themselves, are located lower in the hierarchy than nitrogen and ferulic acid.As detritus ages in the marsh, nitrogen content changes. Similarly, detritus from eutrophied environments shows enhanced nitrogen content. The differences in palatability of new and old detritus, and from eutrophic and non-eutrophic environments, suggests that detritivores respond not to total nitrogen but rather to the availabel nitrogen fractions. The increase in lignin found in old and eutrophic detritus may increase the binding of available nitrogen, and hence reduce the palatability of detritus.