Syngnathids have uniquely evolved male pregnancy and females transfer unfertilized eggs to the male during mating. During mating, eggs are attached to the brooding organ of the males, which is located on the ventral side of the male’s trunk or tail. Brooding organs can be simple patches holding eggs by partially engulfing them with a specialized integument and where eggs remain mostly exposed to the surrounding sea water. But other species feature lateral skin flaps along the brooding organs that partially or completely cover the eggs, provide them with a protected microenvironment. Finally, seahorses feature the most derived brooding organs that likely even facilitate the transfer of nutrients and immunologically active molecules to the embryo. I aim to understand the morphological and molecular basis of the evolution of this organ using histology and single-cell transcriptomics.