In a recent Salt Solutions someone asked if clownfish will accept a tube anemone as a host. The answer was correctly no, and went on to explain that “instead of providing shelter and protection for the clownfish, the tube anemones would quickly consume them!” The column goes on to describe the care of these animals and the answer to that question ends with solid advice as well: “Tube anemones are filter feeders...twice weekly feedings of live or frozen small foods seem to suffice, but the emphasis should be on small and frequent feedings as time will allow.” I think that the advice provided on the care and maintenance of tube anemones was solid, but despite the recommendation that tube anemones are "filter-feeders" and require "small foods" the author of the reply still wanrs that these animals will eat many fish. This is the standard answer provided by nearly every aquarium text, and several well-known references can be cited for describing these animals as dangerous fish killers with "long and extremely poisonous tentacles.” Unfortunately, they all seem to be wrong – I am not sure where the fish-killer reputation for tube anemones originated, but the response I read here is not really any different than that I see other places, and I'm sure that the author was simply repeating the information provided by their favorite reef aquarium text book. Unfortunately, the answers provided by these texts are simply not accurate – the chances of a tube anemone eating a healthy fish are actually much lower than any of the clownfish hosting anemones accomplishing the same feat, but we rarely see a warning about placing fish into a tank with clowns and a host anemone (although James Fatherree had a diary of over $300 worth of tankmates consumed by his “Killer Carpet” recently). As far as I can tell someone at sometime in our distant reefkeeping past decided that these anemones were fish eaters, and every text and column written since that time has repeated that misinformation as fact. I want to discuss these animals in some detail and pose the question: why is it that the tube anemone is always met with such caution while other species that are true fish specialists are not?
To be honest, I simply don’t know, but I would like to share a variety of factoids about tube anemones to try to dispel some of the myth-information about these fascinating animals. First off, you may have noticed that I keep referring to them as “animals” rather than “anemones” unless I specifically say “tube anemone” above. The reason is that despite their appearance, these animals are not anemones at all, and are in fact most closely related to the black corals (of jewelry fame). It is true that they superficially resemble anemones, but the internal differences are so great as to group them in a different subclass (the Ceriantipatharia) from the true anemones – to give you an idea of how different that makes them, true sea anemones, scleractinian (hard) corals, zoanthids and corallimorphs (mushrooms) are all grouped into the same subclass while these animals are not! That’s pretty different!
So why are the cerianthids not anemones? Aside from some internal structural differences that no one cares about, they are the only "anemone_like" animal with two distinct whorls of tentacles on the oral disc __ they have a batch of short "labial" tentacles which surround the mouth and then a ring of very long tentacles which encompass the outer edge of the oral disk. These long tentacles are used primarily in prey capture and defense, while the shorter labial tentacles are used primarily for prey manipulation and ingestion. In many species the tentacles can be bioluminescent, which is thought to be a visual “startle” defense against fishes that may attack the long feeding tentacles of the animal during their nocturnal expansion. Although some species are seen exposed during the day (and more so at depth), in general the species from shallow tropical waters are very stubborn in avoiding light, and rarely come out during daylight hours – even at night, the animals will retract as soon as the edge of a dive light beam passes across them. Their primary predators are not fish, however, but dendronotid nudibranchs which have a sterotypic feeding behavior to latch onto and ingest the feeding tentacles of these animals. These slugs can even be “sucked” into the tube of the “anemone” when it withdraws, but it doesn’t seem fatal to either animal – the slugs can crawl back out of the tube when they have finished their meal, and individual tube anemones have been followed for up to 10 years in areas where they are regularly preyed upon by these nudibranchs. It is impossible to know exactly which tube anemone you have, though, because the specific identification of these guys is virtually impossible without killing them and examining some internal structures.
Another key difference between the cerianthids and true anemones is that cerianthids lack the pedal disk and all associated musculature as well as lacking any sphincter muscles – this may sound minor to you, but it means that unlike true anemones, they cannot attach their base to anything. More importantly, however, although they can withdraw into their tube, unlike true anemones they cannot contract the oral disk or withdraw the tentacles (if you’ve ever seen a true anemone feed, you’ve seen it contract the oral disk to bring it’s tentacles closer to the mouth, and when disturbed the tentacles are retracted, the oral disk contracted and a sphincter muscle closes the top of the animal to protect it). Rather than a pedal disk, the animals end in a blunt point (sometimes called the foot) in which a small hole (often mis-named an “anal pore” but more correctly called a terminal pore because the hole simply allows water to escape the inflated animal during retraction into the tube rather than providing any “anal function”). The tube in which these animals live consists of a material almost akin to fiberglass, which is formed by the discharge of special cnidae (the stinging cells of all cnidarians, with nematocysts being the most common kind) called ptychocysts which are found in only the tube anemones. Depending on the species (there are about 25 world-wide), these tubes can be quite elaborate and some measure >2m in length below the sediment surface! As I mentioned above, they are nocturnal, and some species can expand to form a 30cm-or-so sphere of feeding tentacles at the end of the tube (obviously a large portion of a small tank could be covered by the reach of 1ft long tentacles!).
OK, that's all interesting, but it does nothing to address the question of whether or not these animals are likely to eat fish in an aquarium. The fact is that there are simply no detailed studies of the natural diet of these animals, but studies that examine the distribution and abundance of these animals in the wild link them to high densities of plankton in the water, not to any fish prey. At least one species appears not to feed at all, but rather to live entirely off symbiotic bacteria in their gut. A survey by the National Marine Fisheries Service found that densities of these animals was actually highest in association with polluted waters in which there was significant enrichment and/or resuspension of benthic infauna. Without dteailed studies of the natural diet of these animals it is difficult to answer exactly what they eat, but none of these natural examples seem to support the idea that these animals are effective fish predators. The fact remains that all the clownfish hosting anemones and even some species of corallimorpharians (particularly the Elephant Ear Mushroom, Amplexidiscus enestrafer) are much more effective predators on fish than are tube anemones, yet it is the mention of tube anemone that continually elicits a strong warning among hobbyists to avoid them (and I don’t think I’ve ever seen anyone warned against adding a corallimorph of any sort to their tank).
The “extremely poisonous tentacles” description seems especially unwarranted, in fact, because there are actually a couple of studies on the stinging cells of tube anemones in comparison to other species. One study examined the toxins present in the stinging cells of Aiptasia and compared those to the tube anemone Pachycerianthus torreyi, and found that the toxin of Aiptasia was much more potent and complex than that of the tube anemone. Another study examined the toxic effects of 11 species of anemone-like animals – true anemones, mushroom polyps and tube anemones – to compare the toxicity and effectiveness of the stinging cells of these animals. The researchers found that the toxins of all species tested except the tube anemone P. fimbratus had potent effect on a variety of test vertebrates, and that of the fish predators Urticina lofotensis and U. piscivora was lethal to more than just fish – tests using guinea pig, rat and dog cell cultures all resulted in cell death from tiny amounts of the toxin. In contrast to those results, the sting of the tube anemone P. fimbratus were not found to be lethal to any species in any test these researchers conducted. Hmm, that doesn’t sound very dangerous by comparison to me...
So, those details aside, I can say that I have had a couple of ceriathids in my tanks over the years, and although they have been with a variety of fish all along, I have seen no evidence that they have ever caught anything large in my tank (unlike my carpet anemone which, like the “Killer Carpet” above, has caught several fish over the years!). Sure, it is always possible that a tube anemone may capture a small fish in an aquarium, and I’m sure that the story of the ‘dangerous fish killing tube anemone’ had to start from something untoward at some point, but my point here is that by all accounts these animals are less dangerous to fish than either true anemones or even some species of mushrooms. I simply do not understand why these animals strike fear into the hearts of reefkeepers given that no such response is seen when discussing other species that are potentially much more dangerous.
Tube anemones could be an interesting addition to a reef tank if the proper conditions exist, but more often than not, the conditions preferred by tube anemones are not those maintained in reef aquaria. First, they are called tube anemones for a reason – they should have a well-developed tube (essentially it looks like fiberglass because it is woven of the threads of special stinging cells which only tube anemones possess) when you purchase them. They can rebuild their tube, but it is stressful and energetically costly, and you can be assured that they will be stressed enough when being moved to a new tank and forced to develop a new burrow, so aim for one with a good tube to start with. Second, they live in deep burrows in fine sands and muds. If you have a bare-bottom tank, or one with either a thin bed or coarse gravel, these animals will not be able to find a spot that suits them and will most likely perish in short order in the aquarium. In the wild, the burrows of large animals frequently extend in excess a meters, so a sandbed that is only a couple of inches deep is not just likely to cut it for these animals. Third, as I’ve mentioned twice so far, the vast majority of these species are nocturnal – that means that you will likely only see the animal occasionally after the lights are off, and although some do appear periodically during the day, you shouldn’t expect to see them very often if they are happy and healthy. Finally, because they are nocturnal, they are obviously not photosynthetic, and cannot survive without regular feeding. As I mentioned above, all available evidence suggests that they likely feed primarily on plankton, so if you have a well-developed deep sandbed with lots of critters and/or a refugium in the tank (especially if you develop a good culture of amphipods and mysids) they should do fine (I haven’t specifically fed mine in about a year now), but if not, or if the animal is just becoming established in the tank, krill soaked in Selcon should be offered at least two to three times a week (or as often as you can if it’s not out very much) to keep the animal healthy. The feeding turns out to be one of the biggest problems, because when disturbed, they quickly pull back into their tube. That makes it pretty hard to drop a piece of shrimp or fish on them, and it generally takes a serious effort on your part to ensure that they are well-fed and healthy.
As I said above, the advice in the Salt Solutions was generally sound, and although these animals can be quite challenging, they can also be an interesting addition to a tank if their requirements are properly met. I just wanted to give a little more detail about the group and try to explain why some of the widely-available information on tube anemones (even that in some of most popular and well-respected aquarium texts) is simply misguided...