Macroalgal fouling on the intertidal mole crab Emerita analoga facilitates bird predation

In this work, we studied the effects of predation by birds on the intertidal mole crab Emerita analoga fouled by macroalgae in a sandy beach of central Peru (11° S). The epibiosis affected mostly the larger animals, especially adult females. Epibiosis prevalence for the entire intertidal population was relatively low (1–2%), however, within the size range affected by epibiosis in the intertidal zone (18–23 mm in carapace length), 20–38% of the animals were fouled. Focal observations of birds showed that fouled animals are preferred over those non-fouled of the same size class and hence the same sex, being consumed at a higher rate than their proportion in the intertidal (Chesson’s alpha index of prey selection >0.96), and estimations of mortality rates indicated that more than 35% of the intertidal fouled animals are removed everyday by birds. The effect of epibiosis may be mainly attributed to a higher burrowing time or an increased visual attractive effect of the algae, which make fouled animals more conspicuous to predatory birds, or because of fouling enhances profitability of the animals. The results show that epibiosis has negative effects on E. analoga through increasing predation by birds, which in turn restricts the distribution and abundance of fouled E. analoga in the intertidal zone.