TAPHONOMIC BIAS IN CHESAPEAKE BAY MOLLUSKS: PATTERNS OF SHELL DAMAGE

Years of over-fishing, combined with increased nutrient pollution, have had a catastrophic effect on the ecology of the Chesapeake Bay. The Holocene record of Bay mollusks may provide a useful baseline for ecological restoration, but the effects of preservational bias on these assemblages must first be assessed. This study addresses two questions: (1) What types of shell damage occur in the death assemblages of Bay benthic mollusks and (2) How does shell damage differ according to shell mineralogy, life habit, and geographic location? To conduct this study, we focused on ten sites distributed within the main channel and the major tributaries of the Bay. Death assemblage material was collected using a Young’s modified grab, sieved through a series of four mesh sizes (4mm, 2mm, 1mm, 0.5mm), and temporarily preserved in formalin solution. Identifiable shells and fragments were assigned taphonomic damage states based on categorical scoring systems. Data collection techniques were standardized with past taphonomic studies to allow for comparisons. Data on the following variables were collected from both internal and external shell surfaces (where applicable): (1) bioerosion, (2) encrustation, (3) periostracum loss, (4) drilling, (5) disarticulation, (6) fragmentation, (7) fine-scale alteration, (8) and edge-rounding. Type of encruster, driller, and/or bioeroder was also noted. Preliminary results suggest low levels of bioerosion, encrustation, and drilling and moderate to high levels of periostracum loss, fragmentation, and fine-scale alteration in Chesapeake Bay molluscan death assemblages. Shell damage in these assemblages may differ according to both shell mineralogy and life habit.