"A team of scientists at the school’s Center for Materials Innovation found that crustaceans like crabs and lobsters contain a chemical in their shells called chitin"
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2920538/#:~:text=Henri%20Braconnot%2C%20a%20French%20professor,nitrogen%20in%20chitin%20%5B3%5D.