Acquired thermotolerance in Crassostrea angulata: Effects of priming temperature, recovery temperature and duration on the survival and expression of HSP genes

Acquired tolerance to heat stress is an increased resistance to elevated temperature following a prior exposure to heat. Acquired thermotolerance can significantly increase bivalve's heat adaptation capacity while leaving its basal thermotolerance unchanged. However, its characteristics are not yet fully understood, which limit its potential for utilization in enhancing heat tolerance. This study systematically characterizes the acquired thermotolerance of adult Fujian oyster by observing the effects of different priming temperatures (34 °C, 36 °C, 38 °C, 40 °C, and 42 °C with 1 h), recovery temperatures (16 °C, 20 °C, 24 °C, 28 °C, and 32 °C with 72 h) and durations (0 h, 4 h, 24 h, 3 days, 7 days, 14 days, 21 days, 28 days, 35 days, and 40 days under 20 °C) on the survival rates and the expression of heat shock protein genes (HSP10, HSP27, HSP68 and HSP90) in gill. Results indicated that a priming temperature of 34 °C did not induce tolerance to lethal heat stress. However, thermotolerance increased with increasing priming temperature in the range of 36 °C to 40 °C. In addition, it was found that acquired thermotolerance can be achieved with a lower recovery temperature, but can be reduced by a high recovery temperature (> 24 °C). It was also observed that a recovery durations of 0 h and 4 h are insufficient to induce thermotolerance. The maximum duration of acquired thermotolerance is 40 days in the Fujian oyster and that a high level of thermotolerance, i.e. >75% survival after lethal stimulus, can be maintained for 21 days. As the recovery duration increased from 24 h to 40 days, the expression of four HSP genes decreased with various speed and extent. HSP68 decreased the fastest, while HSP27 and HSP90 decreased moderately. HSP10 decreased the slowest, and its expression was consistently higher than that of the control group. Furthermore, the expression of these four genes at 72 h after the lethal stimulus increased with the extension of recovery duration. This study presents a systematic analysis of the acquired thermotolerance of the Fujian oyster. The findings expand our understanding of oyster thermotolerance and provide a potential strategy to enhance the thermotolerance of oysters by altering the external environment. For example, the temperature could be artificially adjusted during the transfer of spat and intermediates, or the water temperatures could be adjusted by adjusting the water layer in which the oysters are cultured at sea.