Linking Neotropical riparian and stream food webs: nocturnal foraging behavior and facilitation among decapods in response to added palm fruit

Freshwater macroinvertebrates can connect tropical forests and stream food webs by consuming both terrestrially and aquatically produced foods, but direct observations of nocturnal feeding behavior in these natural ecosystems are rare. Observational data on decapods feeding behavior are limited partly because they are cryptic and often forage at night to avoid predators and competitors. We hypothesized that freshwater decapods forage on riparian fruit fall based on its relatively high energy and nutrient content. We used underwater cameras to video record night-time foraging under dim red light. Palm fruits (Acrocomia aculeata (Jacq.) Lodd. ex Mart.) were added to determine rates of feeding responses among crabs (Silviocarcinus australis Magalhães and Turkay, 1996) and shrimp (Macrobrachium amazonicum Heller, 1862) in a spring-fed, clear-water stream in the state of Mato Grosso do Sul, Brazil. Within 30 min, shrimp located the fruit, and within 60 min, crabs began shredding the pulp of the palm fruit. Crabs fed on the palm fruits, on average five times longer than shrimp and produced pieces of palm fruit pulp that some shrimp consumed without aggressively interacting with the crab. Small fish also consumed some palm pulp particles. This interaction network is an example of fruit-fall processing based on indirect facilitation among different types of consumers.