Re:Problem or Just a Weak Swimmer?

Pete Giwojna

Dear Claire:

Yes, I do agree that Sydney’s problem is most likely due to an underinflated swim bladder, and since treating the seahorses with metronidazole/praziquantel did not resolve her problem, the malfunction with her swim bladder is apparently not due to internal parasites. Sometimes bacteria are involved in swim bladder disorders, Claire, so you might consider administering antibiotics orally via gutloaded shrimp, but I really have no idea if that would be helpful at all in this case.

This could be a problem that began shortly after birth. 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.

So it’s possible that Sydney has had an underdeveloped or underinflated swim bladder from an early age. Ordinarily, seahorses can overcome an underinflated swimbladder simply by depositing additional gas into the bladder with the aid of the gas gland. The gas gland provides an acidic environment, and under acid conditions, hemoglobin releases oxygen in the gas gland, which is then deposited in the lumen of the swim bladder via diffusion. Likewise, carbonic acid dissociates into CO2 + H2O within the gas gland, liberating carbon dioxide, which is also deposited in the lumen of the swim bladder through diffusion. (The swim bladder of seahorses is filled primarily with oxygen, supplemented by carbon dioxide; Evans, 1998.) In this way, the gas gland can gradually reinflate the swimbladder to achieve neutral buoyancy, but for some reason this does not appear to be happening with Sydney.

Occasionally, this is because certain parasites and/or bacteria are presenting the gas gland or swim bladder from functioning properly, as in the following case reported by Paul Anderson in which undesirable copepods may have played a role in the problem:

<Open quote>
Hi folks,

I’ve been having some unusual health circumstances arise from my research seahorse population that I thought I’d share with you to see if anybody has encountered any similar problems.

First, I have a problem with harpacticoid copepods in my seahorse tanks. They seem to have gotten in everywhere; all system and quarantine tanks. I’m reasonably sure that they are coming in from live brine shrimp feedings.

Most of the animals seem unaffected by the copepods, but when picking up a seahorse it’s very common to see several of these copepods crawling on the animals. Most of the animals have not developed lesions as a result, but a few have, and I suspect the copepods for contributing to such problems.

Curiously, at least two animals have come down with negative buoyancy problems (sinking to the bottom, unable to keep upright or swim up). Over time, their swimbladders have become so swollen that it is markedly noticeable from the external profile of the animal. We have necropsied both animals that have presented as such and have found HUGE digenes in the swimbladder, coupled with a brownish exudate that, under microscopy, is loaded with bacteria and (presumably) digene eggs. The exudate was examined and cultured and, unfortunately, stained Acid-Fast Base positive and grew on Lowenstein-Jensen Media (suggesting Mycobacterium infection). There are probably other bacterial spp. present but I don’t have that information with me; this case is being worked up with our Veterinary Team here at the Department.

Here come the questions/thoughts:

Because digenes have an indirect life cycle, I suspect that either the copepods or the Artemia themselves are serving as intermediate hosts. If only the copepods are serving as intermediate hosts, the seahorses may be ingesting them when feeding on frozen mysids that the copepods may have attached to prior to ingestion. To help eliminate this problem, I am treating all tanks with Dimilin to eradicate the copepods. Initial treatment did reduce numbers, but the copepods are constantly being re-introduced through the live Artemia feedings. I am not treating the Artemia holding tank with Dimilin because Dimilin may also kill the Artemia.

I have more recently been thinking about biological controls. I was initially hesitant to include other spp. in my tanks to minimize experimental variability, but this problem may merit the introduction of something like a cleaner wrasse or similar animal in each tank to keep copepod numbers down (and off the seahorses).

What are your thoughts? Has anybody had any similar problems? Has anybody tried keeping seahorses with cleaner-type fish?

Looking forward to your responses. Thanks!

Paul Anderson
Ph.D. Candidate/Alumni Fellow
Department of Fisheries and Aquatic Sciences
University of Florida
<close quote>

So it’s very difficult to say precisely what could be causing Sydney to have a problem like this, and your best option may simply be to wait and see if Sydney is eventually able to correct the problem herself the natural way.

I would hesitate to suggest any sort of aggressive treatment without an understanding of exactly what is happening, and right now I am clueless in that regard, Claire…

Best wishes with all your fishes.

Pete Giwojna

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