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

Abstract

Around Sapelo Island, Georgia, E. limosum feed primarily on other crabs and are found where cordgrass Spartina alterniflora 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. The mean (±SD) number of fiddler crabs Uca pugnax milled in 24 h by E. limosum was less (2.0±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. 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 fewer males (4.3±2.55) than females (5.7±2.65) were killed by E. limosum in 48 h. Predation by mud crabs may contribute to male-biased sex ratios commonly observed in natural populations of fiddler crabs. Oyster shell clumps influenced the predator-prey interaction between P. herbstii and the ribbed mussel Geukensia demissa in a simulated intertidal creek environment. Mussels attached to the exterior exposed surfaces of an oyster clump suffered a 3.5 times great mortality than those attached to the inside (concealed) surface of the clump. 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. -from Authors