At the center, six interacting sclerocytes are forming a ‘Mercedes star-shaped’ calcitic spicule, expressing the genes Calcarin1 (magenta), Calcarin2 (cyan), and Spiculin (yellow). Surrounding this central structure are other sclerocytes expressing Calcarin2 and Spiculin while producing spicules of a different shape. Image credit: Voigt et al. (CC BY 4.0)
Many animals – from corals to snails to humans – build hard structures such as shells, teeth, and skeletons through a process called biomineralization. In this process, specialized cells deposit minerals like calcium carbonate with the help of dedicated proteins and their genes.
Biomineralization has evolved independently across different animal lineages, often by reusing or modifying existing genes. Among the earliest animals to produce mineralized structures were sponges, many of which form skeletal elements known as spicules. Calcareous sponges, for example, build spicules from calcite, but the underlying genetic and molecular mechanisms remain poorly understood. By contrast, stony corals, which produce aragonite skeletons, have a relatively well-characterized “biomineralization toolkit.” Studying how sponges construct their spicules, therefore, offers a unique opportunity to explore the origins and diversity of biomineralization strategies across animals.
To address this, Voigt et al. investigated the genes and proteins involved in forming calcitic spicules in the calcareous sponge Sycon ciliatum. This species is an excellent model because each spicule is produced by only a few specialized cells, enabling precise analysis of gene expression during biomineralization in a simple system.
Using a combination of transcriptomic, genomic, and proteomic approaches, and a method called RNA in situ hybridization, the team identified 829 genes that were upregulated in spicule-forming areas in the sponge. Among them were 17 calcarins – a newly described group of galaxin-like proteins (which are proteins also found in corals) that were expressed in mineral-secreting cells known as sclerocytes and incorporated into the spicule matrix.
These proteins appear to be unique to calcareous sponges and absent from other sponge lineages. Different spicule types showed distinct patterns of gene activation, suggesting specialized roles. Evidence of gene duplication and neofunctionalization – that is, duplicated genes adopting a new role – was also found. Together, these results suggest molecular parallels between the calcitic spicules of sponges and the aragonitic skeletons of corals, despite their independent evolutionary origins.
These findings deepen our understanding of the evolution and regulation of biomineralization – a process central to marine ecosystems. They may inform future research into how reef-building organisms such as corals and sponges respond to environmental challenges like climate change and ocean acidification. Insights into how organisms control mineral formation could inspire biomimetic materials and sustainable synthesis techniques. Future studies will be needed to explore the functional roles of these genes and their ecological significance.
