Feeding preferences and food selection of the red swamp crayfish, Procambarus clarkii, in habitats differing in food item diversity

We compared the dietary composition, feeding preferences, and food selection of the introduced crayfish, Procambarus clarkii in a natural and a transformed freshwater marsh (rice field) in the lower Guadalquivir basin (southern Spain). Both habitats differed in hydroperiod and food item diversity. Food item diversity in crayfish stomachs is correlated with prey availability, which is significantly higher in the natural marsh. Herrera's index was low in both areas if compared with the theoretical maximum. P. clarkii behaves like a generalist in this area and individual crayfish have very similar feeding preferences. The most frequent items in the stomach were fresh macrophytes, detritus, and sediment grains. However, especially in the untransformed marsh, insect larvae formed an important part of the crayfish's diet, up to 67% by occurrence, with a marked seasonal pattern: crayfish in this area consumed significantly more animal prey in spring than in winter. P. clarkii positively selects for insect larvae (mayflies, chironomids), snails, and microcrustaceans except ostracodes. Negative selection against oligochaetes is explained by their burrowing behaviour and the difficulty to detect them in the stomachs. Cannibalism is stronger in the rice fields as a consequence of high crayfish densities. Animal food for adult P clarkii is shown to be more important than previously reported. Overestimation of herbivory and detrivory in this omnivorous species might be due to serious methodological drawbacks associated with stomach content analyses.