Two species of mud crabs—the flat mud crab (Eurypanopeus depressus) and the white-fingered mud crab (Rhithropanopeus harrisii)—commonly inhabit subtidal oyster reefs in the Maryland portion of the Chesapeake Bay. Predation of juvenile Crassostrea virginica (spat) by mud crabs could be one of the major contributors to early juvenile spat mortality, hindering natural recruitment and restoration efforts in the Chesapeake Bay. The purpose of this study was to measure the number and size of spat preyed on by E. depressus and R. harrisii. Spat settled on oyster valves were placed in an experimental tank with one crab over a 96-h study period. Every 24 h, the number and size of spat scars were measured. E. depressus consumed nearly 40% of the spat offered whereas R. harrisii consumed less than 10%. E. depressus also consumed significantly more spat than R. harrisii within each spat size class (for all spat size classes, P < 0.0001). Although available spat sizes ranged from 1–29 mm, approximately 75% of spat preyed on by both species were less than 8 mm in size. Our data suggest E. depressus may have the potential to reduce spat survival significantly, with R. harrisii playing a minor role. Considering restoration strategies that reduce spat predation by mud crabs may increase the success of natural recruitment and restoring oyster populations.
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Vol. 30 • No. 2