Rhizocephalan infection modifies host food consumption by reducing host activity levels

• We examined the effects of rhizocephalan infection on crab physiology and behavior.
• Parasitism did not significantly alter crab resting metabolism or gut passage time.
• Infected crabs were considerably less active than uninfected crabs.
• Microhabitat choice substantially differed between infected and uninfected crabs.
• Rhizocephalans found to reduce crab functional response by altering host behavior.

Although castration by rhizocephalans on crab species is well documented, the accompanying effects of infection on behavior and metabolism have remained relatively unstudied. In this investigation, we examined flat back mud crab (Eurypanopeus depressus) physiology and behavior in an attempt to elucidate why infected crabs exhibit a previously documented reduced functional response. Crab respiration and digestion rates were analyzed to determine if infection altered metabolic rate. Laboratory behavioral experiments and a field survey were conducted to determine how infection alters crab feeding behavior and activity levels. Although we found no statistical difference between infected and uninfected crab metabolic or digestive rates, we discovered that, both in the lab and in the field, infected crabs exhibited substantially altered behavior. In the laboratory infected crabs reacted nearly 3 times slower to the presence of prey and spent over 22% more of their time hiding, whereas uninfected crabs were significantly more active. During field sampling, infected crabs were significantly more likely to be found hiding within empty oyster shells while uninfected crabs spent more time in the exposed positions of the habitat. We conclude that rhizocephalans can reduce the host functional response by altering host behavior. Here, these induced changes can impact community structure by altering trophic interactions so that infected crabs spend less time foraging and more time hiding, potentially reducing their predation risk.