Acidification due to anthropogenic CO 2 pollution, along with episodic or persistent acidification that occurs in coastal environments, will likely result in severe seasonal acidification in estuarine environments. Acidification decreases the fitness of individual species, but the degree to which predator-prey interactions will be impacted is largely unknown. This mesocosm study examined the effect of CO 2 acidification on crab-bivalve predator-prey interactions involving two commercially important Chesapeake Bay species, the blue crab Callinectes sapidus and the soft-shell clam Mya arenaria. In particular, the direct effects of CO 2 acidification on clam growth and behavior, and the indirect effects of CO 2 acidification on interactions between crabs and clams were examined. Mya arenaria were grown in CO 2acidified water (pH 7.2) or ambient conditions (pH 7.8) for 30 d. To determine the effect of acidification on clam responsiveness to mechanical disturbance, a probe was slowly moved towards clams until they ceased pumping (a behavior to avoid detection by predators), and the distance between the probe and the clam's siphon was noted. Clams were exposed to predation by C. sapidus, which were held under acidified or ambient conditions for 48 h. Callinectes sapidus handling time, search time, and encounter rate were measured from video. Acidified clams had lighter shells than ambient clams, indicating that shell dissolution occurred. Acidification reduced the responsiveness of M. arenaria to a mechanical disturbance that simulated an approaching predator. As compared to ambient trials, crabs in acidified trials had higher encounter rates; however, this was offset by crabs taking longer to find the first clam in trials, and by increased occurrence of crabs eating only a portion of the prey available. As a result, there was no net change in predation-related clam mortality in acidified trials as compared 3 to ambient conditions. Understanding how acidification will impact food webs in productive estuarine environments requires an examination of the direct impacts of acidification on organism behavior and physiology, as well as indirect effects of acidification mediated through predator-prey interactions.

Bergmann's rule predicts that organisms at higher latitudes are larger than ones at lower latitudes. Here, we examine the body size pattern of the Atlantic marsh fiddler crab, Minuca pugnax (formerly Uca pugnax), from salt marshes on the east coast of the United States across 12 degrees of latitude. We found that M. pugnax followed Bergmann's rule and that, on average, crab carapace width increased by 0.5 mm per degree of latitude. Minuca pugnax body size also followed the temperature–size rule with body size inversely related to mean water temperature. Because an organism's size influences its impact on an ecosystem, and M. pugnax is an ecosystem engineer that affects marsh functioning, the larger crabs at higher latitudes may have greater per‐capita impacts on salt marshes than the smaller crabs at lower latitudes.

Juvenile blue crabs Callinectes sapidus use seagrass and other structured habitats as refuges from predation. Oyster reef habitats provide structural complexity that may offer refuge, but the value of these habitats for juvenile blue crabs has not been examined. We quantified survival of juvenile C. sapidus in structured oyster reef habitat versus unstructured soft-bottom habitat. In a field tethering experiment in the York River, lower Chesapeake Bay (USA), juvenile C. sapidus (10-50 mm carapace width [CW]) were tethered in sand (n = 40) or oyster reef (n = 39) habitats at subtidal sites 1-2 m deep. An underwater camera system was used to record predation activity during 24 h trials. Juvenile crab survival was significantly higher on the oyster reef habitat (53.8%) than on bare sand (15.0%), and tended to increase with crab CW in both habitats. The main successful predators on juvenile blue crabs were northern pufferfish Sphoeroides maculatus in the oyster reef habitat and adult blue crabs in the sand habitat. The high survival rate of juvenile C. sapidus in oyster reef habitats suggests that oyster reefs include physical habitat complexity that may offer refuge from predators. Restored and natural oyster reefs could provide an alternative nursery habitat for juvenile blue crabs, expanding the ecosystem services provided by restored oyster reefs.

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