A predator consuming multiple prey species usually causes indirect effects. Apparent mutualism results when multiple prey species reduce predation risk for each other by altering a predator's functional response. Short-term apparent competition occurs when multiple prey species increase predation risks for each other through the numerical response, i.e. increasing the predator's birth rate, or aggregative response, i.e. attracting a higher density of predators. Our objectives in this study were to determine the aggregative response and 2-prey functional response of a predator and to examine indirect effects over a range of prey densities. We used the clam Macoma balthica and juvenile blue crabs Callinectes sapidus as prey for adult blue crabs. In laboratory experiments, we determined the single-prey functional responses of the crabs to each prey species and the 2-prey functional response. We combined the 2-prey functional response with the known blue crab aggregative response to clams to estimate field predation rates. Our model predicts that at low prey densities, clams and juvenile blue crabs exhibit apparent mutualism, whereas at high clam densities, this relation switches to short-term apparent competition. These unexpected results highlight the need to incorporate multiple aspects of predation at multiple scales when considering indirect effects.KEY WORDS: Predator-prey interactions · Functional response · Indirect effects · Blue crab · Macoma balthicaResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 456: 139-148, 2012 increases the predator density (i.e. changes either the predator's numerical or aggregative response), thus increasing predation on the other species (Holt & Lawton 1994). This occurs either through increasing the predator population growth rate (Settle et al. 1996, Carvalheiro et al. 2008 or through a local increase in predator density due to predator movement (Holt & Kotler 1987, Brown & Mitchell 1989, Murakami & Nakano 2002. Study organismsThe blue crab Callinectes sapidus Rathbun, 1896, is an important benthic predator in Chesapeake Bay (Virnstein 1977). Large adult crabs are opportunistic omnivores, feeding primarily on bivalves (~50% of diet) and crustaceans (~30%) as well as fish and polychaetes (Hines et al. 1990). Blue crabs are cannibalistic, with adults feeding on juveniles (Hines & Ruiz 1995). Because cannibalism represents a major source of mortality for juvenile blue crabs, juveniles congregate in shallow waters (<1 m), where adults are less abundant (Hines & Ruiz 1995, and in structured habitats that provide a predation refuge (Everett & Ruiz 1993).Macoma balthica Linnaeus, 1758, is a small (shell length < 40 mm) tellinid clam that is the dominant biomass in unvegetated mud habitats within the mesohaline regions of Chesapeake Bay (Holland et al. 1977) and is a major source of food for blue crabs (Baird & Ulanowicz 1989). M. balthica avoids predation by burying deeply, up to 35 cm, in the sediment (Hines & Comtois 1985)...