Keratinization-like differentiation process forms chitinous dermal sclerites in the hot-vent snail .

Animals produce diverse hard structures for critical functions such as protection, feeding and detoxification. Most animals use the polysaccharide chitin as a framework for this, while vertebrates have switched to using fibrous proteins like collagen and keratin. Vertebrates make structures like skin and horns through a cellular differentiation process called keratinization where cells accumulating keratin die and compact into hard layers-drastically different from chitinous structures, which are secreted directly by living cells. Here, we report remarkable chitinous dermal sclerites that are not secreted but instead produced by a keratinization-like process, in the deep-sea hot-vent snail . These scales bundle to form 'warts' on the foot, the framework of which we show to be β-chitin. Microscopic observations reveal that scales are not formed by uniform, secreted layers but instead involve cells going through a series of unusual differentiation steps strongly resembling keratinization. The only other gastropod with chitinous dermal sclerites is the phylogenetically distant scaly-foot snail , but the scales of form by secretion. Our finding of a chitinous convergence for keratinization opens a new avenue to unveil how such complex terminal cell differentiation processes evolve and may also inspire biomimetic innovation in material sciences.