Effects of biogenic structure on prey consumption by the xanthid crabs Eurytium limosum and Panopeus herbstii in a salt marsh

The predatory xanthid crabs Eurytjum limosurn and Panopeus herbstii are common components of benthic assemblages in different intertidal habitats within salt marshes around Sapelo Island. Georgia, USA. E. limosum feed primarily on other crabs and are found where cordgrass Spartina altemiflora stems are the dominant structural elements in the environment. P: herbstii feed largely on bivalve molluscs and are abundant in intertidal creeks where oyster Crassostrea virginica reefs are the predominant structural elements. We designed laboratory experiments to measure prey consumption rates by these 2 predators under conditions that simulated those experienced by each species in its respective habitat. Biogenic structure within the environment of each species reduced prey consumption rates. The mean (i. SD) number of fiddler crabs Uca puynax killed in 24 h by E. limosum was less (2.0 t 0.87 individuals) in the presence of stems than in their absence (2.9 + 1.63 individuals). Fiddler crabs had no refuge in size from predation by E. limosum. However, mortality rates of fiddler crabs exposed to predation by E. lirnosum were sex-dependent. Males of U. pugnax sometimes escaped the predator's grasp by autotomizing their prominent major chela and consequently when prey were offered in an equal sex ratio significantly (paired t-test, p = 0.03) fewer males (4.3 f 2.55) than females (5.7 + 2.65) were killed by E. limosurn in 48 h. We believe that predation by mud crabs may contribute to male-biased sex ratios that are commonly observed in natural populations of fiddler crabs. Oyster shell clumps influenced the predator-prey interaction between P herbstil and the ribbed mussel Geukensia dernissa In a simulated intertidal creek environment. Mussels attached to the exterior (exposed) surfaces of an oyster clump suffered a 3.5 times greater mortality than those attached to the inside (concealed) surface of the clump. Results of previous studies in whlch prey were offered as isolated individuals suggest that k? herbstii should have the greatest impact on small (<20 mm shell length) mussels. However, by simulating natural conditions, we showed that predation by P herbstii inflicted the greatest mortality on mussels of intermediate (20 to 40 mm shell length) size because larger mussels have a refuge in size and small mussels have a spatial refuge within the interstices of oyster clumps. Our findings suggest that xanthid crabs have a greater impact within salt marsh communities than previously recognized and emphasize the importance of using 'context-sensitive' experiments to explore predator-prey interactions.