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Mytilus coruscus is a dominant shellfish in the Yangtze estuary and its adjacent sea area. Food deprivation often occurs during their growth due to fluctuations in algal abundance caused by seasonal freshwater flushing and high-density aquaculture mode. To investigate the coping strategies of M. coruscus to starvation stress, electron microscopy and differential proteomic analysis were performed on the critical feeding organ gill of the mussels after 9 days of starvation. The electron microscopy results showed that the cilia of the mussel gills were dissolved, and the gaps between gill filaments widened under starvation. Differential proteomic analysis revealed that phagocytosis-related proteins such as ATPeV1E, ATPeV1C, LAMP1_2 and CTSL were significantly upregulated, and the phagocytosis pathway was significantly enriched (p < 0.05). In addition, the corin content in gill and myeloperoxidase level as well as the number of dead cells in blood were both significantly increased (p < 0.05). What’s more, proteomic data suggested that immune maintenance, cellular transport and metabolism related pathways were significantly enriched, which illustrated an immune and metabolism responses under starvation. This study reveals for the first time that phagocytosis functions as an essential strategy for M. coruscus to cope with starvation, which provides new scientific knowledge and a theoretical basis for understanding the adaptation mechanisms of mussel to starvation and for rational optimization of mussel culture patterns.

Being an industrially valuable species of bivalve, Mytilus is widespread in the area around the eastern coast of China. Starvation is an environmental stress that mussels often encounter in their growth. However, few report carried out on the physiological and biochemical response and molecular regulation and adaptation mechanism of mussels under starvation. To illustrate the molecular mechanism of hosts of Mytilus to starvation, the gill transcriptome was used for analysis before and after 9 days of starvation in Mytilus using the Illumina/HiSeq-2000 deep sequencing platform, accompanied by the study of differentially expressed genes (DEGs). In total, 42.137 GB of clean data were obtained from six sample libraries, the average amount of clean data of each sample is 7.023 GB, and 58,540 unigenes that average 780 bp in length were assembled. Unigenes were illustrated by commenting them against the NR, Swiss-Prot, Pfam, String, GO and KEGG databases. After 9 days of starvation, 2,188 and 2,672 genes were determined to be significantly up- or down-regulated expression genes, separately. Amongst, 4,860 genes were associated with 260 pathways, contains vital enrichment pathways, such as “Metabolic”, “PI3K-Akt signaling pathway”, “Phagosome”, “Apoptosis” and “Lysosome”. In inclusion, autophagy cells were investigated by an electron microscopy, alongside further observations of the expression of autophagy-related genes in gill tissue before and after starvation. The data indicates that the amount of autophagosomes increased and found that atg2, atg6 and atg13 were significantly up-regulated. These results indicated that Mytilus may use autophagy to cope with their damage after starvation. With our research results, we have contributed to a deeper understanding of the role of the molecular mechanisms of immune defence in Mytilus under environmental stress, which has given insights into mussel breeding and the understanding of the molecular mechanisms of Mytilus immune system.

Microalgae are the primary source of food and energy for marine bivalves, and hence play a vital role in their development and growth. To characterize the microalgal feeding preference and the effects of fatty acid composition and digestive gland microorganisms on Mytilus, four microalgal species (Chlorella vulgaris, Chaetoceros muelleri, Isochrysis zhanjiangensis and Platymonas helgolandica) were used in a mix and single feeding experiments. The feeding preference in the mix feeding was determined using quantitative PCR analysis of water samples obtained at various time periods from biodeposits. The results demonstrated that C. muelleri and I. zhanjiangensis were selectively filtered by M. coruscus, while C. vulgaris and P. helgolandica were preferentially rejected. Individuals of M. coruscus fed with C. muelleri and I. zhanjiangensis had higher levels of fatty acids contents, particularly some unsaturated fatty acids like eicosapentaenoic acid (EPA), which rose by 63.6% after 28 days feeding with C. muelleri. In the digestive glands of M. coruscus fed these two microalgae diets, the relative abundance of digestion-related bacteria such as Polaribacter was also dramatically enhanced, indicating that they had a greater ability to degrade and metabolize microalgal polysaccharides. The metabolic pathway may also play a dominant role as a feedback reaction to microalgal diet feeding, according to potential function prediction. In comparison to C. vulgaris and P. helgolandica, our findings showed that C. muelleri and I. zhanjiangensis had a better nutritional value and could be utilized more efficiently by M. coruscus, implying that C. muelleri and I. zhanjiangensis may be employed to boost M. coruscus growth and output.