| Abalone is considered high-value seafood with a continuously rising global market demand.Meanwhile,China is the world’s leading producer and consumer of abalone,and the Pacific abalone,Haliotis discus hannai is the predominantly cultured species.However,many environmental factors threaten the successful production of the species in China,leading to the introduction of new species such as hybrid abalones.Luckily,hybrids have gained popularity in Chinese abalone aquaculture because they offer a faster growth rate and higher survival rate and demonstrate superior tolerance to many environmental conditions.Salinity is a critical environmental factor affecting many marine invertebrates and determining their survival,growth,distribution,behavior,and many other physiological parameters.Generally,abalones were weaker osmoconformers,and the Pacific abalone is particularly sensitive to low salinity.Meanwhile,current climate change forecasts a persistent decline in ambient salinity of estuarine and coastal regions due to a prospective melting of polar ice caps,increased precipitation,typhoons and enormous flood occurrences.Therefore,the capability to adapt and recover from salinity stress is a vital indicator of the quality of aquaculture species and immense importance to the aquaculture industry.Besides,more research remains a prerequisite to further understanding the mechanism of salinity stress tolerance in abalone to aid genetic improvement and Selection of aquaculture sea area.Consequently,this study investigated the response of H.discus hannai(herein referred to as DD)and two of its hybrids,H.discus hannai ♀ x H.fulgens ♂(DF)and H.gigantea♀ x H.discus hannai ♂(SD),to salinity variations.Overall,the study assessed the influence of various salinities on the survival,growth and development at the embryonic,larval,juvenile,and adult stages of the abalone.Furthermore,under fluctuating salinities,the study adopted video recordings to understand the behavior of the abalone and used flow cytotometric analysis of hemolymph to understand some cellular mechanisms of low salinity adaptation.Moreover,this study adopted the non-invasive heart rate detection method to study the cardiac performance of abalone under fluctuating salinities and transcriptome analysis of the gill tissues to evaluate some candidate pathways involved in low-salinity adaptation in abalone.Finally,the effect of salinity variations on the meat quality of abalone was investigated using the proximate analysis method.The data suggest that juveniles and adult abalone could achieve unexpected survival rates at low salinities of 18(≈ 40%for DD and≈50%-60%for DF and SD)and 21(≈60%for SD and DD and≈80%for DF)for 60 days;however,at a significantly reduced growth rate.Thus,survival might be possible at low salinities that do not support growth.Meanwhile,survival and growth rate were significantly higher in the hybrid DF than in SD and DD,though SD’s performance was relatively better than DD.It can be seen that the hybrids showed obvious heterosis of low salt tolerance.However,below 27,salinity caused a significant increase in deformity,reduced development,settlement failure,and eventual death at the embryonic and larval stages.Therefore,the data hints that salinity 27 is the salinity cutoff for proper development and settlement of the embryos and larvae of the Pacific abalone and its hybrids.Moreover,the data suggest that phenotypic difference between the species is evident later in their embryonic development,and the capacity to maintain ionic and osmotic balance supposedly increases with age in the hybrid DF.Furthermore,a significant behavior change was observed when abalone was allowed a relatively limited time to adjust to the salinity changes.The movement was significantly reduced at low salinity than control.However,movement and survival rate at low salinity was significantly higher in the hybrid DF than DD,suggesting a better capacity of the hybrid to tolerate low salinity.Altogether,DD exhibited a "moving" behavior at control salinity but showed"staying and sedentary" behavior at low salinity,while DF demonstrated a "wandering and homing" behavior at the control and low salinity.The cardiac performance of the abalone suggests a pattern with three phases and two breakpoints of salinity(BOS1&BOS2),BOS1 representative of isosmotic point and BOS2 representative of lethal salinity.Meanwhile,the hybrid DF recorded the lowest BOS1 and BOS2,which was also significantly different(P=0.01)from DD at BOS1,suggesting a better tolerance potential for low salinities than the other species.Moreover,the data hints that abalone exhibits salinity-insensitive metabolism as one adaptive mechanism against low-salinity stress.Furthermore,the cellular mechanism of low salinity adaptation in abalone involves increased phagocytic activity and migration of hemocytes to the neighboring tissues that might be predisposed to injury.Meanwhile,at the molecular level,abalone engages the fluid shear stress and atherosclerosis(FSS)pathway and consequently up-regulates the genes that promote anti-oxidation and anti-apoptosis,such as calmodulin-4(CaM-4)and heat-shock protein90(HSP90).On the contrary,abalone significantly down-regulates the genes that promote oxidation and apoptosis,such as tumor necrosis factor(TNF),bone morphogenetic protein-4(BMP-4),nuclear factor kappa B(NF-kB),and mitogenactivated protein kinase 14(MAPK14/p38),thereby preventing gill epithelial cells(ECs)damage.Also,the hybrids showed significantly higher and sustained expression of CaM and HSP90,significantly higher phagocytosis,and significantly lower hemocyte mortality at low salinity,suggesting a more active molecular and hemocyte-mediated immune response and a more efficient capacity to tolerate low salinity than DD.Finally,assessing the abalone’s meat quality suggests that the low salinity of 28 will not negatively impact the crude protein,total lipid,total minerals,moisture content,and total carbohydrate.Meanwhile,the hybrid SD demonstrated a significantly high content of most essential minerals than DD and DF,while both hybrids showed somewhat higher lipid content than DD.Overall,the Pacific abalone and its hybrids could maintain good meat quality and survival under a range of 28-34 of salinity,promoting the growth of cultured abalone under the low salinity of 28.Altogether,this study reveals the salinity limit of the Pacific abalone and its hybrids at various developmental stages,shows some mechanisms of low salinity adaptation in abalone,and hints that hybridization could be a route to breed the more salinity stressresilient abalone species. |
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