Anthropogenic modifications and their impacts on shellfish physiology

Humans have been modifying marine habitats for centuries to enhance productivity and facilitate the collection of natural food sources such as fish and shellfish. Anthropogenic alterations and impacts on marine habitats include coastal development, aquaculture, fishing, agriculture, transportation and waste disposal, which have led to a decrease in habitat complexity and a loss of biological diversity. The maintenance, regulation and protection of healthy aquatic habitats and the ecosystem services they provide is a global concern. In this study transcriptional analysis was utilized to investigate physiological responses of shellfish to two different types of anthropogenic marine impacts; clam garden habitat modifications and microplastic pollution. \nClam gardens are examples of ancient anthropogenic modifications built by the Northwest Indigenous Coastal peoples of America to enhance clam habitat productivity, providing secure and reliable food sources. Physiological differences of Leukoma staminea (Littleneck clams) transplanted in unmaintained clam garden beaches for 16 weeks compared to clams in unmodified reference beaches were investigated using metrics of gene expression, growth and survival. This study found no statistically significant differences in growth and survival but did find statistical differences in expressed biological pathways in clams between clam gardens and reference beaches. Most biological pathways in both groups were associated with environmental stress, suggesting both habitats contained their own unique multiple stressors. There were also no statistically significant differences in sediment carbonate, organic content, or grain size distributions between the sediment from clam garden beaches compared to reference beaches. An interesting finding in this study was a significant negative correlation between sediment carbonate content and survival. The presence of several highly upregulated viral transcripts from the Dicistroviridae family had significant correlations with geographical proximity and survival, further confirming that other factors (such as geographical location and sediment characteristics) had a greater influence on Littleneck clam survival and immune status if a beach had been modified or not \nMicroplastics are emerging anthropogenic pollutants found in marine habitats worldwide, including key aquaculture and fisheries species such as bivalves. To examine the impacts of environmentally relevant concentrations of microplastics on the highly commercial Pacific oyster (Crassostrea gigas), 102 adult oysters were exposed to microplastics (5 microplastic fibers per litre) in microalgal diets for 30 days and impacts were assessed using gene expression, condition index, microplastic load and lysosomal membrane stability. Results were compared to control (n= 102) oysters receiving microalgal feed and held in the same experimental conditions but with no microplastic exposure, and background counts of microplastic load in seawater and microalgal production were also assessed. There were no statistically significant differences observed in survival, condition index or lysosomal membrane stability between control and exposed oysters. However, there were statistically significant differences in microplastic load and gene expression between the exposed and control oysters. There was an upregulation in biological pathways associated with immunity and stress and a downregulation in pathways associated with reproduction in the exposed oysters, highlighting the potential long-term negative consequences of environmental microplastics on long-term population stability, especially if microplastic concentrations continue to increase. \nThis study found that previous beach modifications (clam gardens) did not positively affect clam growth, survival or physiology, and that regional environmental stressors played a greater role in survival. Environmentally relevant microplastic exposures over the 30-day study period was found to elicit an immune response and have negative implications for reproductive success.