Single-cell RNA sequencing reveals 40 different cell types in Stylophora pistillata, a reef-building stony coral native to the Indo-Pacific oceans. The calcium carbonate skeleton of stony coral colonies are the main habitat for a huge diversity of marine species. The study has detected the presence of specialized immune cells in corals or any cnidaria. The findings will aid present and future conservation efforts to protect coral reef ecosystems threatened by rising temperatures and ocean acidification.