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Like all cnidarians, hydroids are capable of stinging. There are very many types of hydroids, but the kinds that you cannot feel stinging with your bare hands and forearms are generally harmless to the greater seahorses.
However, some hydroids and hydromedusae can produce a notable sting to your bare skin when you’re working in the aquarium, and such varieties can be a source of irritation to be even the larger seahorse species, so if you have hydroids established in your aquarium and they are one of the varieties with potent nematocysts, it’s possible that they could be stressing your seahorses to the point where their immune system is compromised and they become susceptible to various health problems.
Sooner or later hydroids will appear in any marine aquarium that is receiving regular feedings of rotifers, copepods, or baby brine shrimp or plankton suitable for filter feeding invertebrates. It’s inevitable because they can gain entry into the aquarium in many ways. For example, they are notorious hitchhikers. Both the colonial polyp stage and the free-swimming micro-jellies can thumb a ride on live rock, macroalgae, hitching posts, sand or gravel, specimens of all kinds, or within so much as a single drop of natural seawater (Abbott, 2003). Beware of fuzzy looking seashells! Very often hydrozoans come in on the shells of the hermit crabs or snails we purchase as aquarium janitors (Abbott, 2003). Or they may be introduced with live foods, or even among Artemia cysts, in some cases it seems. They can even be transferred from tank to tank in the aerosol mist arising from an airstone or the bubble stream of a protein skimmer.
It can be very challenging to identify hydroids because there are about a zillion different species of hydrozoans and the different types have different characteristics and are often vary remarkably in appearance. There is considerable variation within the species as well, and the same type of hydroids can appear vastly different depending on the size of the colony and its stage of development, conditions in the aquarium, and their predominant diet. And, of course, the different stages of the life cycle of these amazing animals are so entirely different that they were long believed to be different types of cnidarians altogether, and different species names were often assigned to the same hydroid in different phases of its life cycle. Because they are so difficult to identify and are not easy to distinguished with the naked eye during their initial stages, hydroids often go undetected in nursery and rearing tanks until they begin to take a toll on the fry.
The typical hydroid colony has a stem with a variable number of polyps growing on it, and each of these polyps bears numerous tentacles that are liberally studded with knobby nematocysts (batteries of deadly stinging cells). There are many different kinds of hydroids and they appear in the aquarium in many different guises: many colonies are stalked; some have fingerlike projections, others look like tiny pink fuzzy balls or appear like cobwebs (the webbing kind usually spread along the bottom or grow on the aquarium glass along the substrate). The “snowflake” type of hydroids seem to be particularly common in aquaria, whereas other species look more like crystal chandeliers, and some species form bushy colonies as they grow that serve as microhabitats for Caprellid skeleton shrimp and other tiny crustaceans.
Even a large hydroid colony appears harmless to the naked eye of the untrained observer. It takes a much closer look to reveal the dreaded ‘droid’s lethal nature, as described below:
“Studying the colony under high magnification, one soon becomes lost in an extraordinarily complex, living world–a microcosm in which a beautiful but deadly ballet is conducted on a microscopic scale (Rudloe, 1971). Hungry polyps, some resembling snapdragons, others looking more like daisies or tulips, expand their knobby, translucent tentacles, slowly flexing and languidly waving them about, lulling the observer with their slow-motion ballet — until they abruptly and quite unexpectedly snap up a bit of planktonic life, stinging it, drawing it in with one violent contraction, digesting it, and then re-expanding like a blossoming flower to hunt again (Rudloe, 1971). There are many such polyps in a colony, hundreds of them, each of which is armed with many tentacles and countless nematocysts, and at any given moment, some of them will be dormant and still, some will be expanded and lazily casting about for prey (Rudloe, 1971), and still others actively feeding (Abbott, 2003).”
The feeding or nutritive zooids are the distinct individual animals in a hydroid colony that are responsible for capturing and digesting prey; as such, they bear the nematocyst-studded tentacles. But you need high magnification in order to appreciate the true beauty of living hydrozoans, or to differentiate between different species of hydroids, or to observe the zooids going about their deadly business.
Hydroids are insidious because they start out so small and insignificant, yet spread so quickly under ideal conditions (e.g., a nursery tank or dwarf seahorse tank receiving daily feedings of Artemia nauplii). Many species can spread asexually by fragmentation as a microscopic speck of the parent colony. All of the troublesome types have a mobile hydromedusae stage, which look like miniscule micro-jellyfish, and can spread sexually in this way as well (Rudloe, 1971). The mobile medusae swim about with a herky-jerky, pulsating motion and are often mistaken for tiny bubbles due to their silvery, transparent, hemispherical bodies (Rudloe, 1977). These tiny jellies often go unrecognized until they begin to settle and are discovered adhering to the tank walls. They will have a large “dot” in the middle of their bodies and smaller ones at the base of their nematocysts (Abbott, 2003). Both the polyp stage and the medusa stage sting (Rudloe, 1977) and are capable of killing or injuring seahorse fry. Multiple stings can kill the babies outright, but they are often only injured by the nematocysts, which damage their integument and leave them vulnerable to secondary infections. Many times it is a secondary bacterial or fungal infection that sets in at the site of the injury which kills the fry.
Once they find their way into a dwarf seahorse setup or nursery tank, hydroids can explode to plague proportions very quickly because conditions are ideal for their growth: perfect temperatures, an abundance of planktonic prey that is renewed every few hours, and a complete absence of predators.
When hydroids become a problem in the nursery or dwarf tank, there are a couple of good ways to eradicate them and get the situation under control again:
Hydroids can be controlled in the aquarium by using a medication known as fenbendazole to treat the tank over a period of days. Fenbendazole (brand name Panacur) is an inexpensive anthelmintic agent (dewormer) used for large animals such as horses, and the de-worming granules can be obtained without a prescription from stores that carry agricultural products (e.g., farm and ranch equipment, farming supplies and products, veterinary supplies, livestock and horse supplies, livestock and horse feed). If you live in a rural area, those would be good places to obtain it as well.
You can also fenbendazole granules in small quantities online from the following vendor:
However, there are a couple of things you should keep in mind when treating an aquarium with fenbendazole, Kaie. Administering a regimen of fenbendazole (FBZ) or Panacur will eradicate any hydroids, Aiptasia rock anemones, or bristleworms from live rock or live sand, thereby rendering them completely seahorse safe. The recommended dose is 1/8 teaspoon of the horse dewormer granules (22.2% fenbendazole) per 10 gallons of water. Dose aquarium with 1/8 teaspoon/10 gallons every other day until you have administered a total of 3 such treatments (Liisa Coit, pers. com.). Even one dose will do a fine job of eradicating bristeworms, but Aiptasia rock anemones and hydroids are a bit tougher and may require 2-3 doses to eliminate entirely.
Because fenbendazole is essentially a de-worming agent, it will destroy any bristleworms, flat worms, spaghetti worms or the like. The FBZ or Panacur treatments are best administered to the live rock in a bucket or hospital tank before the LR is introduced in the main tank. Otherwise, the massive die-off of the worm population in the aquarium may require large water changes in order to prevent a dangerous ammonia spike! And after the treatment is completed, its a good idea to add a portion of newly purchased live sand to the system in order to help restore its normal diversity of fauna and microfauna again (Liisa Coit, pers. com.).
Fenbendazole does not have any adverse effects on biological filtration, but be aware that it is death to many Cnidarians besides hydroids. Mushrooms and related corals are generally not affected, but expect it to have dire effects on other corals (e.g., sinularias), polyps, gorgonians, and anemones. In general, any Cnidarians with polyps that resemble the stalked family of Hydrozoans are likely to be hit hard by fenbendazole, so don’t use this treatment in a reef tank!
Also be aware that fenbendazole seems to soak into the porous live rock and be absorbed indefinitely. I know one hobbyist who transferred a small piece of live rock that had been treated with fenbendazole (Panacur) months earlier into a reef tank, where it killed the resident starfish and Astrea snails. So enough of the medication may be retained within treated live rock to impact sensitive animals months after the fenbendazole was administered. Don’t treat live rock intended for reef systems with fenbendazole (Panacur)!
But this can actually be a good thing in the dwarf seahorse tank. The fenbendazole that soaks into live rock and is then leached back out again in very small quantities can provide the dwarf seahorse tank with protection against another outbreak of hydroids for many months after the tank is treated.
At the lower dosage recommended for nursery tanks and dwarf seahorse tanks with fry (1/16 tsp. per 10 gallons), fenbendazole normally does not harm cleaner shrimp and decorative shrimp. It will kill starfish but copepods, hermit crabs, and shrimp are normally not affected.
When it comes to snails, Nerites, Ceriths, and Nassarius snails are not affected by the medication and can remain in the aquarium during and after treatment with fenbendazole.
On the other hand, Trochus or turbo snails, Astrea snails, and especially Margarita snails are sensitive to fenbendazole/Panacur and should be removed from the aquarium until the treatment regimen has been completed and the fenbendazole has been pulled from the aquarium using activated carbon and/or polyfilter pads for chemical filtration.
Macroalgae such as the feathery or long-bladed varieties of Caulerpa or Hawaiian Ogo (Gracilaria) are not harmed by exposure to fenbendazole at even triple the normal dose. In fact, if you will be using Caulerpa in your nursery tanks to provide hitching posts for the fry and serve as a form of natural filtration, it’s a very wise precaution indeed to treat them with a regimen of fenbendazole beforehand.
So fenbendazole (FBZ) or Panacur is primarily useful for ridding bare-bottomed nursery tanks and dwarf seahorses setups of hydroids and Aiptasia anemones, ridding Caulerpa and other macroalge of hydroids or Aiptasia before its goes into the aquarium, and cleansing live rock of bristleworms, hydroids, and Aiptasia rock anemones before it is introduced to the aquarium.
It can also be used to eradicate bristleworms, hydroids, an Aiptasia rock anemones from an established aquarium if it does not house sensitive animals such as live corals and gorgonians, starfish, Astrea snails, or tubeworms and other desirable worms that may be harmed by FBZ, providing you monitor the ammonia levels closely and are prepared to deal with the ammonia spike that may result from the sudden death of the worm population.
In summation, if the aquarium with a hydroid problem will be housing live corals at some point, it would be best not to treat it with fenbendazole. And you may not need to be concerned about the hydroids in the first place, since as long as you keep them out of your nursery tanks, most types of hydroids won’t present a problem for large seahorses like Mustangs and Sunbursts (Hippocampus erectus) at all. They are impervious to their stings. In fact, in the wild, seahorses often encourage algae, small bryozoans and other encrusting organisms, including hydroids, to grow on their exoskeleton in order to enhance their camouflage (Vincent 1990). The hydroids are only a problem for baby seahorses in nursery tanks or for the tiny dwarf seahorses, which are susceptible to their stings. So as long as you will be keeping large seahorses in the aquarium that has hydroids, there is really no need to eliminate them.
If it becomes necessary to eradicate hydroids from an aquarium that will be used for live corals and delicate invertebrates, there is another alternative to killing them with fenbendazole that you can consider. Hydroids of many kinds can be eradicated from the aquarium by raising the water temperature to 92°F or above for period of 3-5 days (Liisa Coit, pers. com.). Keep all of the filters and equipment operating and sterilize your brine shrimp nets, so that the hot water circulates throughout them and destroys any hydroids or hydromedusae that may be present in the filtration system. Be sure to sterilize your brine shrimp nets, etc., at the same time as you are heating up the infested aquarium. (Seahorses and their tankmates, including snails and the cleanup crew, must be removed to a temporary holding tank while the heat treatment is carried out.) Maintaining the water temperature at 92° for this period does not harm the beneficial nitrifying bacteria in your biofilter, injure marine plants or macroalgae, or kill off copepods and other beneficial microfauna (Liisa Coit, pers.com.).
After the treatment period, perform a large water change to assure that the die off of hydroids does not degrade your water quality, and adjust the water temperature back to normal, and all the animals can be returned to the aquarium. The tank will not undergo a “mini cycle” and there will be no ammonia or nitrite spikes (Liisa Coit, pers. com.).
However, not all types of hydroids respond to the heat treatment method of eradication. Unfortunately, the snowflake type of hydroids that are all too common seem to have no difficulty surviving the heat treatment. So generally speaking, then Panacur (i.e., fenbendazole) is a more reliable way to eliminate them. But if your hydroids do not resemble snowflakes, then there is a fair chance that the heat treatment will be effective.
Okay, Kaie, that’s the quick rundown on hydroids and when they need to be controlled.
The problems you could be having could serve may also be an indication that the seahorses provided by your local fish store (LFS) are not the healthiest. It’s possible that your pet-shop ponies may be wild-caught or may have been pen raised, rather than being captive-bred-and-raised seahorses
Net Pens are a low-tech, low-maintenance method of farming seahorses that basically involves raising them in large enclosures in coastal waters. In some cases, entire lagoons may be fenced off for that purpose. In the simplest form of pen rearing, broodstock are released into these enclosures, and then they and their progeny are pretty much allowed to fend for themselves thereafter. Any offspring that survive to marketable size are periodically harvested from the holding pens or lagoons.
Such operations are controversial with environmentalists for a number of reasons. Since the enclosures are open to the ocean, there is a real risk that adults or their fry may escape from the pens and establish colonies in the wild that may pose a threat to endemic seahorse populations. The pens are no barrier to disease organisms or parasites, so pathogens and parasites imported on foreign broodstock may spread to fishes in the wild (or vice versa). Wastes from the high density of penned animals are carried directly to ocean on prevailing tides and currents and may have a negative environmental impact on the surrounding area. There is no way to monitor the penned animals, hence no way to determine whether the seahorses they contain are actually born and raised in captivity or are merely wild-caught seahorses maintained in holding pens prior to being shipped off to unsuspecting consumers.
Pen-grown ponies can thus be risky for the hobbyist because of the circumstances under which they were raised. In essence, a mesh barrier is all that separates them from wild seahorses. There is no guarantee they will be disease free. Although many of them learn to accept frozen Mysis, there is no guarantee they will eat frozen foods since they are often accustomed to foraging for live prey. There is no guarantee they will be able to adjust to aquarium conditions since they are essentially raised in the sea. There is no guarantee that they are even captive bred, since the pens are not secure and livestock is introduced and removed from the pens and lagoons on a continuous basis. There is no guarantee they will be friendly and sociable rather than shying away from their keepers, since they are unaccustomed to the human presence.
Seahorse keepers that are shopping for their livestock at their LFS should therefore proceed with caution nowadays. Don’t assume that the new CITES regulations protecting the genus Hippocampus automatically assures that the seahorses sold at your LFS are cultured animals.
If the seahorses in question are over 4 inches in total length, there is a strong possibility the seahorses were harvested from the wild. It’s a mistake to assume that since CITES regulations to protect the genus Hippocampus went into effect last year that all the seahorses now being exported to the US have been captive-bred-and-raised. Plenty of wild-caught seahorses are still entering the country, destined for the pet trade.
The CITES regulations currently use a size limit to manage the seahorse fishery. Seahorses smaller than 10 cm or about 4 inches in length are illegal under CITES and cannot be imported or exported by member nations. However, wild-caught seahorses larger than 4 inches can still be imported legally if the necessary permits are obtained. Also, CITES regulates only the international trade in seahorses, so Hippocampus erectus and Hippocampus zosterae collected in US coastal waters are exempt from the regulations and can still be marketed freely within the US.
So when you purchase seahorses from your LFS there is a good possibility that they may have been procured from the wild or pen-raised rather than born and raised in captivity at an aquaculture facility. It’s fine if you want to give the new seahorses at your LFS a try, but because of the uncertainties surrounding them, be sure to play it safe and quarantine them before introducing them to your main tank, just as if they were wild seahorses. Assume they are wild or at best pen-raised and take all the necessary precautions so you won’t get burned. And I would also be a very sensible precaution to set up a new tank just for them, rather than mingling them with your existing captive bred livestock.
In general, if you’re new to seahorse keeping, you will increase your chances for success if you start off with hardy, easy to feed, captive-bred-and-raised livestock you obtained directly from a reputable breeder such as Ocean Rider.
Consider Ocean Rider’s strains of captive-bred-and-raised Hippocampus erectus, for example, Kaie. Ocean Rider has been working with this species intensively since 1998, and they have now been selectively bred for traits such as adaptability, disease resistance, vigor, aggressive feeding habits, and rapid growth for dozens and dozens of generations. Far from being recessive characteristics that could eventually result in inbreeding, these are all adaptive traits that increase the line’s fitness and improve survivorship. In fact, they are the same sort of traits Mother Nature herself selects seahorses for in the wild to assure survival of the fittest. When nature culls out the weakest and least fit, it is known as “natural selection.” It is nature’s way of keeping a species strong, vigorous, and adaptive (i.e., evolving to better fit its niche). The only difference is that Mother Nature is selecting for suitability to their natural habitat, whereas aquaculturists are selecting seahorses for fitness to captive conditions. In both cases, the selection process assures that the specimens become ever stronger and better adapted to their environmental niche, whether that is the aquarium or the ocean itself.
Practiced in this manner, selective breeding actually strengthens and improves a strain generation by generation, producing seahorses that are progressively hardier and better suited for aquarium life. This level of domestication not only improves their general health but also eliminates much of the stress wild seahorses experience in captivity, allowing cultured seahorses not only to live longer and healthier but to live better as well.
So Mustangs and Sunbursts (H. erectus) are now very hardy specimens that are supremely well adapted for aquarium life.
I will attach a document to this e-mail that explains all about Ocean Rider Mustangs and Sunbursts, including dozens of full-color photographs, so that you can download the document, save it on your computer, and then read through the information at your convenience, Kaie. You should have a much better idea of what the Sunbursts are really like in a home aquarium once you’ve had a chance to look through the document.
In addition, it would be a good idea for you to participate in the Ocean Rider seahorse training program considering the problems you’ve been having with your ponies lately. The Ocean Rider seahorse training manual is very comprehensive, consisting of several hundred pages of text with more than 250 full-color illustrations, and it will explain everything you need to know in order to keep Ocean Rider seahorses successfully in a home aquarium. We provide a free copy of the seahorse training manual to all first-time buyers and customers to assure that home hobbyists are well prepared to give their ponies the best possible care before they make a purchase.
All things considered, you may want to consider administering a regimen of fenbendazole (brand name Panacur) to eliminate any hydroids or Aiptasia that could possibly be stressing your seahorses. In the meantime, you may want to obtain a copy of the Ocean Rider Seahorse Training Manual to assure that you are providing your ponies with the best possible care. If so, just contact me via e-mail offlist, and I will send you your free copy of the training manual right away and do my best to answer any other questions you may have about the care and keeping your seahorses. You can always reach me at the following e-mail address:
Please let me know if I can be of any further service in the meantime, kaieocean.
Pete Giwojna, Ocean Rider Tech Support