Sexual reproduction is a fundamental process essential for species persistence, evolution, and diversity. However, unprecedented oceanographic shifts due to climate change can impact physiological processes, with important implications for sexual reproduction. Identifying bottlenecks and vulnerable stages in reproductive cycles will enable better prediction of the organism, population, community, and global-level consequences of ocean change. This article reviews how ocean acidification impacts sexual reproductive processes in marine invertebrates and highlights current research gaps. We focus on five economically and ecologically important taxonomic groups: cnidarians, crustaceans, echinoderms, molluscs and ascidians. We discuss the spatial and temporal variability of experimental designs, identify trends of performance in acidified conditions in the context of early reproductive traits (gametogenesis, fertilization, and reproductive resource allocation), and provide a quantitative meta-analysis of the published literature to assess the effects of low pH on fertilization rates across taxa. A total of 129 published studies investigated the effects of ocean acidification on 122 species in selected taxa. The impact of ocean acidification is dependent on taxa, the specific reproductive process examined, and study location. Our meta-analysis reveals that fertilization rate decreases as pH decreases, but effects are taxa-specific. Echinoderm fertilization appears more sensitive than molluscs to pH changes, and while data are limited, fertilization in cnidarians may be the most sensitive. Studies with echinoderms and bivalve molluscs are prevalent, while crustaceans and cephalopods are among the least studied species even though they constitute some of the largest fisheries worldwide. This lack of information has important implications for commercial aquaculture, wild fisheries, and conservation and restoration of wild populations. We recommend that studies expose organisms to different ocean acidification levels during the entire gametogenic cycle, and not only during the final stages before gametes or larvae are released. We argue for increased focus on fundamental reproductive processes and associated molecular mechanisms that may be vulnerable to shifts in ocean chemistry. Our recommendations for future research will allow for a better understanding of how reproduction in invertebrates will be affected in the context of a rapidly changing environment.
Hypoxia is one of the most important environmental stressors in aquatic ecosystems. To deal with the hypoxia environment, fishes exhibit a series of physiological and molecular responses to maintain homeostasis and organism functions. In the present study, hypoxia-induced changes in gene expression profiles and alternative splicing (AS) events in spotted sea bass (Lateolabrax maculatus), a promising marine-culture fish species in China, were thoroughly investigated by RNA-Seq analysis. A total of 1,242, 1,487 and 1,762 differentially expressed genes (DEGs) were identified at 3 h, 6 h and 12 h in gills after hypoxia stress. Functional enrichment analysis by KEGG and GSEA demonstrated that HIF signal network system was significantly activated and cell cycle process was remarkably suppressed in response to hypoxia. According to the temporal gene expression profiles, six clusters were generated and protein-protein interaction (PPI) networks were constructed for the two clusters that enriched with hypoxia-induced (cluster 2) or -suppressed genes (cluster 5), respectively. Results showed that HIF signaling related genes including vegfa, igf1, edn1, cox2b, cxcr4b, ctnnb1, and slc2a1a, were recognized as hubs in cluster 2, while mcm2, chek1, pole, mcm5, pola1, and rfc4, that tightly related to cell cycle, were down-regulated and considered as hubs in cluster 5. Furthermore, a total of 410 differential alternative splicing (DAS) genes were identified after hypoxia, which were closely associated with spliceosome. Of them, 63 DAS genes also showed differentially expressed levels after hypoxia, suggesting that their expression changes might be regulated by AS mechanism. This study revealed the key biological pathways and AS events affected by hypoxia, which would help us to better understand the molecular mechanisms of hypoxia response in spotted sea bass and other fish species.
High temperature and hypoxia greatly threaten marine life and aquaculture. Scallops, a diverse and ecologically important group of high economic value, mostly thrive in fluctuating environments, and are vulnerable to environmental stress. In the present study, the molecular response mechanism of scallops to a combination of environmental stressors was investigated via transcriptome analysis of the gill tissues in three scallop species, the Yesso scallop (Patinopecten yessoensis), Zhikong scallop (Chlamys farreri) and bay scallop (Argopecten irradians) that were exposed to transient heat, hypoxia and a combination thereof. The Yesso scallop had the most differentially expressed genes (DEGs) compared with the other two scallop species, indicating the highest sensitivity of the Yesso scallop to environmental stress. With increased temperature and decreased dissolved oxygen, the number of DEGs was greatly increased in the three scallop species, indicative of the enhancement in gene expression regulation in scallops in response to severe environmental changes. Heat and hypoxia had a synergistic effect on scallops. GO and KEGG enrichment analysis of DEGs under different stressors revealed overlapping molecular mechanisms of response in scallops following exposure to heat and hypoxia. Several immune and apoptosis-related pathways were highly enriched in the upregulated DEGs of the three scallops, suggesting that immune system activation and apoptosis promotion occurred in scallops in response to environmental stress. Heat shock proteins (HSPs) were significantly upregulated under heat and hypoxia, which likely assisted in correct protein folding to facilitate the adaption of the scallops to the altered environment. Additionally, the HIF-1 signaling pathway—the key pathway associated with hypoxia response—was triggered by extremely acute environmental changes. Comparative transcriptome analysis revealed 239 positively selected genes among the different scallops, including those involved in immune system and environmental adaptation, suggesting a long-term mechanism of environmental adaptation. The present study provides new insights into the molecular response mechanism in scallops to multiple environmental stressors and improves our understanding of the adaptive mechanisms of marine organisms under changing global climate conditions.
Heat shock protein 70 (HSP70) members participate in a wide range of housekeeping and stress-related activities in eukaryotic cells. In marine ecosystems, bivalves encounter abiotic stresses, including high temperatures and low dissolved oxygen. Here, 133 MmHSP70 genes were identified through combined methods including Blastp, HMM and manual filtration, based on the whole Mercenaria mercenaria genome. The MmHSP70 genes were unevenly distributed, and 41 genes (33.08%) were located on Chr 7. Phylogenetic analyses indicated that the MmHSP70 gene family mainly consisted of two clusters and the Hspa12 subfamily underwent lineage-specific expansion. A high-density collinear gene block was observed between M. mercenaria Chr 7 and Cyclina sinensis Chr 14. Tandem duplication MmHSP70 gene pairs experienced different levels of purifying selection, which could be an important source of sequence and functional constraints. MmHSP70 genes showed tissue-specific and stress-specific expression. Most tandem duplication HSP70 gene pairs had high expression under hypoxia stress. HSP70 B2 tandem duplication gene pairs showed significantly increased expression under heat plus severe hypoxia stress. This study provided a comprehensive understanding of the MmHSP70 gene family in the M. mercenaria and laid a significant foundation for further studies on the functional characteristics of MmHSP70 genes during exposure to heat and hypoxia stress.
The silver carp (Hypophthalmichthys molitrix) is an economically, as well as environmentally, important fish that harbors low environmental hypoxia tolerance and frequently contributes to a loss of aquaculture productivity. The gill is the first tissue attacked by hypoxia; however, the response of the gills of H. molitrix to hypoxia stress at the tissue, cellular, and molecular levels has not been clearly established. The influence of hypoxia on histological features along with gene expression in silver carp gills were explored in this research. The hematoxylin and eosin-stained sections and electron microscopy examinations of gills indicated that the gill lamellae were seriously twisted, gill filaments were dehisced, and the swelling and shedding of epithelial cell layer in the gill tissue were intensified along with the degree of hypoxia. In the hypoxia, semi-asphyxia, and asphyxia groups, the gill transcriptomic assessment of shifts in key genes, as well as modulatory networks in response to hypoxic conditions revealed 587, 725, and 748 differentially expressed genes, respectively. These genes are abundant in immune response signaling cascades (e.g., complement and coagulation cascades, Nucleotide-binding and oligomerization domain (NOD)-like receptor signaling cascade, and differentiation of Th1 along with Th2 cells) and oxygen transport [e.g., MAPK, PI3K-Akt, and hypoxia-inducible factor 1 (HIF-1) signaling cascades]. Genes linked to immune response (e.g., c2, c3, c6, klf4, cxcr4, cd45, and cd40) and oxygen transport (e.g., egln1, egln3, epo, ldh, and vegfa) were additionally identified. According to our findings, the silver carp may be using “HIF-1” to obtain additional oxygen during hypoxia. These findings illustrate that hypoxia stress might damage gill tissue, trigger an immunological response, and activate HIF-1 signaling to increase oxygen availability under hypoxic situations. The findings of this work will help scientists better understand the molecular mechanisms driving hypoxia responses in hypoxia-sensitive fish and speed up the development of hypoxia-resistant varieties.
Frontiers in Marine Science
Impacts of Climate Change on Seaweeds