Prevalence, Patterns, and Effects of Shell Damage on Geukensia Demissa in South Carolina Estuarine Habitats

Geukensia demissa, ribbed marsh mussel, collected from intertidal marsh habitats in Murrells and Hog Inlet, South Carolina, commonly had 20 to 60% of the total shell area damaged. Shell damage was identified by removal of the periostracum exposing the inner white calcite layers. Mid-marsh mussels had significantly greater amounts of damage than mussels collected from intertidal oyster reefs. The effects of shell damage on mussel survivorship and growth were determined in long-term (10-mo.) and short-term (5-mo.) studies. The long-term experiment was conducted from January to September 2007 to examine integrated seasonal growth effects and the short-term study was conducted from January to May (non-growing) and May to September (growing) 2007 to examine seasonal effects. The experiments included size classes (50 and 70-mm) and shell damage treatments (undamaged and damaged). Mussels with minimum shell damage were collected from a common mid-marsh field site. Half of the mussels received an extra 20% of shell damage using a rotary tool to sand off the shell. Initial shell dimensions and photographs were collected and all mussels placed in a mid-marsh location in Hog Inlet, South Carolina. Predator-exclusion cages were used to minimize additional shell damage and predation. Damaged mussels had a significant 4 to 10% increase in mortality compared to undamaged for both long-term and short-term studies. Growth in shell length was significantly greater for 50-mm mussels compared to 70-mm mussels in all experiments. Shell damage had inconsistent results on the change (final - initial) in shell mass for each damage and size treatment. Shell mass change was significantly greater for damaged mussels of both size classes in the long-term and winter short-term data. However, summer short-term damaged mussels grew significantly slower than undamaged mussels. Tissue mass of damaged mussels consistently grew slower in the 50-mm size class; whereas changes in tissue dry mass for damaged 70-mm mussels was significantly greater for long-term and winter short-term experiments. The reduced tissue growth in 70-mm mussels during the summer of the short-term experiment may reflect the fall collection of experimental mussels coincidently occurring after a spawning event. Decreased survivorship resulting from shell damage can have long-term effects on marsh mussel population dynamics. Shell damage, requiring expenditure of energy for repair while increasing tissue mass, likely influences reproductive output and will also affect the overall energy budget of the mussels. Shell damage represents an under recognized impact on the mussel populations and their overall contribution to marsh ecosystems