The calcifying interface in a stony coral primary polyp: An interplay between seawater and an extracellular calcifying space

•Primary polyps may have high tissue permeability.•External seawater is delivered to the mineralization site.•Large paracellular spaces and filopodia networks observed in the calicoblastic layer.•Newly recruited corals may be more vulnerable than adult to different stressors.•Implications for the use of coral skeletons in paleoceanography.