Seahorse reproduction is a legendary case of parental role reversal.
When the female produces her eggs, she gives them to the male to care for. The male seahorse is a devoted father. He carries the eggs inside a special brood pouch, undergoes bodily changes that bear an uncanny resemblance to pregnancy, and when he gives birth appears to virtually go into labor.
The reproductive process begins with a pre-courtship ritual which lasts several days. During this ritual the male and female seahorses link tails and swim together, and they may even change color. Then they engage in a courtship dance which can last as long as eight hours. The male expands his brood pouch with seawater and opens it to show the female that it is empty, and the female insert her eggs into it. The female may produce up to a third of her body weight in eggs, and may need to take several rests from egg laying. During her rest periods, the male twists and turns in an attempt to place the eggs properly inside his brood pouch.
Once the eggs are in place, the male releases sperm to fertilize them. The fertilized eggs then embed themselves into the wall of the pouch. This is lined with a spongy, placenta-like tissue which provides the growing eggs with nourishment. The male also secretes prolactin, the same hormone which is responsible for mammalian milk production, and feeds and protects the developing eggs until they are ready to hatch. At first, the fluid inside the brood pouch resembles the male seahorse's own body fluid, but the composition gradually changes until it resembles sea water.
The female's involvement with the growing eggs consists of daily morning visits which she pays to the male while he is carrying the eggs. The male may carry the eggs anywhere from 9 and 45 days depending on the species.
The male usually give birth at night. The mature eggs hatch into fully developed miniature seahorses inside the male's pouch, which becomes extremely swollen. The male performs what appear to be excruciating muscular contortions for about ten minutes before explosively ejecting his young into the water. Most species of seahorses give birth to about 200 young, although some species may give birth to as many as 2000. Here is a link for a YouTube video of a male seahorse giving birth to approximately 1500 babies, all of which are fully formed miniature versions of their parents.
The pair will be ready to mate again almost immediately after the male gives birth.
After the baby seahorses are born, the parents are no longer involved in their care. They will float about in the ocean as plankton, and many of them will provide food for other marine creatures. In fact, less than 0.5% of the babies will survive into adulthood.
Researchers believe that the male's role as protector of the seahorse eggs improves the survival rate for the baby seahorses, and increases the number of broods that the seahorse can produce within the breeding season.