The female deposits unfertilised eggs into the male's ventral brood pouch through an ovipositor. He fertilises them inside the pouch, which then seals. Over roughly two to four weeks the pouch lining becomes highly vascularised, and the male regulates the internal environment: oxygen is supplied across the pouch wall, salinity is progressively adjusted from internal fluid toward seawater, and immune and nutritive proteins are secreted.
Birth is muscular and strenuous. The male contracts repeatedly to expel fully formed juveniles, sometimes hundreds at once, which are independent from the moment of release. Both parents can be ready to mate again within days.
The common framing that the male is simply a container is wrong; the physiology has real parallels with placental gestation, including hormonal control of parturition. What is often overstated in the other direction is lifelong monogamy: seahorses form durable pair bonds within a breeding season and greet each other daily, but multi-season fidelity varies by species and is not universal. Hippocampus kuda is heavily traded for traditional medicine and the aquarium market and is assessed as Vulnerable.