Mustang pregnant or not

Dear Mandy:

If your Sunburst stallion has been courting and now has an enlarged brood pouch and continues to perform daily greetings with the female, but no longer performs his pumping pouch displays, those are pretty good indications that he may be pregnant.

However, sometimes not all of the eggs make it into the male’s pouch during the egg transfer, and the fact that some of the eggs may have been spilled could account for his marsupium being smaller than you would expect at this stage in his pregnancy. It’s not uncommon for inexperienced pairs to produce smaller than normal broods during their first few mating attempts due to spilled eggs. Also, a gravid male can re-absorb infertile eggs that fail to implant or develop properly, so that’s another possibility to consider.

The best way to begin such a discussion is to review how the implantation of the eggs and gestation ordinarily takes place during a normal pregnancy, and then go over some of the factors that can influence gestation and disrupt the pregnancy.

As you know, Mandy, male seahorses nourish the developing young in a special brood pouch. 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 development of these structures is thus under testicular control and takes place primarily in the offseason when the seahorse is not breeding (Vincent, 1990). 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).

These placenta-like changes accelerate after the actual mating and transfer of the eggs take place. The male releases his sperm as the eggs are deposited. The moment the last egg is nestled safely inside, the male’s pouch deflates, compressing the eggs against the pouch lining in order to facilitate implantation. The male then perches and attempts to settle the eggs properly in his pouch, often undergoing a series of agitated contortions, swaying, twitching, or wagging his tail from side to side, and perhaps stretching as though trying to rearrange the eggs more comfortably (Vincent, 1990). He is dispersing the eggs uniformly throughout his pouch, giving each one the best chance to be fertilized and implant in the septum or wall of the marsupium. The fertile eggs implant in the wall or septum of the pouch, triggering a spongelike expansion of its tissues as the capillaries and blood vessels swell and multiply. Epithelial and connective tissue proliferate around the embedded eggs, enveloping each ovum within a tiny chamber or alveolus of its own (Vincent, 1990). Eventually 7/8 of every embryonic sac is embedded in the spongy tissue lining the pouch (Vincent, 1990). Each compartment or alveolus opens into the central cavity of the pouch. About 1/8 of each embryo remains exposed, protruding through this opening, and is immersed in a special placental fluid within the pouch (Vincent, 1990).

In this way, the brood pouch is prepared to maintain the pregnancy by carrying out the following vital functions:

(1) Protection. The brood pouch protects the young in a number of ways. It shields them from harmful ultraviolet radiation, which can destroy unprotected eggs and larvae (Vincent, 1990). It shelters the eggs and fetal fry from predators, and protects them from siltation and suffocating algae (Vincent, 1990).
(2) Aeration. A dense network of capillaries forms in the connective tissue that surrounds each of the embedded eggs, delivering oxygen to the fetal fry through the membrane of the embryonic sac (Vincent, 1990) and carrying away their metabolic wastes in the same manner.
(3) Control of osmotic pressure. The sealed pouch creates a watertight environment for the developing young and, over the course of the gestation, the male adjusts the osmotic pressure from that of his bodily fluids to that of seawater (Vincent, 1990). The young are thus gradually acclimated to full-strength saltwater over the course of the pregnancy, so the newborns will be right at home when they are expelled from the pouch. This is crucial for the survival of the delicate fry, since sudden changes in osmotic pressure are known to cause stunting, breathing abnormalities, and physical deformities in teleost fish larvae (Vincent, 1990).
(4) Nourishment. A portion of each embryonic sac is bathed within a nourishing placental fluid containing calcium and other inorganic ions contributed by the male (Vincent, 1990). The placental fluid also contains organic ions derived from the female via the yolk. The male secretes enzymes that dissolve away the outer covering of the eggs known as the chorion shortly after incubation begins (any infertile eggs that fail to implant are normally resorbed in the same manner), and the organic ions thus contributed by the female diffuse across the exposed membrane of the embryonic sac into the pouch fluid (Vincent, 1990). There the inorganic ions are transformed into amino acids by a special enzyme (protease) secreted by the pouch epithelium (Vincent, 1990). These amino acids eventually become proteins incorporated within the embryos (Vincent, 1990). The calcium provided by the male is similarly taken up by the embryos and infused into their skeletons (Vincent, 1990).

In short, the brood pouch enfolds, protects, aerates, osmoregulates, and nourishes the developing embryos as the male undergoes a true pregnancy (Vincent, 1990). After the fertilized eggs implant in the walls of the marsupium, the normally soft, flaccid pouch firms up and often darkens in coloration due to the placenta-like changes taking place within. And as the embryonic young and fetal fry grow and develop, the pouch typically becomes increasingly distended as the pregnancy progresses, especially when the stallion is carrying a large brood of young.

Okay, Mandy, that’s what should take place during a normal pregnancy. With that in mind, let’s examine some of the many factors that can affect gestation and cause a pregnancy to fail.

To begin with the basics, gestation in seahorses is largely determined by water temperature, is controlled by the levels of key hormones, and can be influenced to a lesser degree by diet and nutrition.

In general, the warmer the water, the shorter the gestation period, and vice versa. Relatively cool water temperatures are generally desirable during pregnancy, since they prolong the incubation period and the newborns are larger and better developed when they are subsequently born.

Aside from water temperature, gestation may vary due to hormonal influences as well. For example, in seahorses, a hormone known as fish isotocin, which is the equivalent of oxytocin in mammals, triggers parturition or giving birth. Thus anything that stimulates excess secretion of isotocin can result in premature births, whereas anything which decreases or delays the secretion of isotocin can postpone delivery and prolong a pregnancy abnormally.

In a similar manner, disruption of other hormones can cause a male to spontaneously abort a pregnancy or to actually resorb the eggs. The placenta-like changes that take place in the brood pouch, the development of the embryonic young, and the pregnancy itself are all controlled by various hormones — testosterone, adrenal corticoids, prolactin, and isotocin — so basically anything that influences the secretion of those key hormones can have a profound effect on the pregnancy

Some of the factors that influence these hormonal responses are the presence of the female, low oxygen levels, diet and, of course, stress. The presence of the female most definitely influences the gestation and brood success of her mate. Numerous studies indicate that the presence of female fishes visually or hormonally stimulates male sexual activity such as courtship, nest building, and the development of androgen-dependent sexual characteristics (Vincent, 1990). Research has also shown conclusively that male seahorses which have been with the same female for more than one mating cycle are markedly more successful in brooding young (Vincent, 1990). It is believed that one of the reasons for this is that the presence of their mate stimulates the secretion of the corticoids (steroid hormones produced by the adrenal cortex) and prolactin that control the pouch environment and maintain the incubation (Vincent, 1990). The male is thought to further expand his pouch and develop the placenta-like internal structures to a greater degree as a result (Giwojna, Feb. 2002). More of the eggs can then be successfully implanted and carried to full term (Giwojna, Feb. 2002). Separating a gravid male from his mate can therefore have a negative impact on his pregnancy and should be strictly avoided.

Low oxygen levels during pregnancy can likewise be disastrous. They result in respiratory distress for the gravid male, putting the embryonic young at risk, as well as directly altering the hormones we have been discussing, which can further disrupt the pregnancy.

Poor water quality — especially ammonia and/or nitrite spikes — are one of the most common aquarium stressors that can disrupt hormones and interrupt a pregnancy. Stress hormones such as cortisol will be released in response to such stressors, at the expense of other adrenal hormones, which can have a negative impact on the pregnancy and the developing fry.

Heat stress is doubly bad news for gravid males. Not only can abnormally warm temperatures disrupt the secretion of these key hormones and shut down breeding, they can also directly denature long chain polymers and macromolecules (e.g., proteins, enzymes and hormones) by altering certain bonds and changing the three-dimensional shape of the molecule on the atomic level. And, of course, water temperature also directly affects the metabolism of the seahorse and therefore its gestation period. Up to a certain point, increasing water temperatures will shorten the normal gestation period, just as decreasing water temperature will prolong or extend gestation.

Past a certain point, however, when the increasing temperatures exceed the comfort range for the seahorses, elevated temperatures will bring reproduction to an abrupt halt. For example, the Mexican population of H. ingens begins breeding in late September when the water temperatures decreases below 81°F (27°C), and keep breeding until late May when the water temperatures increase above 80°F again (Eliezer Zúñiga, pers. comm.).

An inadequate diet can also be detrimental to a gravid male for obvious reasons. Maintaining a large brood of developing young can be a big drain on the male’s bodily resources, and a nutritious diet rich in HUFA and essential fatty acids is necessary at this time to help the male keep up his strength. That is why male seahorses have an intestinal tract that’s 50% longer than that of females (Tamaru, Aug. 2001). They need the extra food absorption ability and digestion a longer intestine provides in order to sustain the metabolic demands of up to 1600 rapidly growing fry. It might be helpful for you to provide your pregnant male with some choice live foods at this time, Mandy.

When factors such as these disrupt the pregnancy, a gravid male may not be able to carry his brood of embryonic young and fetal fry to full term due to the sorts of developments we have been discussing. This is how Carol Cozzi-Schmarr describes the situation: “If… conditions are not optimum and the general stress level of the male is too high, the male will simply re-absorb the eggs or abort them. You will sadly think that he was never really pregnant.”

So the actions of the aquarist and the aquarium conditions can have a big effect on how well a pregnancy progresses, and whether or not the fetal fry develop normally and are brought to full term, or are aborted, delivered prematurely, or resorbed as embryos. In short, Mandy, it’s important for you to handle your gravid male with care during his pregnancy, to keep him and his mate together, and to provide him with a nutritious diet, optimal water quality, and a stress-free environment at all times.

Those are the things that you should concentrate on to help assure that your Sunburst will have a successful pregnancy. Here’s hoping that he soon provides you a brood of newborns.

Respectfully,
Pete Giwojna, Ocean Rider Tech Support