Investigating byssus self-assembly of Mytilus coruscus: synergy of biological regulations and spatial heterogeneity in mussel foot

Mussels fabricate byssus to anchor themselves on diverse substrates, a process that is crucial for their survival in dynamic marine environments, as well as for aquaculture productivity. Currently, many studies have focused on the composition of mussel adhesive proteins (MAPs) and their self-assembly in vivo and in vitro. However, the biological regulation of byssus fabrication other than MAPs has been largely overlooked. Given the established roles of Ca in cellular regulation, as well as its frequent detection in byssus and the environment, Ca (50 mmol/L) was applied as an environmental stimulus to investigate the regulatory mechanisms of byssus in thick-shelled mussels, Mytilus coruscus. After 7 days of exposure, the secretion behavior of mussels, histology of the three foot sections (anterior, middle, and posterior), anterior-section transcriptomics, and section-specific gene expression were integratively analyzed. The results revealed that Ca exposure inhibited macroscale byssus self-assembly, but the self-assembly of microscale byssus precursors and histological status in the different foot sections were not affected. Moreover, no negative effects on MAP expression were identified. However, the cytoskeleton-based secretory process became imbalanced, and the gene expression in the three foot sections was disordered (e.g., genes encoding actin, tubulin, and calponin). In addition, Ca-induced foot paralysis may be accompanied by negatively regulated neurotransmission and muscle contraction, which may account for the absence of macroscale byssus. Overall, this study demonstrated that MAPs were not decisive in the self-assembly of byssus. Other biological regulatory processes, as well as the spatial heterogeneity of the mussel foot, should be considered when exploring the self-assembly mechanisms of byssus. This may then provide cues for mussel aquaculture and the development of novel biomimetic materials.