Mollusc aquaculture is advocated as a highly sustainable food source and may play an important role in future food security globally. With production increasing worldwide, it is timely to appraise all aspects of aquaculture when considering its expanding role as a food source. In this regard, one regularly overlooked aspect of mollusc aquaculture is waste generation: namely the production of calcareous shells. Shells from the aquaculture industry are widely regarded as a nuisance waste product, yet at the same time, calcium carbonate is mined in the form of limestone and viewed as a valuable commodity. In a time of increased awareness of the need for a circular economy, the aquaculture and seafood industry should consider shells as a valuable biomaterial that can be reused for both environmental and economic benefit. This review discusses the current waste shell issue and identifies large‐scale shell applications that are already in place. Further, it highlights proposed applications that have the potential to be scaled up to address the problem of waste shell accumulations and reduce our reliance on environmentally damaging incineration and landfill disposal. Of the plethora of shell valorisation techniques proposed in the scientific literature, this review will focus only on those that can incorporate large‐scale shell utilisation, and do not require high‐energy processing, and are thus; simple, sustainable and potentially economically viable. Further, this review questions whether, in many cases, shells can provide more inherent value being returned to the marine environment rather than being used in land‐based applications.

There are few factors more important to the mechanisms of evolution than stress. The stress response has formed as a result of natural selection, improving the capacity of organisms to withstand situations that require action. The ubiquity of the cellular stress response suggests that effective mechanisms to counteract stress emerged early in the history of life, and their commonality proves how vital such mechanisms are to operative evolution. The cellular stress response (CSR) has been identified as a characteristic of cells in all three domains of life and consists of a core 44 proteins that are structurally highly conserved and that have been termed the 'minimal stress proteome' (MSP). Within the MSP, the most intensely researched proteins are a family of heat-shock proteins known as HSP70. Superficially, correlations between the induction of stress and HSP70 differential expression support the use of HSP70 expression as a nonspecific biomarker of stress. However, we argue that too often authors have failed to question exactly what HSP70 differential expression signifies. Herein, we argue that HSP70 up-regulation in response to stressors has been shown to be far more complex than the commonly accepted quasi-linear relationship. In addition, in many instances, the uncertain identity and function of heat-shock proteins and heat-shock cognates has led to difficulties in interpretation of reports of inducible heat-shock proteins and constitutive heat-shock cognates. We caution against the broad application of HSP70 as a biomarker of stress in isolation and conclude that the application of HSP70 as a meaningful index of stress requires a higher degree of validation than the majority of research currently undertakes.

Most molluscs possess shells, constructed from a vast array of microstructures and architectures. The fully formed shell is composed of calcite or aragonite. These CaCO3 crystals form complex biocomposites with proteins, which although typically less than 5% of total shell mass, play significant roles in determining shell microstructure. Despite much research effort, large knowledge gaps remain in how molluscs construct and maintain their shells, and how they produce such a great diversity of forms. Here we synthesize results on how shell shape, microstructure, composition and organic content vary among, and within, species in response to numerous biotic and abiotic factors. At the local level, temperature, food supply and predation cues significantly affect shell morphology, whilst salinity has a much stronger influence across latitudes. Moreover, we emphasize how advances in genomic technologies [e.g. restriction site‐associated DNA sequencing (RAD‐Seq) and epigenetics] allow detailed examinations of whether morphological changes result from phenotypic plasticity or genetic adaptation, or a combination of these. RAD‐Seq has already identified single nucleotide polymorphisms associated with temperature and aquaculture practices, whilst epigenetic processes have been shown significantly to modify shell construction to local conditions in, for example, Antarctica and New Zealand. We also synthesize results on the costs of shell construction and explore how these affect energetic trade‐offs in animal metabolism. The cellular costs are still debated, with CaCO3 precipitation estimates ranging from 1–2 J/mg to 17–55 J/mg depending on experimental and environmental conditions. However, organic components are more expensive (~29 J/mg) and recent data indicate transmembrane calcium ion transporters can involve considerable costs. This review emphasizes the role that molecular analyses have played in demonstrating multiple evolutionary origins of biomineralization genes. Although these are characterized by lineage‐specific proteins and unique combinations of co‐opted genes, a small set of protein domains have been identified as a conserved biomineralization tool box. We further highlight the use of sequence data sets in providing candidate genes for in situ localization and protein function studies. The former has elucidated gene expression modularity in mantle tissue, improving understanding of the diversity of shell morphology synthesis. RNA interference (RNAi) and clustered regularly interspersed short palindromic repeats ‐ CRISPR‐associated protein 9 (CRISPR‐Cas9) experiments have provided proof of concept for use in the functional investigation of mollusc gene sequences, showing for example that Pif (aragonite‐binding) protein plays a significant role in structured nacre crystal growth and that the Lsdia1 gene sets shell chirality in Lymnaea stagnalis. Much research has focused on the impacts of ocean acidification on molluscs. Initial studies were predominantly pessimistic for future molluscan biodiversity...

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