Scott Rikard | USGS Science

USGS2024

Rates of osmoconformation in triploid eastern oysters, and comparison to their diploid half-siblings

Triploid eastern oysters ( Crassostrea virginica ) suffer greater mortalities than diploids in the U.S. Gulf of Mexico estuaries when extreme low salinities (< 5) and elevated temperatures (≥ 28 °C) coincide. To investigate potential causes, changes in plasma osmolality, hemolymph pH, valve opening and mortality in diploid and triploid oyster half-siblings were compared during a step-down gradual acclimation from a salinity of 5 to 1 (5, 2.5, 2.0, 1.5, 1.0) at 23 °C (expt 1) and at 28 °C (expt 2). To further explore differences in diploid and triploid oyster responses to changing salinity, we compared their plasma osmolality after abrupt decreases in salinity from 20 to 10 and 5, followed by increases in salinity from 5 and 10 to 20 once oysters had osmoconformed to the lowered salinities (expt 3). Lastly, changes in wet weights of mantle tissue explants were compared between diploid and triploid oysters every 10 min for 40 min after being transferred from a salinity of 20 to 10 (expt 4). Oysters of both ploidies were able to osmoconform to water at salinities between 5 and 1.5. After a decrease in salinity, triploid oysters were slower to open their valves and osmoconform, were less efficient in maintaining acid-base status enduring longer periods of acidic hemolymph pH, and were less efficient in regulating tissue water content compared to half-sibling diploid oysters. At a salinity of 1.0, plasma of both diploid and triploid oysters remained hyperosmotic, their hemolymph acidic and their valves closed. Oysters osmoconformed faster at 28 °C than at 23 °C, but the combination of low salinity (≤ 1.5) and higher temperature caused rapid mortalities regardless of ploidies. Triploid oysters, however, started dying earlier and at greater percentages when salinity of 1.5 and temperature of 28 °C were combined. Triploids have been embraced as a means to support higher production, but results indicate superimposed stressors, such as low salinity and high temperature, may be more lethal to triploid than diploid oysters.

Aquaculture↗

USGS2023

Hypoxia and anoxia tolerance in diploid and triploid eastern oysters at high temperature

Increasing reliance on the use of triploid oysters to support aquaculture production relies on their generally superior growth rate and meat quality over that of diploid oysters. Reports of elevated triploid mortality have generated questions about potential trade-offs between growth and tolerance to environmental stressors. These questions are particularly relevant as climate change, coastal activities, and river management impact water salinity, temperature, nutrients, pH, and oxygen levels within key estuarine oyster growing areas. In particular, the co-occurrence of warm water temperatures and low dissolved oxygen concentration (DO) events are increasingly reported in estuaries, with potentially lethal impacts on sessile, oyster resources. To investigate potential differences in DO tolerance, diploid and triploid market-sized or seed oysters were exposed to continuous normoxia (DO > 5.0 mg L –1 ), hypoxia (DO < 2.0 mg L –1 ), and anoxia (DO < 0.5 mg L –1 ) at 28°C and their mortalities were monitored. The hemolymph of the market-sized oysters was collected to measure cellular and biochemical changes in response to hypoxia and anoxia, whereas their valve movements were also measured. In general, about half of market-sized oysters died within about 1 wk under anoxia (LT 50 : 5.7–8.9 days) and within about 2 wk under hypoxia (LT 50 : 11.9–19.4 days) with diploid oysters tending to die faster than triploid oysters. Seed oysters took longer to die than market-sized oysters under both anoxia (LT 50 : 9.5–12.1 days) and hypoxia (LT 50 : 21.8–25.0 days) with diploid oysters (LT 50 : 9.5–11.8 days) dying slightly faster than triploid oysters (LT 50 : 11.8–12.1 days) under anoxia. Hemolymph pH decreased and plasma calcium and glutathione concentrations increased with decreasing DO, with values under anoxia being different than those under normoxia. Hemocyte density was also lower under anoxia than under either normoxia or hypoxia. Overall, few differences in physiological responses to hypoxia and anoxia were found between diploid and triploid oysters suggesting that ploidy (2N versus 3N) had limited effect on the tolerance and response of eastern oysters to low DO.

Journal of Shellfish Research↗