Reply To: Slithering on the bottom

Pete Giwojna

Dear Lisa:

It sounds like your seahorse was suffering from chronic pouch emphysema, which was causing the buildup of gas in its brood pouch. I think you get the right thing by treating your stallion with Diamox to resolve the issue with Gas Bubble Disease, which is a potentially fatal condition.

However, your seahorse is now apparently dealing with negative buoyancy, the tendency to sink, which is a much less serious issue. Many times a problem with negative buoyancy is simply the result of an underinflated gas bladder or swimbladder, as explained in more detail below, Lisa:

As in many other bony fishes, the seahorse’s gas bladder functions as a swim bladder, providing the lift needed to give them neutral buoyancy (Seahorse Anatomy, 2004). In essence, the swim bladder is a gas-filled bag used to regulate buoyancy. Because the seahorse’s armor-plated body is quite heavy, this organ is large in Hippocampus and extends well down into the body cavity along the dorsal boundary (Seahorse Anatomy, 2004). It will have a whitish to silvery appearance and is a simple, single-chambered sac that begins at the bend in the neck and extends to about 1/3 of the length of the coelomic cavity (Bull and Mitchell, 2002).

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, and they can no longer inflate their gas bladders this way. Consequently, fry that miss this early opportunity to gulp air — 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. On the other hand, accidentally ingesting air after the pneumatic duct closes off, or over inflating the swim bladder by gulping too much air while feeding at the top or entrapped by the surface tension, result in fatal buoyancy problems that leave them bobbing helplessly at the surface, again unable to feed.

Past the newborn stage, the seahorse’s swim bladder is completely self-contained, with no duct connecting it to the esophagus. As a result, they can only regulate their buoyancy by resorbing gas from the swim bladder or secreting more gas into the bladder, which is a relatively slow process (Jobling, 1995).

The composition of the gas contained within the swim bladder is about 80% oxygen, with much lesser amounts of carbon dioxide and nitrogen (Evans, 1998). The oxygen that fills the swim bladder is delivered via the bloodstream, but in order to do this, the oxygen must be secreted from the blood to the lumen of the swim bladder against a strong gas pressure gradient, and once deposited therein, the gas must be prevented from diffusing back into the blood (Evans, 1998).

This is accomplished with the aid of the gas gland, a very sophisticated organ located in the wall of the swim bladder, and the rete mirable or “miraculous net,” which delivers blood to the gas gland (Evans, 1998; Jobling, 1995). With the help of the rete mirable, the gas gland is capable of extracting gases from the blood stream and concentrating them into the swim bladder. The rete mirable is basically a dense network of blood vessels running parallel to each other, which function as a countercurrent exchanger. Capillaries carrying oxygen-rich arterial blood from the gills to the gas gland run parallel to and directly alongside capillaries carrying oxygen depleted venous blood from the gland in the opposite direction (Evans, 1998). It is countercurrent exchange in the rete mirable that acts to retain the swim bladder gases. To the naked eye, the rete mirable appears as one or more circular patches of blood vessels on the surface of the swim bladder (Diseases of Ornamental Fish, 2004).

Gasses and solutes in the venous blood leaving the gas gland move into the incoming arterial blood through the rete mirable via passive diffusion and are returned to the gas gland (Evans, 1998). In this way, the rete acts as a trap that retains the gases in the swim bladder.

The respiration of epithelial cells in the gas gland releases lactic acid and CO2, and these substances are then trapped in the rete via countercurrent exchange and returned to the gas gland where they accumulate (Evans, 1998). As a result of this multiplying effect of the rete mirable, conditions within the gas gland can become 10 times more acidic than normal (Evans, 1998). This is important because hemoglobin loses the ability to bind oxygen under acidic conditions, so the oxygen-rich arterial blood flowing into the gas gland releases the oxygen it is carrying in the gland (Evans, 1998). The oxygen that’s offloaded due to the acidification of the blood becomes concentrated in the gas gland until it is finally secreted into the swim bladder itself.

Removing excess gas from the swim bladder is an entirely different matter. The gas gland plays no role in gas resorption, which occurs in an entirely different area of the swim bladder, called the oval (Evans, 1998). The surface of the swim bladder in the oval region is covered with a meshwork of thin blood vessels, which receive a different blood supply altogether than that of the gas gland (Jobling, 1995). It is there, in the oval, that gas resorption occurs. Gas removal takes place only when a fish is rising in the water column and thus experiences reduced hydrostatic pressure. At other times, the blood vessels that supply the oval are closed off by a series of muscular valves; with no significant blood flow to the oval, there can be no gas resorption (Evans, 1998).

Okay, Lisa, that’s a quick rundown on how the seahorse’s gas bladder or swimbladder regulates its buoyancy. The mechanisms described above allow the seahorse to maintain neutral buoyancy, the point at which is weightless in the water, and can therefore maneuver and swim about effortlessly. Many times when a seahorse is experiencing a problem with negative buoyancy, it will be able to resolve the problem itself by secreting more oxygen from the gas gland into the swimbladder. But this is a slow, gradual process that may take several days. As long as the seahorse is still eating and keeping up its strength, you can afford to wait and see if the problem corrects itself. So for the time being, I would recommend doing nothing as long as the seahorse is able to feed normally and keep its strength up. Just take your time and wait to see if your stallion is able to gradually secrete more gas into its swim bladder and counteract the negative buoyancy.

Good luck.

Pete Giwojna, Ocean Rider Tech Support

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