My smallest seahorse, Debelius, has started developing a pouch. Click on the pictures to see a super close-up. The first photo is him in late March. The second photo was taken yesterday. The third is a photo of him when he was a juvenile and didn't have a pouch yet. You can see how his pouch has grown, and his chest is getting deeper. Even before he had a pouch, his chest started getting deeper than his sisters'.
He has always been close to Ellis, the large adult female. He constantly hitches on her and stays near her. Now that he has a pouch, he is always showing it off to her. But Ellis is not impressed. She and Juniper, the large adult male, still court once every couple weeks. They've had one egg transfer so far, but Juniper lost the eggs. He had the eggs safely in his pouch after the transfer, but as soon as Ellis came near him again, he got excited, started doing pouch flushes, and pushed all the eggs out. Maybe next time they will figure it out.
If you ever find a sheet of orange eggs in your seahorse aquarium, they are probably seahorse eggs that didn't make it into the pouch. This can happen for a number of reasons. When seahorses mate, they rise to the top of the aquarium and then slowly make their way down while transferring the eggs. If the tank is not tall enough, and they are hurrying the transfer, they may not get them in the pouch. Also, if the flow in the aquarium is too strong, it will be difficult for them to make the transfer. Sometimes a female seahorse will have eggs ready, but can't find a receptive male. The eggs will not be fertilized until they are inside the male's pouch. The eggs are a nutritious food source for the other fish and inverts in your aquarium.