Results

Results from morphometric data on the left valves of Stono River-collected oysters are presented in the figures below: valve length (upper left), shell depth (upper right), valve height (lower left), and anterior scar height (lower right). Y-bars are standard error and letters represent significant differences among years based on post-hoc Tukey's analysis. We measured a total of 531 shells and 365 of these were left-handed. For some samples, we were unable to measure all four morphometric variables due to shell brokenness. The sample sizes for each response variable are as follows: valve length (year 0: n=30; 1700s: n=10; 1800s: n=169; 2013, n=128); shell depth (year 0: n=32; 1700s: n=10; 1800s: n=145; 2013: n=78); valve height (year 0: n=32; 1700s: n=10; 1800s: n=174; 2013: n=127); and anterior scar height (year 0: n=28; 1700s: n=5; 1800s: n=157; 2013: n=129).

Our data suggest modern oysters are narrower (reduced valve length), shallower (reduced shell depth), and longer (increased valve height) than historical oysters. There were no observed differences in anterior scar height with time. These results contrast the findings of Lunz (1938) who found that modern oysters were not only narrower but also shorter than pre-colonial shells collected in the Charleston estuarine system.  He contributed the decrease in size to pollution and the intensity of commercial harvesting.  His conclusions are consistent with hypotheses described in Claassen (1998) that propose a reduction in shell size with time may be due to increased harvesting pressure from humans, increased predation by other organisms, and/or deteriorated habitat. 

The narrower and longer shell morphology of the modern oysters measured in our study may be indicative of a change in suitable habitat over time.  Adams et al (1991) showed that oysters growing in overcrowded habitats develop narrower and longer shells relative to their un-crowded conspecifics.  Overcrowding of oyster clusters will result if larval recruitment is plentiful, but available substrate to settle upon is not.  Though we are unable to discern if historic oysters experienced more favorable settlement conditions, we do know that “high recruitment but limited substrate” correctly describes the current state of oyster populations in Charleston Harbor (SCDNR SCORE program).    

Sampling methods may also bias these results. We cannot know the specific harvesting methods of the societies that collected the oysters near the year 0 C.E. or in the 1700s and 1800s.  Historic oysters were most likely selected for a larger size and accessibility.  Though we attempted to replicate this bias in our field-collections of modern oysters, there still may be a wide disparity in collection methods across time.  Additionally, oysters found at a historic site may be a compilation of multiple populations collected from different locations.  Growth rates of oysters may be dependent on local and regional conditions especially in regards to temperature, sedimentation, and parasite load (Moroney and Walker 1999).  The position of an individual on a cluster of oysters (Moroney and Walker 1999) and the individual’s age class (Claassen 1998) may also ultimately determine their size; both factors were not controlled for in this study.

Despite these caveats, our results do suggest a change in the shape of individual oysters over time.  Futures studies that complement morphometric analyses with methods to infer change in environmental conditions (e.g., use of stable isotopes, sediment coring) and studies that increase our understanding of the societal uses of oysters may clarify what is driving this change.