Skeletal growth, ultrastructure and composition of the azooxanthellate scleractinian coral Balanophyllia regia

International audienceThe biomineralization process and skeletal growth dynamics of azooxanthellate corals are poorly known. Here, the growth rate of the shallow-water dendrophyllid scleractinian coral Balanophyllia regia was evaluated with calcein-labeling experiments that showed higher lateral than vertical extension. The structure, mineralogy and trace element composition of the skeleton were characterized at high spatial resolution. The epitheca and basal floor had the same ultrastructural organization as septa, indicating a common biological control over their formation. In all of these aragonitic skeletal structures, two main ultrastructural components were present: “centers of calcification” (COC) also called rapid accretion deposits (RAD) and “fibers” (thickening deposits, TD). Heterogeneity in the trace element composition, i.e., the Sr/Ca and Mg/Ca ratios, was correlated with the ultrastructural organization: magnesium was enriched by a factor three in the rapid accretion deposits compared with the thickening deposits. At the interface with the skeleton, the skeletogenic tissue (calicoblastic epithelium) was characterized by heterogeneity of cell types, with chromophile cells distributed in clusters regularly spaced between calicoblasts. Cytoplasmic extensions at the apical surface of the calicoblastic epithelium created a three-dimensional organization that could be related to the skeletal surface microarchitecture. Combined measurements of growth rate and skeletal ultrastructural increments suggest that azooxanthellate shallow-water corals produce well-defined daily growth step

Cnidaria: Anthozoans in the Hot Seat

Cnidarians comprise a diverse and phylogenetically basal phylum, some of which—such as scleractinian corals (Anthozoa)—are responsible for the formation and maintenance of ecosystems. Anthozoan immunology is a relatively new field, yet has great potential to inform invertebrate immunology, medicine, as well as coral reef conservation and restoration. Here we review cnidarian innate immune mechanisms in the context of invertebrate effector responses. We focus on anthozoans and discuss the blurred boundary between immune and stress responses. We conclude by high 1ighting unique aspects of coral biology and exploring the role of immunology in coral reef conservation and restoration through climate change