Re:seahorse fry

#5261
Pete Giwojna
Guest

Dear Karen:

That’s a very good question and I can understand why this is a confusing issue for so many seahorse keepers. Yes, in most species, newborn seahorses need to have an opportunity (access to the surface) in order to properly inflate their swim bladders. At the same time, pelagic seahorse fry that go through a free swimming or planktonic phase of development need to be drawn away from the surface of the aquarium as soon after they have inflated their air bladders as possible, in order to prevent them from swallowing access their and developing buoyancy problems once their pneumatic duct has closed off.

The reason for this, and the reason that exposure to the air is a serious problem for newborn seahorses but of no concern at all for older seahorses, is due to the presence or absence of the pneumatic duct. Allow me to elaborate.

In seahorses as in other fish, the gas bladder arises as a simple pouch or outgrowth from the foregut (Evans, 1998). In newborn seahorses, this connection with the gut is retained as an open tube, called the pneumatic duct, and seahorse fry gulp air at the surface to fill their gas bladder initially. There is only a short window of opportunity to do this, since the fry lose this open connection very early in life. As a result, the air bladder is often completely closed off (physoclistous) in fry that are more than a few days old (sometimes after they are only a few hours old), and they can no longer inflate their gas bladders this way.

Newborn seahorses are therefore physosymotous, and their gas bladders open into their esophagus via the pneumatic duct. If they are exposed to the air for any length of time in this condition, chances are great that they will swallow too much air and overinflate their gas bladders. Or they may accidentally ingest air when feeding at the surface, after they have already inflated their air bladders. When that happens, the fry develop fatal buoyancy problems and become the infamous "floaters" that bob helplessly at the surface until they starve to death. If you examine a floater carefully, you can actually see its overinflated swim bladder, which appears as a silvery bubble in its neck at the base of the throat.

In seahorses, the pneumatic duct closes off after a few days of development, and this open connection to the esophagus is lost. At that point, exposure to the air is no longer a great threat to the physoclistous fry since they can no longer overinflate their gas bladders by gulping air.

I don’t have any data indicating at what age the pneumatic duct closes off in various species of seahorses, so I cannot answer your question much more precisely than that. Most likely, any seahorse fry that are more than several days old are safe in that regard.

Interestingly, denying newborn seahorses access to the surface after birth is just as harmful to them as lifting them out of the water and exposing them to the air. This is because newborns that miss the opportunity to gulp air at the surface while their pneumatic duct is still open — perhaps as the result of an oily or greasy film at the surface of the water — suffer from underdeveloped swim bladders. As they grow and become heavier, they sink to the bottom and are unable to swim or feed normally.

If denied access to the surface to inflate their swim bladders, the fry behave normally while they are small and their weight is still negligible. But over the weeks, as they grow and put on weight, their underdeveloped swim bladders and inability to achieve neutral buoyancy increasingly handicap them. Once they gain a little weight, they sink like rocks. Unable to swim, they are reduced to slithering along the bottom on their bellies and are commonly referred to as sliders. This deficiency does not become apparent until the fry are several weeks old. Needless to say, this hinders their swimming ability and severely limits their feeding opportunities, delaying their growth and development, and rendering entire broods useless. In several cases, the problem was traced back to an oily film on the surface of the nursery tank, which prevented the newborns from filling their swim bladders with air (Silveira, 2000). A protein skimmer will prevent this by removing filmy surface layers and surfactants in general.

All of this creates a tricky situation for the seahorse keeper. Newborns that are unable to reach the surface and inflate their gas bladders suffer from negative buoyancy and become sliders. On the other hand, seahorses that are exposed to the air may swallow too much air and develop positive buoyancy as a result of their overinflated gas bladders, becoming floaters. It can sometimes be difficult to achieve the right balance between these two extremes because physosymotous fry can also accidentally ingest air while feeding on newly-hatched brine shrimp which tend to congregate at the surface where they are drawn by the light.

The situation is often more complicated for the home hobbyist because many times the aquarist is not present when a gravid male delivers his brood. For example, when rearing seahorses that go through a pelagic stage, the problems often begin even before the aquarist realizes the fry have been born. Gravid males normally give birth in the early morning hours (Vincent, 1990), and the hapless hobbyist is apt to be confronted with his first crisis immediately upon arising in the form of a writhing mass of newborn sea horses, hopelessly tangled together at the top of the tank (Giwojna, Jan. 1997).

This dangerous situation develops because a newborn’s first instincts are to head to the surface to fill its air bladder and then to anchor itself to something solid. In the vastness of the ocean this is not a problem, since strong currents rapidly disperse the young, but in the confines of an aquarium, the first hitching post it finds will very likely be the tail or snout of one of its siblings (Giwojna, Jan. 1997). The same mistake is apt to be repeated by the rest of the pelagic fry, as they cluster at the surface, until the entire spawn is snarled together tail-to-tail, head-to-tail, tail-to-snout and so on (Giwojna, Jan. 1997). This is a very common experience when raising seahorses such as Hippocampus erectus, H. reidi, and H. ingens, which produce large broods of pelagic fry.

So if you should awaken to find a swarm of newborn seahorses in the aquarium, or return home from work or school only to discover the newborns clustered at the top of your tank, go ahead and transfer them to your nursery tank at once. They will already have had ample time to fill their air bladders and the danger is that they may continue to gulp air accidentally while feeding at the surface.

However, if you should be fortunate enough to witness your pregnant male giving birth, give the newborns perhaps an hour and then transfer them to your nursery tank, if they are pelagic fry. (If you’re fortunate enough to have benthic babies, which orient to the substrate and begin hitching near the bottom shortly after birth, you need not take immediate action. The benthic babies are in no danger of gulping too much air and cannibalism is an extreme rarity among seahorses, so the benthic fry can remain in the main tank until you have a chance to transfer them to your nursery at your leisure. That is one reason that seahorses which produce benthic fry, such as Hippocampus zosterae, H. fuscus, H. capensis, H. barbouri, etc., are considered so much easier to raise than seahorses that produce fry that go through an extended pelagic phase.)

When transferring the newborns into the waiting nursery tanks, take care NEVER lift the newborns out the water. They may swallow air and develop fatal buoyancy problems that leave them bobbing helplessly at the surface, unable to submerge or eat (Giwojna, Jan. 1997). Netting them out or otherwise exposing the newborns to the air is one of the most common mistakes inexperienced breeders make, and it can result in the loss of the entire brood (Giwojna, Jan. 1997). The proper way to move the babies is to carefully scoop them up in a small cup or bowl, and gently immerse the cup in the nursery tank to release the fry (Giwojna, Jan. 1997). Or a common turkey baster works well for gently sucking up one or two of the fry at a time along with a little water, and then releasing them into their nursery, but you may have to cut off the tip of the baster to increase the size of the opening when using this technique (Giwojna, Jan. 1997).

Okay, Karen, those are some guidelines you can use when it comes to moving the newborns to the nursery tanks. It’s very difficult to determine why many of the newborn H. reidi from your first brood had descended to the bottom so quickly, whereas very few of the newborns from your second and third broods have done so. Hippocampus reidi fry normally go through a prolonged pelagic phase and it usually takes weeks before they begin to settle down and orient to the bottom.

Best wishes with all your fishes, Karen!

Respectfully,
Pete Giwojna

Post edited by: Pete Giwojna, at: 2011/01/27 07:18


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