Description
The intertidal species, Donax gouldii displays band stripping. Band stripping is the partial or complete removal of growth bands from the outer shell layer of the valve. Three questions were posed to address this taphonomic occurrence: 1) Where, in terms of geography and environment, does band stripping occur? 2) Can band stripping be emulated in the laboratory? 3) Are there changes in shell composition or structure that cause band stripping. Both field and laboratory tests were performed to try to answer these questions. To determine where band stripping occurs geographically and environmentally, five sites were selected along the coast of San Diego, California. Three of the sites experienced significantly higher percentages of stripping (mean 22-24%) than the other two sites (mean 1-5%). Statistical pair-wise comparisons were performed. The comparison indicated that Ocean Beach Pier/Dog Beach and La Jolla Shores/Dog Beach co-vary for the majority of the study period. The similarity between Ocean Beach Pier and Dog Beach may be due to their proximity. No environmental variable common to La Jolla Shores and Dog Beach can be identified; however, similarity may be explained through the time valves remain on the beach, population viability and age range. In order to determine whether mechanical or chemical processes could potentially cause band-stripping, tests were performed to try emulating band stripping in the laboratory. Band stripping did not result from utilizing a rock tumbler which simulated mechanical processes. Timed acetic acid tests resulted in preferential dissolution of specific growth bands suggesting that band stripping results from a chemical process. Shell composition may be composed of both calcite and aragonite and when both minerals are in the same shell layer, a sharp mutual boundary without microstructural integration is formed (Carter, 1980). XRD revealed that D. gouldii valves are composed solely of aragonite. Microstructural groups used to classify shell microarchitecture differ in how well they interdigitate along their boundaries. Depending on the type of microarchitecture present or if there are changes in microarchitecture across shell interfaces, shell integrity may vary, resulting in band stripping. SEM depicted changes in valve microarchitecture. Backscattered images of the inner shell layer display a highly interdigitated aragonitic cone complex crossed lamellar microstructure. Secondary electron imaging of the outer shell layer displays a poorly interdigitate nondenticular composite prismatic microstructure. Microgrowth increments within the outer shell layer vary in thickness. Thicker increments consist of larger crystals and tend to strip. Narrower increments with smaller crystals appear better organized: the organization may be why these bands do not strip.
