It’s quite rare but I’ve heard of a few other cases of asymmetrical swelling or enlargement of the brood pouch. It can happen because the internal anatomy of the pouch is much more complex than most folks suspect, and in breeding males, it includes a septum that divides the marsupium into left and right hemispheres, as discussed below:
The marsupium consists of four tissue layers forming an enclosed pouch located under the abdomen on the front of the tail (Vincent, 1990). The aperture or opening of the pouch is located directly under the anal fin, and is fully dilated during mating to allow the female easy access to deposit her eggs. The pouch opening is ringed with a powerful sphincter muscle that acts as a valve to create a watertight seal when the pouch is closed (Vincent, 1990).
The development of the internal pouch structures necessary to support the growing embryos is under testicular control via testosterone, but corticoids (steroids secreted by the adrenal cortex) and the hormone prolactin maintain the actual incubation (Vincent, 1990). The elaboration of internal pouch structures takes place primarily during the offseason when the seahorses are not breeding, but just before mating occurs, there is an active proliferation of epithelial tissue, which forms the innermost layer of the pouch (Vincent, 1990). The walls of the pouch thus thicken in preparation for the embryos and a longitudinal wall of tissue or septum grows up the middle of the pouch to increase the surface area in which fertilized eggs can implant. (Most seahorses have a single septum or membrane that divides the pouch roughly into left and right halves, but Hippocampus abdominalis has anywhere from 3-5 septa to accommodate even more eggs.)
It is the presence of this septum that allows one half of the pouch to be swollen with gas or fluid, or just possibly fetal fry, while the other half remains flaccid and unexpanded, Jerry. Several years ago Bart Goedegebuur, a very successful seahorse breeder in the Netherlands, contacted me about a case that was remarkably similar to yours. Pasted below is Bart’s e-mail regarding this incident:
One of my H. ramulosus seems to develop a problem, which I have never seen
before within my own stock. Pouch of a male is enlarged on one side of the
body. Initially I thought the animal was pregnant however the situation
already exist for more then 6 weeks, and duration of pregnancy should be
approximately 3 weeks. The animal’s behavior is normal in all aspects
including neutral buoyancy. So I don’t suspect that air is trapped in his
pouch. Do you have any idea what can course this non-symmetric pouch
As you can see, Jerry, in the case of Bart’s Hippocampus ramulosus, he also believed that his mail was pregnant initially since it was showing no problems with positive buoyancy or any other unusual symptoms.
There are three possibilities in a situation like this, sir:
(1) Gas is beginning to build up in the stallion’s pouch unilaterally. In that event, the seahorse will eventually develop problems with positive buoyancy (i.e., the tendency to float), indicating a problem with pouch emphysema. That’s a possibility because sexually mature male seahorses that are actively courting and breeding are especially vulnerable to pouch emphysema and other forms of gas bubble syndrome (GBS) as a result of the placenta-like changes taking place in their physiologically dynamic, heavily vascularized marsupium.
(2) The right side of the stallions pouch is swollen with fertilized eggs that managed to implant only on one side of its brood pouch, and the pouch is gradually enlarging as the fetal fry and embryonic young grow and develop. In that event, he may indeed be pregnant and could still deliver healthy, viable newborns when the time comes, but it is likely that the bulk of the female’s eggs were spilled and never found their way into his marsupium, so the brood is likely to be smaller in number than usual.
(3) One side of the male’ pouch is filled with water as a result of performing pouch displays during courtship (i.e., “Pumping” and/or “Ballooning”). In that event, the pouch may remain swollen asymmetrically for the time being, but the seahorse will nevertheless remain neutrally buoyant and have no problem swimming and feeding normally.
Right now, it’s still too early to determine which of these outcomes will come to pass, Jerry, but time will tell the tale.
Pete Giwojna, Ocean Rider Tech Support