Selection of low-salinity resistant strains and the underlying mechanisms in Bohai Red scallops

Seasonal or regional low salinity in estuarine and coastal waters, the key zones for bivalve aquaculture, often impose severe challenges to the survival and reproduction of shellfish, whose salinity tolerance range could potentially restrain their distribution and successful culture. Thus, the selection of low-salinity strains is essential for the aquaculture industry, especially for the most cultured species such as the Bohai Red scallops. In this study, we first determined the 96-h median lethal salinity (LS50 = 14.42 ± 0.07 ‰) for Bohai Red scallops and then exposed the Bohai Red scallops to seawater with a salinity of LS50 (14.42 ‰) until about 90 % of the scallops died. The surviving scallops were considered as the low-salt resistant group (Group L) and used as broodstocks to reproduce the low-salt resistant F1 offspring (F1L) while the Bohai Red scallops not exposed to low salinity stress were designated as the control group (Group C) and used as broodstocks to produce the control F1 offspring (F1C). Low salt tolerance experiments revealed that the LS50 of the F1L group (14.24 ± 0.03 ‰) was significantly lower than that of the F1C group (14.52 ± 0.09 ‰). Histological examination of the gills showed that, while the gill filament structure in the control group became extended under low salinity, that of the F1L group largely remained intact, indicating better adaptability to low salinity in the F1L. Measurement of the activities of the antioxidant enzymes showed that the F1L group maintained higher antioxidant capacity than the control group at low salinity. Transcriptomic analyses and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses revealed that glutathione metabolism is an important pathway for resistance to low salinity stress. Weighted Gene Co-expression Network Analysis (WGCNA) indicated that the most abundant module (blue module) was associated with ion channel activity, suggesting that maintaining osmotic pressure homeostasis may be a key mechanism in scallops. We also found that the Duox gene may play a crucial role in the resistance to low salinity stress in the Bohai Red scallops. Thus, we have shown that it is possible to select low-salinity resistant strains for culture areas with low salinity by stressing the broodstocks and have uncovered the mechanisms underlying low-salt stress tolerance in the scallops. Our results may help the selection of low-salinity resistant strains and the expansion of culture areas for the Bohai Red scallops.