GROWTH AND LONGEVITY OF THE SCALLOP CHESAPECTEN NEFRENS (CALVERT CLIFFS, MARYLAND) DURING THE MIDDLE MIOCENE CLIMATIC OPTIMUM AND SUBSEQUENT RAPID COOLING EVENT

We know little about how scallop growth and longevity are affected by rapid climate change at a single location. An excellent site to test such differences occurs in the stratigraphically well-known Calvert Cliffs of Maryland. At this locality, bed 14 represents a tropical climate during the middle Miocene Climatic Optimum (MMCO) and bed 19 represents ensuing, rapidly cooling global temperatures likely affected by the expansion of the East Antarctic Ice Sheet (EAIS). Growth analysis of the abundant scallop Chesepecten nefrens, present in both beds, can provide valuable insight concerning ecological responses of scallops to climate change.

Bottom valves (n = 53) of C. nefrens from field and museum collections were binned into two size classes (SC) for each bed: Size class A (100-120 mm shell length) and size class B (120-150 mm). Scallop age was estimated by counting the prominent annuli on the valves and the von Bertalanffy growth equation was used to estimate growth (k) and maximum shell length (Linf). MMCO scallops were expected to have constant, uninterrupted growth resulting in higher k values, smaller shells and shorter longevity during warmer conditions, while EAIS scallops were expected to have lower k values, larger shells and higher longevity.

Results indicate that the largest MMCO scallops had higher average growth than EAIS scallops (SC B: k = 0.47 and 0.32, respectively), but smaller scallops had similar growth in both beds (SC A: k ~ 0.41). Estimates for maximum size indicate that MMCO C. nefrens were smaller than EAIS individuals for the largest size class but similar for the smaller size class (SC B: Linf 134 mm and 144 mm, respectively; SC A: Linf ~ 120 mm). Longevity estimates were similar: the smaller size class yielded ages of 5.24 yrs (MMCO) and 4.71 yrs (EAIS); larger size classes yielded 7.80 yrs (MMCO) and 7.50 yrs (EAIS). We found that size class strongly affects the outcome of growth analysis for C. nefrens. However, for the largest size class, MMCO scallops do appear to grow faster and obtain a smaller size than EAIS scallops perhaps indicative of higher temperatures. Despite differences in climatic regimes, however, both scallops appear to live up to 8 yrs in age, suggesting a genetic control.