Okay, judging from your description of his pouch, it certainly sounds promising! I’m pulling for you, and hoping he is indeed carrying a brood of healthy Zulu babies.
However, a full, enlarged pouch doesn’t always mean that a male is pregnant. There are three possibilities when a male’s pouch is obviously very swollen and enlarged:
1) He is pregnant and carrying a large brood of developing young;
2) He is courting and performing pouch displays known as "Pumping" and "Ballooning;"
3) He is ailing and his pouch is filled with gas or swollen with accumulated fluid. When the brood pouch is bloated with gas, we would certainly expect the male to be experiencing positive buoyancy and having severe difficulty swimming, if not actually floating and bobbing at the surface like a cork. On the other hand, if his brood pouch was distended with accumulated fluid (ascites), he would be very likely to have difficulty swimming due to negative buoyancy. When that happens, the seahorse tends to hang downward from his hitching post, rather than assuming the normal upright posture, and he may spend periods of time lying prone on the bottom. Since your male is not having any buoyancy problems or difficulty swimming, Nigel, I think we can probably rule out the third possibility.
Ballooning is a simple display in which courting males inflate their brood pouches with water to the fullest possible extent and parade around in front of the female in all their glory as though trying to impress her with the sheer dimensions of their pouches. The pumped up paramours perform proudly, putting on quite a show for the flirtatious fillies.
Hippocampus abdominalis, H. breviceps, and H. tuberculatus, in particular, have developed enormous pouches that are all out of proportion to their bodies when fully expanded. Their oversized pouches look like over-inflated balloons ready to burst when these stallions come a courting. Take the tiny Hippocampus breviceps, for example. With its brood pouch expanded to the maximum, a courting male looks like a fuzzy 3-inch pipe cleaner that swallowed a golf ball! Courtship in temperate/subtemperate species generally centers around pouch displays more than color changes, dancing or prancing.
Ballooning is much more common in temperate seahorses than tropical species. Thus, male Brazileros (Hippocampus reidi) or male Mustangs and Sunbursts (Hippocampus erectus), for example, are much more likely to engage in a different type of pouch display known as "Pumping" instead.
Pumping requires a series of coordinated movements. Bending vigorously, the aroused male jackknifes his tail to meet his trunk, thereby compressing his inflated brood pouch in the middle. The male then straightens up again, suddenly snapping back to "attention" so as to relieve the pressure on his severely compressed midsection. This rapid pumping motion has the effect of forcing water in and out of the brood pouch in a manner that is virtually identical to the way the young are expelled at birth (Vincent, 1990).
The strenuous pumping action is the stallion’s way of demonstrating his pouch is empty of eggs and that he is a strong, healthy, vigorous specimen capable of carrying countless eggs (Vincent, 1990). By so doing, he assures the female that he is ready, willing, and able to mate, and that he can successfully carry and deliver her entire brood. The male’s marsupium also becomes grossly distended during displays of Pumping, but in that case, it is obvious the male is courting because it looks like he’s doing abdominal crunches as the vigorously pumps water in and out of his brood pouch. Zulu-lulus (H. capensis), like your male, will perform both types of pouch displays when courting, and often concentrate on dancelike displays when flirting with fillies.
During displays of Ballooning and Pumping, the male’s pouch is inflated with seawater, so he maintains neutral buoyancy and can swim normally. As a rule, males don’t perform these pouch displays if they are pregnant. The intrusion of saltwater into the pouch can be harmful to the embryonic young in the early stages of pregnancy because it dilutes or displaces the marsupial fluid that is bathing the fetal fry, and opening the pouch in the later stages of pregnancy is equally disastrous since it may result in the expulsion of pug-nosed preemies still attached to yolk sacs. Ordinarily, once a male seals the aperture of his pouch after the transfer of the eggs, he will not open it again until the hormone isotocin triggers parturition and the birth spasms.
In general, then, it is safe to assume that a male that is performing pouch displays is not pregnant, and it is likewise fair to presume that a gravid male will not indulge in pouch displays. But under captive conditions where flirtatious fillies are always present and sex pheromones from other courting couples may be wafted through the water, just occasionally, a gravid male’s instincts may get the better of him and he may attempt pouch displays regardless of his delicate condition.
Pregnant males do continue to perform the preliminary phases of courtship with their mates. The pair will continue to flirt and dance and brighten in coloration as part of their Daily Greetings, but the male will no longer pump (no pouch displays) and neither the female nor the male will point. The pair will make no more copulatory rises. So a mated pair’s usual morning greeting ritual continues throughout pregnancy, but is limited to the preliminary dancelike courtship displays and normally always stop short of pouch displays. But if a fickle female should begin flirting with another male during her partner’s pregnancy, as commonly happens under captive conditions where monogamy tends to breakdown, it would not be unusual for the pregnant male to intervene and attempt to break up that budding romance.
Close observation of your male over the next several days will hopefully make it clear which of these possibilities is correct in your case, Nigel. Here’s hoping he’s carrying a big brood and will soon present you with a fine new batch of healthy fry.
Yes, all that dancing you observed is a sure sign that your H. reidi have paired up. Hippocampus reidi are famous among seahorse keepers for two things: brilliant colors and making babies. The Brazilian breeding machine is the most prolific of all the seahorses (Abbott 2003). They have a well-deserved reputation for churning out brood after brood every two weeks with relentless regularity, and hold the world record for delivering ~1600 young in a single brood (anecdotal reports of broods up to 2000 fry are not uncommon)! Not bad for a livebearer. But with that many fetal fry crammed into one incubator pouch, the inevitable tradeoff is that the young are born at a considerably smaller size than most seahorses (Abbott 2003). They also go through a lengthy pelagic phase, drifting freely with the plankton for up to 1-2 months, which makes H. reidi fry notoriously difficult to raise (Abbott 2003).
Zulus (Hippocampus capensis) are another prolific species that breed like bunnies in the aquarium when conditions are favorable, so with both your Zulus and your H. reidi are getting serious about breeding now, you are going to be up to your eyeballs in seahorse fry before you know it, Nigel! That’s a nice problem to have and you’ll find the Zulu babies are much easier to raise than the smaller H. reidi fry.
Best of luck with your prolific ponies and all our progeny, Nigel!