No, sir, there is no evidence to suggest that seahorses are hermaphroditic or that they can change sex the way some other marine fish (e.g., clownfish in certain wrasse) can do under certain circumstances. But it can sometimes seem that way due to late bloomers and females that have developed a "pseudo-pouch."
One of the things that can come found the issue of gender in Hippocampus is the fact that a certain percentage of females have a subanal structure that can be easily mistaken for an incipient pouch (Vincent, 1990). This is misleading because the pseudo-pouch seen on many such females is merely a pigmented patch of skin, not a functional brood pouch or even a pocket of tissue (Vincent, 1990). Although they are very often presumed to be male, at least initially, females having these subanal structures produce viable eggs, pair off with males, and mate normally just like all the other fillies.
Mustangs and Sunbursts (Hippocampus erectus) are usually shipped around the age of 5-7 months, which is about when they begin to hit sexual maturity and pair off with one another. But there is considerable variation in that regard — some precocious ponies begin to show pouch development at the tender age of 3-4 months white at the opposite end of the spectrum there may be a few late bloomers that may not develop fully functional pouches until they’re yearlings.
It’s like puberty in humans; some youngsters begin to develop while they’re still in elementary school but others don’t hit puberty until after high school. Most are somewhere in between.
Sexing such late bloomers is always problematic. As I said, the greater seahorses typically reach sexual maturity around the age of 4-6 months (Warland, 2003), so it’s natural to assume that a 6-month seahorse that lacks an obvious brood pouch is a female. Many hobbyists are therefore very surprised when a specimen they were quite certain was female suddenly develops a full-blown pouch at the age of 9-12 months. On the other hand, it’s only natural to assume that a 6-month old seahorse with a subanal patch of skin that’s colored entirely different from the rest of its abdomen is blossoming into a fine young stallion right on schedule, and it can thus be a bit of a shock to hobbyists when their presumed male drops its first clutch of eggs. More than a few aquarists have ended up renaming their seahorses when it became clear that Victoria was actually a Victor (or vice versa).
So it can sometimes appear as though seahorses change sex when a female with a pseudo-pouch produces a clutch of eggs or when a late blooming male matures into a fine stallion later than expected, but seahorses are not gender benders or hermaphroditic in nature.
If you can e-mail photographs of your four seahorses to me off list ([email protected]), I will be happy to see if I can clarify the situation for you, Gordon. I am pretty good at identifying the sex of adult seahorses, although I am lousy at sexing the juveniles. Let me take a look at the photographs and we can go from there, sir.
Best wishes with all your fishes, Gordon!