Most molluscs possess shells, constructed from a vast array of microstructures and architectures. The fully formed shell is composed of calcite or aragonite. These CaCO 3 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 CaCO 3 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. However, more sophisticated experiments incorporating selective breeding and multiple generations are identifying subtle effects and that variability within mollusc genomes has potential for adaption to future conditions. Furthermore, we highlight recent historical studies based on museum collections that demonstrate a greater resilience of molluscs to climate change compared with experimental data. The future of mollusc research lies not solely with ecological investigations into biodiversity, and this review synthesizes knowledge across disciplines to understand biomineralization. It spans research ranging from evolution and development, through predictions of biodiversity prospects and future-proofing of aquaculture to identifying new biomimetic opportunities and societal benefits from recycling shell products.
Bibliographical noteFunding Information:
This manuscript was partly funded by the European Union Seventh Framework Programme [FP7] ITN project ?CACHE: Calcium in a Changing Environment? (www.cache-itn.eu) under REA grant agreement 605051. The authors thank everyone involved in this project including Mark Blaxter, Xushuai Zhang, Yan Wang-Duffort, Nina Fox, Rita Pereira, Nicola Munro, Elaina Ford, Lucy Gonzalez, Christina Chatzela, Rachel Ramirez, our EU Project Officer Giuliana Donini and EU Mid-Term Review Expert Guy Duke. Also, many thanks to our Project Partners: the Association of Scottish Shellfish Growers, specifically Nick Lake, Janet Brown and Walter Speirs, Coastal Research and Management, Kiel especially Peter Krost, and our External Experts Professor Catherine Boyen, Station Biologique de Roscoff and Professor Mike Thorndyke. We also thank Laura Gerrish and Jamie Oliver (British Antarctic Survey) for drawing figures. Additional funds were provided by Funda??o para a Ci?ncia e a Tecnologia (FCT) through project UID/Multi/04326/2019 and the European Marine Biological Research Infrastructure Cluster-EMBRIC (EU H2020 research and innovation program, agreement n? 654008) and a Natural Environment Research Council Studentship (Project Reference: NE/J500173/1) to V.A.S.
© 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
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- integrative biomineralization
- ion channels
- phenotypic plasticity