Biodiversity and distribution patterns of blooming jellyfish in the Bohai Sea revealed by eDNA metabarcoding.

BACKGROUND: The mass occurrence of scyphozoan jellyfish severely affects marine ecosystems and coastal economies, and the study of blooming jellyfish population dynamics has emerged in response. However, traditional ecological survey methods required for such research have difficulties in detecting cryptic life stages and surveying population dynamics owing to high spatiotemporal variations in their occurrence. The environmental DNA (eDNA) technique is an effective tool for overcoming these limitations. RESULTS: In this study, we investigated the biodiversity and spatial distribution characteristics of blooming jellyfish in the Bohai Sea of China using an eDNA metabarcoding approach, which covered the surface, middle, and bottom seawater layers, and sediments. Six jellyfish taxa were identified, of which Aurelia coerulea, Nemopilema nomurai, and Cyanea nozakii were the most dominant. These three blooming jellyfish presented a marked vertical distribution pattern in the offshore regions. A. coerulea was mainly distributed in the surface layer, whereas C. nozakii and N. nomurai showed a upper-middle and middle-bottom aggregation, respectively. Horizontally, A. coerulea and C. nozakii were more abundant in the inshore regions, whereas N. nomurai was mainly distributed offshore. Spearman's correlation analysis revealed a strong correlation between the eDNA of the three dominant blooming jellyfish species and temperature, salinity, and nutrients. CONCLUSIONS: Our study confirms the applicability of the eDNA approach to both biodiverstiy evaluation of blooming jellyfish and investigating their spatial distribution, and it can be used as a supplementary tool to traditional survey methods.

Physiological and transcriptomic responses of Aurelia coerulea polyps to acidified seawater conditions.

Scyphozoan jellyfish, known for their evolutionary position and ecological significance, are thought to exhibit relatively notable resilience to ocean acidification. However, knowledge regarding the molecular mechanisms underlying the scyphozoan jellyfish response to acidified seawater conditions is currently lacking. In this study, two independent experiments were conducted to determine the physiological and molecular responses of moon jellyfish (Aurelia coerulea) polyps to within- and trans-generational exposure to two reduced pH treatments (pH 7.8 and pH 7.6). The results revealed that the asexual reproduction of A. coerulea polyps significantly declined under acute exposure to pH 7.6 compared with that of polyps at ambient pH conditions. Transcriptomics revealed a notable upregulation of genes involved in immunity and cytoskeleton components. In contrast, genes associated with metabolism were downregulated in response to reduced pH treatments after 6 weeks of within-generational acidified conditions. However, reduced pH treatments had no significant influence on the asexual reproduction of A. coerulea polyps after exposure to acidified conditions over a total of five generations, suggesting that A. coerulea polyps may acclimate to low pH levels. Transcriptomics revealed distinct gene expression profiles between within- and trans-generational exposure groups to two reduced pH treatments. The offspring polyps of A. coerulea subjected to trans-generational acidified conditions exhibited both upregulated and downregulated expression of genes associated with metabolism. These physiological and transcriptomic characteristics of A. coerulea polyps in response to elevated CO2 levels suggest that polyps produced asexually under acidified conditions may be resilient to such conditions in the future.

Environmental and molecular regulation of diapause formation in a scyphozoan jellyfish.

Understanding the mechanisms underlying diapause formation is crucial for gaining insight into adaptive survival strategies across various species. In this study, we aimed to uncover the pivotal role of temperature and food availability in regulating diapausing podocyst formation in the jellyfish Aurelia coerulea. Furthermore, we explored the cellular and molecular basis of diapause formation using single-cell RNA sequencing. Our results showed cell-type-specific transcriptional landscapes during podocyst formation, which were underscored by the activation of specific transcription factors and signalling pathways. In addition, we found that the heat shock protein-coding genes HSC70 and HSP90a potentially act as hub genes that regulate podocyst formation. Finally, we mapped the single-cell atlas of diapausing podocysts and identified cell types involved in metabolism, environmental sensing, defence and development that may collectively contribute to the long-term survival and regulated excystment of diapausing podocysts. Taken together, the findings of this study provide novel insights into the molecular mechanisms that regulate diapause formation and contributes to a better understanding of adaptive survival strategies in a variety of ecological contexts.