Re:fry and salinity

Pete Giwojna

Dear Nigel:

Yes, sir, A newborn seahorse’s first instinct is to head to the surface to fill its swim bladder. Pelagic seahorse fry, which undergo a free-swimming or planktonic stage of development, tend to remain near the surface afterwards. Benthic seahorse fry, however, typically make a quick trip to the surface to inflate their swim bladders and then immediately settle down to a bottom-dwelling existence.

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 (physosymotous), 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.

This is the case with most seahorse fry. 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.

I have never reared Hippocampus capensis myself, so I cannot say for certain whether the newborns are physosymotous or physoclistous at birth. But the Zulu babies are as benthic as they come, which is what the phrase "drop like rocks" is meant to indicate. To play it safe, you should allow them an opportunity to inflate their swim bladders at the surface, as many other benthic seahorse fry do.

Yes, providing your in-tank refugium is large enough and is free of hydroids and Aiptasia rock anemones, it will make an acceptable nursery tank for seahorse fry. The abundant pods and microfauna that populate a refugium can supplement the daily feedings of newly-hatched brine shrimp (Artemia nauplii) you provide for the fry. Just allow the newborns an opportunity to fill the air bladders at the surface, and then transfer them to your in-tank refugium once they are a day or two old and they should be fine. How large is a refugium, Nigel?

When you order a mated pair of seahorses, they have been paired up for one or more breeding cycles. Beyond that, there’s no way of knowing for sure how experienced they are or how many broods they may have produced overall. In general, the seahorses you receive will be young adults that have recently become sexually mature and begun breeding.

It’s a misnomer to speak of a breeding pair as inexperienced or experienced parents, since the young are completely independent and left to their own resources from the moment of birth. So there’s really no such thing as parental care among seahorses…

Ocean Rider has been in existence since 1998, and to my knowledge, they have never had to deal with a hurricane. Unlike Florida, the Eastern Seaboard, and the Gulf of Mexico, which may face one severe storm after the other during hurricane season, such events are quite rare in Hawaii. The vast majority of hurricanes die well before they get that far West. The Ocean Rider aquaculture facility is uniquely situated at the National Energy Lab, which means they have unlimited access to ultra-pure natural seawater at any temperature they desire from deep beneath the ocean, which is completely unaffected by conditions at the surface. As long as they have power, the seafarm is assured of clean, recirculating ocean water, and with backup generators to assure an uninterrupted source of power, the facility is relatively weatherproof.

Best of luck with your Zulus, Nigel!

Happy Trails!
Pete Giwojna

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