The nautilus, commonly known as a "living fossil," is endangered and may be at risk of extinction. The lack of genomic information hinders a thorough understanding of its biology and evolution, which can shed light on the conservation of this endangered species. Here, we report the first high-quality chromosome-level genome assembly of Nautilus pompilius. The assembled genome size comprised 785.15 Mb. Comparative genomic analyses indicated that transposable elements (TEs) and large-scale genome reorganizations may have driven lineage-specific evolution in the cephalopods.Remarkably, evolving conserved genes and recent TE insertion activities were identified in N. pompilius, and we speculate that these findings reflect the strong adaptability and long-term survival of the nautilus. We also identified gene families that are potentially responsible for specific adaptation and evolution events. Our study provides unprecedented insights into the specialized biology and evolution of N. pompilius, and the results serve as an important resource for future conservation genomics of the nautilus and closely related species.

In recent years, suboptimal environmental conditions have been frequently reported to negatively impact on aquaculture organisms. Salinity is an important environmental factor, and its fluctuation will affect survival, growth and, potentially, the meat quality of the organisms with narrow salt tolerance cultured in coastal and inner bay. The aim of the study was to ascertain and report the influence of sub‐low salinity (28 and 30) on the production traits and nutrient composition of the Pacific abalone Haliotis discus hannai (DD), and two of its hybrids (DF and SD). In view of this, 360 abalones were subjected to four salinity levels (28, 30, 32 and 34) for ninety days. The data showed that the weight gain and the specific growth rates in wet weight between the groups followed the trend of DF > SD > DD. DF recorded the highest meat yield, which was significantly higher than SD and DD (p < .05). The sub‐low salinities tested did not influence moisture content, crude protein, total lipid and total carbohydrate (p > .05). Whilst lipid levels were fairly higher in SD and DF than in DD, at sub‐low salinities, the opposite was true for carbohydrate content. Total minerals (ash), however, were influenced by salinity (p < .05). Individuals reared at 28 significantly differed from those of the other three treatments, but there was no significant difference between the groups (p > .05). Further analysis showed sodium and potassium as the predominant minerals—with a higher concentration of sodium at higher salinities, though no significant difference was found between individuals treated at 30, 32 and 34. Zinc was the only mineral not affected by salinity (p > .05), and the accumulation of discrete minerals by the species significantly differed from one mineral to the other and proved to be species‐specific. Notwithstanding, SD demonstrated superior content of most essential minerals over DF and DD. The results in this paper infer that 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 sub‐low salinity of 28.

No abstract

Background Transcriptome sequencing is an effective tool to reveal the essential genes and pathways underlying countless biotic and abiotic stress adaptation mechanisms. Although severely challenged by diverse environmental conditions, the Pacific abalone Haliotis discus hannai remains a high-value aquaculture mollusk and a Chinese predominantly cultured abalone species. Salinity is one of such environmental factors whose fluctuation could significantly affect the abalone’s cellular and molecular immune responses and result in high mortality and reduced growth rate during prolonged exposure. Meanwhile, hybrids have shown superiority in tolerating diverse environmental stresses over their purebred counterparts and have gained admiration in the Chinese abalone aquaculture industry. The objective of this study was to investigate the molecular and cellular mechanisms of low salinity adaptation in abalone. Therefore, this study used transcriptome analysis of the gill tissues and flow cytometric analysis of hemolymph of H. discus hannai (DD) and interspecific hybrid H. discus hannai ♀ x H. fulgens ♂ (DF) during low salinity exposure. Also, the survival and growth rate of the species under various salinities were assessed. Results The transcriptome data revealed that the differentially expressed genes (DEGs) were significantly enriched on the fluid shear stress and atherosclerosis (FSS) pathway. Meanwhile, the expression profiles of some essential genes involved in this pathway suggest that abalone significantly up-regulated calmodulin-4 (CaM-4) and heat-shock protein90 (HSP90), and significantly down-regulated tumor necrosis factor (TNF), bone morphogenetic protein-4 (BMP-4), and nuclear factor kappa B (NF-kB). Also, the hybrid DF showed significantly higher and sustained expression of CaM and HSP90, significantly higher phagocytosis, significantly lower hemocyte mortality, and significantly higher survival at low salinity, suggesting a more active molecular and hemocyte-mediated immune response and a more efficient capacity to tolerate low salinity than DD. Conclusions Our study argues that the abalone CaM gene might be necessary to maintain ion equilibrium while HSP90 can offset the adverse changes caused by low salinity, thereby preventing damage to gill epithelial cells (ECs). The data reveal a potential molecular mechanism by which abalone responds to low salinity and confirms that hybridization could be a method for breeding more stress-resilient aquatic species.

Hybridization and backcrossing are classical breeding methods that can effectively improve the genetic characteristics of organisms through heterosis. The hybrid abalone (Haliotis discus hannai ♀ × H. fulgens ♂, DF) and backcrosses are cultured on a large scale due to their high survival rates under heat stress in summer. The thermal resistance of purebreds, hybrid F1 and backcross populations were evaluated through Arrhenius break temperature (ABT) of cardiac performance in this study. The results showed that the hybrid F1 had the best thermal resistance, and the ABT gradually decreased with the increase in recurring number. In addition, a growth experiment of DD × DF (H. discus hannai ♀ × DF ♂) populations showed that DD × DF‐Y (presence of black spots in the visceral mass of DD × DF) grew faster than DD × DF‐N (absence of spots in the visceral mass of DD × DF) (p < 0.05), but the ABT of DD × DF‐Y was lower than that of DD × DF‐N (p < 0.05). The results suggest that the presence or absence of black spots in the visceral mass could be used as a breeding trait for the thermal resistance and growth of backcross abalone. This study provides basic data for further study of the response of abalone to heat stress for the healthy and sustainable development of the abalone industry.