Establishing Energetic Cost of Acute Exposure to Select Anthropogenic-Caused Environmental Conditions in the North Gulf of Mexico Derived Eastern Oyster (Crassostrea virginica)

Abstract

Oyster aquaculture is an important industry in the Gulf of Mexico and has seen a significant increase in interest over the last 20 years. Because oyster farming takes place in ambient waters, farmed oysters are subjected to uncontrollable environmental conditions such as PFAS pollution from industrial discharge, extreme temperature swings, and rapid salinity shifts. Farmers have been reporting massive mortalities in their oyster crops across the United States coastline with no direct cause. Many commercially available oyster lines are locally adapted and, while there are studies addressing some of the suspected primary causes in the northeastern United States, there is significantly smaller body of work that focuses on oysters the Gulf of Mexico. As a result, there is a clear need to study the energetic cost that environmental conditions have on oysters when ambient conditions become unfavorable in the Gulf of Mexico. To address some of these knowledge gaps, two studies were conducted to determine the impact of PFAS bioaccumulation and depuration on energetic cost. Longer-chained PFAS compounds bioaccumulated more rapidly than shorter chained compounds but, regardless of chain length, PFAS concentrations were at undetectable levels within 24 hours after being placed in clean water. Respirometry identified that the act of rapidly depurating PFAS was not energetically expensive in adult oysters. When under combined stressors (PFAS exposure and elevated temperature (33°C)), oysters under thermal stress exhibited a higher metabolic rate but there was no compounded energetic cost to PFAS exposure, suggesting that, at temperatures above what is considered optimal for GOM oysters, PFAS is likely not contributing to mortalities in farmed and wild oyster populations. To address concerns related to environmental stress on polyploidy oysters, two studies were conducted on diploid and triploid oysters to identify which conditions increased susceptibility of triploids to acute stressors commonly associated with Mobile Bay. Acute thermal ramps revealed that diploid and triploids did not have a significant difference in the temperature at which metabolic, behavioral, and lethal temperatures occurred. It was observed that triploid metabolic rates were lower than diploid oysters, possibly due to inefficient gas exchange across characteristically larger triploid oyster cells. When acute thermal stress was conducted under hyposaline conditions, triploid oysters showed an increased susceptibility by having significantly increased closure frequency and lower respiration rate compared to diploid oysters. Regardless of ploidy, hyposaline events coinciding with high temperature resulted in increased closure frequency and a significant decrease in the temperature at which metabolic peak occurred, demonstrating that metabolic stress during these coinciding stressors likely play a role in oyster mortality.