Shell Repair in Geukensia demissa and Predation Preferences of Callinectes sapidus: Do Crabs Target Mussels with Weakened Shells?

Atlantic blue crabs, Callinectes sapidus, are voracious predators that often leave damage on the shells of unconsumed ribbed marsh mussels, Geukensia demissa. The extent of shell damage and size-dependent tradeoffs in marsh mussel growth and repair, as well as the effects of shell damage on crab predation preferences, were determined in this thesis. A preliminary experiment investigated characteristics of damaged mussels in the field. Mussels (n = 30) were collected in the fall of 2011 within two ocean-dominated inlets along the South Carolina coast and were measured for size (length, width, height), area of damage, shell thickness, mass, and strength (crushing resistance). Shell damage was significantly different between inlets and shell repair was evident in damaged mussels. During the summer of 2012 three sizes of field-collected mussels (small: 20-30 mm, medium: 50-60 mm, large: \u3e60 mm) were damaged (undamaged 0%, moderate 33%, extensive 66% shell surface removed), caged in the mid-marsh, and sampled monthly. Changes in mussel characteristics (e.g., shell length, strength), were measured. In most cases, increased damage suppressed growth, however, only medium, moderately-damaged mussels repaired shells. Medium, moderately-damaged mussels also experienced a greater mortality rate, suggesting mussels enter a critical stage around 55 mm with increased energy demands for both growth and repair. Small mussels eschewed repair and focused entirely on growth, as larger sizes create a refuge from predation. Large mussels did not exhibit any signs of shell repair and had minimal growth, possibly instead prioritizing reproduction. A series of wet lab mesocosm experiments and field trials were conducted to determine if blue crabs target damaged mussels. In the wet lab mesocosms, crabs showed a significant preference for damaged and first-touched mussels. Crab consumed damaged mussels in 68% of all successful predation attempts and mussels touched-first in 73% of successful predation attempts. Unsuccessful crabs targeted undamaged mussels first more frequently than successful crabs (55% vs 33%). However, a preference for damaged mussels was not observed consistently in the field and may have been masked by various mitigating factors. Undamaged mussels survived significantly longer than damaged mussels in the mid-marsh but were consumed at equal rates on mudflats, oyster reefs, and in the low-marsh. Mussel survival was greater overall in the mid-marsh with large mussels (\u3e 60 mm) surviving significantly longer than medium (50-60 mm) and small (20-30 mm) mussels. Limited tidally-influenced inundation and densely distributed Spartina alterniflora stems likely increased survival by impeding access of large predators (e.g., blue crabs). The generally thicker shells of larger mussels also will increase predator time and effort required to breach shells successfully and should increase survival rates for large mussels. Both mussels and crabs play a vital role in maintaining healthy salt marsh systems and reductions in either population have dramatic consequences. Salt marshes are structured in part by the top-down control of blue crabs and recent die-offs of Spartina is suspected to be caused by declining blue crab numbers while salt marsh loss due to sea level rise is suspected to be exacerbated by declining mussel numbers. Pollution, overfishing, habitat destruction, and the various effects of climate change (e.g., temperature rise, ocean acidification, sea level rise, etc.) threaten crab and mussel populations. Mussel response to shell damage and the ability of crabs to detect weakened mussels may be increasingly important as environmental conditions deteriorate. Further research should investigate the effect of shell damage on mussel pumping and if changes in pumping influences crab predation. The latitudinal differences in crab and mussel growth and behavior should also be examined, as additional insight into mussel-crab dynamics would be useful for salt marsh conservation