Congratulations on your new Sunbursts!
No, sir, male seahorses are not born with pouches. The brood pouch or marsupium develops as the juvenile seahorses hit sexual maturity, just as you surmised. This usually happens around the age of 4-6 months or so in cultured Mustangs and Sunbursts (Hippocampus erectus). But it’s like puberty in human beings — there are always some precocious youngsters that develop much earlier than the others and some late boomers that lag behind and develop much later than the others, as discussed below.
Sexing adult, fully mature seahorses in breeding condition is normally simple and straightforward — the males have a brood pouch slung under their abdomens at the base of their tails and the females do not. This difference will be most obvious when the seahorses are courting and breeding, since the stallions perform vigorous pouch displays in which they inflate their pouches with water to the bursting point, making them unmistakable.
However, in actual practice, things are often more confusing, especially when attempting to sex immature (subadult) seahorses or mature seahorses that are not actively breeding. A myriad factors can confound the issue. For example, some fully functional females possess a pseudo-pouch and in some stallions, especially virgin males, the brood pouch shrinks away to almost nothing in the off season when their hormones stop flowing, becoming all but unnoticeable. And late bloomers are always problematic.
In some seahorse species, adult males and females can be very difficult to tell apart when they are not breeding because the male’s pouch shrinks to almost nothing in the offseason and does not become obvious again until hormonal changes triggered by courtship and mating cause it to grow and expand (Bull and Mitchell, 2002).
For example, this is how Michael Payne (Seahorse Sanctuary) describes this phenomenon:
"Temperature may effect whether or not you can see the pouch of a male. In H. breviceps, it is very difficult to sex adults that are not in breeding condition. At low temperatures (17°C), the males’ pouch deflates such that you can hardly see it. Increase the temp (22°C) and the
pouch appears and mating starts."
During the breeding season, the male’s brood pouch undergoes elaborate changes to prepare it for pregnancy. Often called the marsupium, this remarkable organ is much more than a simple sack or protective pocket or a mere incubator for the eggs. Think of it as an external womb, which undergoes placenta-like changes throughout the pregnancy in order to meet the needs of the fetal fry. Its internal architecture is surprisingly complex. In fact, the male must begin preparing his pouch to receive his next brood long before gestation begins (Vincent, 1990). The elaboration of the internal pouch anatomy that is necessary to support the developing young is triggered by the male hormone testosterone. The four layers of tissue that comprise the pouch undergo increased vascularization at this time (Vincent, 1990) and a longitudinal wall of tissue or septum grows up the middle of the pouch, separating it into left and right halves. This increases the surface area in which fertilized eggs can implant, and enriches the blood supply to the lining of the pouch in which they will imbed. Just before mating occurs, this is enhanced by a surge in the active proliferation of the epithelial tissue that forms the innermost layer of the pouch (Vincent, 1990).
In the offseason, the levels of gonadotropin, testosterone and adrenal corticoids in the bloodstream are reduced, and the pouch deflates and shrinks accordingly, reversing these placenta-like changes. The male’s marsupium becomes much less conspicuous at this time as a result.
You can also attempt to sex immature seahorses with no indications of an incipient pouch using the position of the anal fin and a few other subtle indicators as a guide, but the younger the seahorses are the more difficult this is to accomplish with any degree of accuracy. Suffice it to say, sexing juvenile seahorses that are much younger than 5 months can be a very challenging adventure. This is typically done by noting the position and shape of the anal fin as well as the curvature of the abdomen. In immature females, the anal fin is situated right at the very base of the abdomen where it meets the tail and points more or less straight downward, almost flush up against the tail. In immature males that lack a brood pouch, the anal fin is located higher up on the abdomen, allowing room for the brood pouch to subsequently develop, and protrudes outward at an angle from the tail. This makes it appear as if there is more space between the tail and anal fin in juvenile males.
The difference in the position of the anal fin is due to the way the vent is situated in males and females. The seahorse’s vent is the cleft formed by the combined openings of the anus and urogenital pore (Seahorse Anatomy, 2004). It is the simple recessed passage located just above (cranial to) the anal fin in females; in males, the anal fin is located in the middle of the vent where it separates the anal opening from the urogenital pore. The male’s anus is therefore situated above the anal fin while the genital opening of the male is located below the anal fin at the mouth of the brood pouch. In females, however, both the anus and the urogenital pore are located above the anal fin.
Adolescent males will also sometimes show a thick dark line near their vent where their pouch will eventually form as they mature.
There is also a noticeable difference in the profile of the abdomen. In females, the abdomen curves inward more sharply, so that the base of the belly almost forms a right angle to the tail. In males, the abdomen slopes inward toward the tail more gently, giving the base of the belly a more rounded appearance in profile. In addition, in some species, the anal fin tends to be pointed in females and rounded in males. One can attempt to sex adolescents or even seahorse fry according to these subtle differences, but the younger the specimen, the more likely mistakes are to be made and the greater the chances that the gender that is assigned will prove to be wrong.
The position of the anal fin is thus a fairly good indicator, but it takes a great deal of experience to become proficient at sexing young seahorses this way, and I must confess that I’m pretty lousy at it. I’ve come to the conclusion that it’s really more of an art form than an exact science, and that there’s definitely more than a little intuition involved; I guess I just don’t have the best "feel" for it.
Further confounding the issue of gender is the fact that a certain percentage of females also 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 9-10 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).
Since you’re new Sunbursts are still youngsters, eaglestour, and the males are likely virgins, they may not display an obvious pouch at this time. You may need to wait for them to mature a bit before their gender becomes apparent. When they have adjusted to their new surroundings and begin to get serious about courtship and breeding, the hormonal changes that occur should make it easy to tell the stallions from the fillies. Don’t be too surprised if one of the three females actually has a nascent pouch and blossoms into a fine young stud within a few months.
Best of luck with your herd of ponies, eaglestour! Here’s hoping you end up with two male and two female Sunbursts upon further development.