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By Bob Goemans
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Q&A - Octopus

Authored by: Rob Toonen

Rob,

I just recently saw a Reef Octopus at the local pet shop and immediately fell in love. I have to get one of these guys, and wanted to know if it would be safe to add into my reef tank. I have a big tank (90g, 500W MH lighting, 70+ pounds live rock, soft and LPS corals) with plenty of hiding places for it and all my water parameters are perfect, so I think it will do great. My only real concern at this point is that I have several power heads in my tank and am worried that the octopus may get sucked into one of them. Is there anything else I need to worry about?

Thanks,

Jon.

Well, Jon, I certainly understand your fascination with these incredible animals, because I feel the same way myself! I have kept and bred octopus as pets for many years, and I am still constantly amazed by their intelligence and personality. However, despite my appreciation for your feelings, and the fact that I feel the same way about them, I simply cannot recommend one for a reef aquarium. While you are correct that an octopus needs a lot of hiding places and good water flow, a reef tank is not really the best way to provide for the needs of your animal. Furthermore, most octopuses are nocturnal, and even those that are active during the day prefer subdued lighting rather than the 500W of metal halide lighting that your reef tank provides! Although octopuses are invertebrates, and some species are certainly found on coral reefs, these animals are not particularly suitable for a reef aquarium for a wide variety of reasons that I will try to outline below. But before I get to that, I will (as usual) try to give you a bit of background about the biology of these animals.

Octopuses are members of the Phylum Mollusca, making them the distant cousins of a wide variety of more familiar reef aquarium animals including the Polyplacophorans (chitons), Gastropods (snails & slugs), and Bivalves (clams & mussels). For anyone interested in more detail on these groups, I provide a simplified introduction to some of the issues of current taxonomic classifications and basic biology of the Phylum Mollusca in my Introduction to the Molluscs article (Toonen 1998). Interested readers who wish to dig deeper into the relationships among the major types of Molluscs (or any other groups for that matter) are encouraged to get onto the Internet and check out the Tree of Life web pages (http://tolweb.org/tree/eukaryotes/animals/mollusca/cephalopoda/cephalopoda.html). Together with the squid, cuttlefishes and nautilus, octopuses are members of the Cephalopoda (or foot-heads), and for anyone interested in more detail than will be presented in this article, I very strongly recommend that you check out some of the great information about these groups and their biology on the Cephalopod web page (http://is.dal.ca/~ceph/TCP/index.html)(http://is.dal.ca/~ceph/TCP/index.html) and the National Resource Center for Cephalopods (http://www.nrcc.utmb.edu/).

I also think it is worth a quick discussion of why I use “octopuses” rather than “octopi” for the plural of octopus. I frequently hear people talking about “octopi” and it is somewhat of a pet-peeve of mine. Although many medical and scientific names tend to be Latin, for which the “i” ending is the correct plural (e.g., bronchus and bronchi), the name octopus comes from Greek roots (oktÇ = eight, and pous = foot), and should not be pluralized in the same way. Technically, the proper pluralization of the “-us” ending for a word such as octopus should be the suffix “-odes”, but octopodes is generally considered a bit of a mouthful, and in both the Oxford Dictionary and among the scientific community, the most common pluralization of octopus is simply “octopuses.” Even though this could be considered incorrect by purists, it is certainly preferable to the obviously incorrect pluralization “octopi.”

OK, having gotten that out of the way, let’s move on to discussing the octopus you would like to add to your reef tank. In general, the cephalopods are highly active predators that have an unusually high metabolism for an invertebrate, and as such have much more particular demands on some aspects of water quality (in particular, the oxygen content of the aquarium water - I will come back to water quality when discussing filtration below) than do most reef invertebrates. Octopuses are no exception to this generalization. In that respect, a reef tank is actually likely to provide excellent water conditions to keep one of these animals. Having said that, however, very few reef aquaria provide for the other requirements of keeping an octopus in captivity. For example, there are a plenty of good reasons that glass tops are not popular for reef tanks, including such issues as light penetration, gas exchange, and heat retention. However, with very few exceptions octopuses are phenomenal escape artists, and even a large octopus (some of mine have had a head the size of a grapefruit and been nearly 2 feet in diameter when spread out) can easily make a home out of a long-neck beer bottle! For a simple rule-of-thumb to suggest to hobbyists, I generally say that if you can stick your pinky finger into a hole, your octopus will be able to crawl through it. Obviously, this amazing ability to ooze through such tiny holes (despite an apparently large body size) makes it much more important that you provide a carefully designed and tightly fitting cover on your tank. Without such a tightly fitting cover to your tank, there is a very good chance that your new pet will eventually end up on the floor rather than in your aquarium.

This leaves you with something of a dilemma: how do you leave the water surface open to allow access for filtration and efficient gas exchange while simultaneously making an escape-proof tank to prevent your octopus from climbing out (or removing the top to let itself out) and ending up on the floor? Yes, you read that right - there are reports of octopuses not only being smart enough to figure out how to open the lid on their tank, but strong enough to actually do just that! Even more than simply being smart enough to escape the tank, there are numerous reports of octopus making “shopping trips” by crawling out of their own tank into nearby tanks to feed, and then returning to their own tank once they have captured something tasty to eat. The repeated disappearance of crabs or fish from nearby tanks has often turned out to be “late night snacks” for a hungry octopus that is able to figure out that there is food a short distance away, and figures out how to escape it’s tank, capture a quick snack, and then return home to enjoy it’s meal. Sadly, not all animals seem quite motivated or intelligent enough to return to their aquarium, and many octopuses have eventually met an untimely end by drying out on the kitchen floor. Obviously, an open-topped reef aquarium is simply not going to do the trick when it comes to providing a safe home for keeping an octopus in captivity.

So, if you’re still with me, and still determined to keep an octopus, how do you go about setting up a tank for one of these amazing animals? Well, many people have come up with different strategies for preventing these extremely clever escape-artists from taking a walk across the kitchen floor. For example, some people use open tanks that are very deep, but only fill the tank part of the way such that it is a long way for the animal to climb before it can get over the top of the aquarium. This may work well for public aquaria in which a couple of feet (or more, depending on the size of the animal) of empty aquarium is easily hidden behind a wall and only the lower water-containing portion of the aquarium is open to viewing, but is probably not overly practical for most aquarists at home. Another strategy is to have less space left open at the top of the aquarium (say 8-12" or so, again depending on the size of the octopus), but line the upper portion of the aquarium with Astro-Turf, which tends to discourage most octopuses from climbing across it. However, I have seen some particularly determined octopuses climb across Astro-Turf barriers, as well, so this technique is not always effective. Furthermore, aside from the fact that there is no guarantee that this technique will always work, most people find a tank with 8-12" of Astro-Turf lined space above the water line pretty unattractive! The final common strategy, and the one that I tend to recommend to most people for a home aquarium, is to use a well-sealed tank (with the top held firmly in place using bungie cords, heavy weights or even duct tape) that overflows into an open sump which provides a simple way around the problems a sealed tank presents for gas exchange, protein-skimming and filtration.

Now we can come back to the issues of filtration and water quality. Most aquarium books that mention octopus at all claim that these animals are extremely sensitive to water quality and cannot survive without reef-quality water conditions (e.g., Haywood and Wells 1989). In fact, I have said more-or-less that same thing in this very article (above). However, that statement is not entirely true. For example, Hanlon, Forsythe and colleagues have done numerous experiments with culturing octopuses in captivity and shown that these animals are pretty tolerant of a variety of water conditions that would generally be considered unacceptable for a coral reef tank (e.g., DeRusha et al. 1989; Hanlon and Forsythe 1985). In fact, in the five species cultured by these researchers to date, there was no significant decrease in the rate of feeding or growth of octopuses cultured at a pH as low as 7.5, salinities in the range of 32-38 ppt (roughly a specific gravity of 1.022 - 1.028 at 80EF), and even ammonia and nitrite concentrations as high as 0.2 ppm! In the case of nitrate, even concentrations as high as 500 ppm did not seem to affect growth or feeding significantly (although reproduction was decreased at nitrate concentrations higher than about 100 ppm). Regardless of the parameter we choose to examine, these are certainly not the water parameter values that we expect to see when discussing “excellent water quality.”

So where does this idea that octopuses require extremely high quality water come from? Well, there are two primary requirements that are similar between water parameters needed by both coral reef aquaria and by octopuses. First, elevated concentrations of heavy metals, especially copper, are deadly. And second, reef habitats typically have highly oxygenated water (see my June 2000 column - Toonen 2000) for a detailed explanation of dissolved oxygen in the reef aquarium), and octopuses are extremely sensitive to low concentrations of dissolved oxygen. Experiments with the common octopus, Octopus vulgaris, have shown that these animals will die when the concentration of dissolved oxygen drops to 2.5 ml/L (e.g., Nesis 1982). I know that number will mean very little to most people, but water coming out of a high-speed aeration protein skimmer is close to 100% saturated (there is as much oxygen in the water as it can hold), and this water would contain something close to 5 ml/L in an average reef aquarium (depending on the exact temperature and salinity of the water – again, see my dissolved oxygen article mentioned above for more detail). As soon as that well-oxygenated water enters the aquarium, however, your animals and even the bacteria in your filter begin to use the oxygen within it, and the more animals and bacteria there are in the tank, the faster that oxygen is used up.

In the simplest terms, the primary way that oxygen gets into an aquarium is when the water comes into contact with the air, and accumulated carbon dioxide is able to leave the water and new oxygen is also taken up. Without sufficient turn-over and current to allow efficient gas exchange, the rate of oxygen replacement into your tank water is not able to keep up with the rate at which is being removed by the animals and bacteria in the tank and filter. Obviously, the smaller the tank, the larger the bioload and the lower the flow rate and surface area available for exchange, the bigger problem that low oxygen is likely to be in your octopus tank. Sadly, many people do not realize that oxygen concentration is low in their aquarium until it is too late for the octopus. That extreme sensitivity to low oxygen concentration is the main reason that I recommend high turn-over skimmers for any octopus tank – although the skimmer tends to maintain better water quality, and is therefore likely to be healthier for your animal, the highly efficient gas exchange that takes place in the skimmer and maintains a flow of highly oxygenated water into the tank is well worth the cost of a skimmer on any octopus aquarium. As I said above, I generally prefer to use a well-sealed aquarium with an open sump in which to keep the skimmer and filtration, but a carefully sealed tank without a sump can work just as well, provided you have a reasonably large tank (say 30 gallons or so for an octopus whose head is about the size of a mandarin orange) and good filtration.

My recommendation here is obviously not the only way that an octopus can be kept in captivity, but I am offering you my opinion for the way in which you are most likely to have success in keeping an octopus happy and healthy throughout it’s lifespan. Regardless of the design you decide to use, however, I think that using a protein skimmer and power filter with carbon are always a good idea for an octopus. In addition to the reasons listed above, an efficient skimmer and some carbon will help in the event that your pet is scared into expelling it’s ink within the confines of the aquarium. Inking is a normal defensive strategy in cephalopods, and although it is natural it can be a serious concern in a closed system. The ink itself is not toxic (Wood 1994), but the ink can mechanically coat the gill surfaces of the octopus, and effectively suffocate the animal if it is not immediately removed. The use of activated carbon and an efficient skimmer can help to remove the ink from an aquarium, and give you more time to discover the problem and do a water change. In fact, octopuses can control the amount of ink that they expel, and if the amount of ink expelled is small and the tank is large, use of carbon and skimming can eliminate the need for immediate water changes (Johnston and Forsythe 1993). Having said that, however, I always think it is a good idea to err on the side of caution, and I usually recommend that early water change anyways.

In terms of increasing flow in your tank, most reef aquarists use powerheads, but I cannot really recommend that solution for an octopus tank. Octopuses have extremely sensitive arms, and they tend to be extremely tactile animals: like small children, they like to touch, explore, and play with everything they encounter in their environment. If the impeller of your powerhead is easily accessible (and most are), I would expect that they would function as a tentacle blender for the curious animal. The strength of these animals makes properly shielding the intake of a powerhead that much more difficult, because even after being glued on, my experience has been that the friction-fit filter baskets typically used to diffuse the intake flow (and prevent animals from being sucked into a powerhead intake) are easily removed by an octopus. Personally, I prefer to have only an overflow for the open sump in my tank and a remote pump that turns over the tank volume at least 5-6 times per hour. Even with a mesh-covered overflow, my octopuses have tended to stick their arms into these intakes on a regular basis, so I am very hesitant to use a powerhead in any octopus tank.

The last problem that I will mention in detail with regard to keeping an octopus in a reef tank is that any molluscs, crustaceans and likely even fish will eventually be eaten by your new pet. This is of particular concern in a reef tank, because in general, reef aquarists use a variety of herbivorous snails, hermit crabs or other small crustaceans to control nuisance algae in the aquarium, and these “clean-up” crews are often fairly expensive. The addition of an octopus to your reef tank will quickly eliminate this population of clean-up animals from your tank, which is certainly an expensive way to feed an octopus. Even more expensive would be the loss of giant clams (Tridacna spp.) that would eventually prove too tasty a treat for your octopus to resist. The eventual loss of most of the animals that typically perform as the clean-up crew in your tank, especially when coupled with the additional nutrient input from a relatively large and highly active animal such as an octopus (even a pygmy octopus produces an enormous amount of nitrogenous waste in comparison to a similarly-sized coral), makes it very likely that you’ll experience a pretty significant increase in algal growth in your reef tank. Even if you don’t mind them eating the majority of their tank mates, there are other problems you’ll face with trying to keep an octopus in a reef aquarium. Even the cnidarians (corals, anemones, hydroids and the like), which an octopus is unlikely to try to eat turn out to be problematic, but in the other direction this time – as I mentioned above, octopuses have extremely sensitive arms, and the constant stinging by the cnidarians is both a source of stress and potential infection for your octopus. All-in-all, the vast majority of reef aquaria quite simply provide an inappropriate place to try to keep an octopus alive in captivity...

So after all that, if you are still determined to get one of these animals, what can I suggest for you to provide the best care possible for your new pet? First of all, as I’ve belabored in detail above, an octopus needs its own tank, and one that will prevent it from being able to escape. Second, these animals are active marine predators and they require quite a bit of high quality food. “High quality food” does not mean feeder goldfish! Although many people are fascinated by the ability of an octopus to hunt down prey, feeder goldfish make a lousy prey item for a marine predator (please see my October 2001 column - Toonen 2001 - for more detail), and will contribute to an early death for your animal. The natural prey of octopuses consists primarily of other molluscs (especially when young, but in some species throughout their lives) and crustaceans (Boyle 1987). Obviously, you should make an effort to provide such foods, and you should also try to get the freshest (live if possible) variety of seafood prey to ensure your animals health. If fresh or live crustaceans are not easily available, the best alternative is to feed your pet good quality frozen shrimp (after you have thawed them, of course), provided that you try to vary the diet from time to time with other prey that is available (e.g., DeRusha et al. 1989).

Aside from the nutritional issue, live prey has another benefit over fresh or frozen seafood: your octopus will have to hunt it. This may seem unimportant to you, but it turns out to be very important to your pet. As I mentioned above, these animals are extremely intelligent predators, and spending their life in a completely predictable and unchanging glass box is not only boring for them, it has a strong impact on both their health and behavior (e.g., Wood and Wood 1999). In fact, during his research with baby octopuses at Dalhousie University, James Wood discovered that he could prevent “suicides” (animals crawling out of their tanks and drying out) by adding sufficient numbers of toys and challenges to keep the animals interested (Wood and Wood 1999). Professional zookeepers have long recognized that captive animals housed in unnatural and unstimulating enclosures can develop abnormal, repetitive and neurotic behaviors, and most zoos have made a serious effort to enrich the environment in which their animals are housed to avoid such behaviors (which has led to the development of a trade journal for zookeepers and professional aquarists titled The Shape of Enrichment to exchange ideas to better stimulate captive animals). Octopuses are no exception to this issue, and a captive animal housed in a tank without sufficient hiding places and without sufficient stimulation can develop a number of stress behaviors including white color patterns, inking, frequent deimatic displays (these are sometimes called “startle flashing” in which false eye spots or brilliant colors are suddenly displayed in an attempt to startle a potential predator), autophagy (eating the tips of their own arms), hiding all the time (depending on species, this may be natural in some particularly timid species), and rapid jetting into the side of the tank, among others (Wood and Wood 1999). It is important to try to provide sufficient stimulation to your pet in order to avoid such behaviors and prolong their life in captivity.

There are many options for enriching the environment in which your animal is housed, but providing an abundance of live prey periodically is one of the simplest and most satisfying for your animal. Even in locations where live crustaceans are not easily obtained (either as bait or from a seafood shop or Asian grocery), there are usually alternatives such as “feeder glass shrimp” available from pet shops that can provide a nutritious and stimulating treat for your animal. Other options are to periodically feed your octopus in a novel way such as using a bamboo skewer through a cork to feed fresh shrimp, or placing the food item in a plastic bottle with a drilled cork sealing it. Some animals will figure out these games very quickly and become bored with them again, whereas others may never figure them out and simply give up (Wood and Wood 1999). In any case, it is important for you to continue to come us with ideas to challenge your pet, and clever aquarium safe (e.g., no metal!) toys (such as a ping-pong ball or some cat toys) for your pet to amuse themselves.

Beyond enrichment being important to the physical and mental health of your pet, it will also make your octopus a much more enjoyable and interactive pet. For example, in the article I mentioned above, James Wood also found that animals that were initially very shy about feeding became much more interactive in response to such games. Within two months of starting these enrichment experiments, his octopuses went from constantly hiding to immediately coming out of their lairs and moving about on the glass while flashing “excitement” colors as soon as he entered the room (Wood and Wood 1999). Although it is impossible to know what an octopus is really thinking, this behavior has been compared to that of a dog getting excited and jumping about when you return home from work. I’m sure that anyone who is interested in keeping an octopus as a pet would much rather have the latter (excited interaction) than the former (constantly hiding) behavior in their pets.

So, if you are serious about trying to keep an octopus, I would encourage you to set up a special tank that provides for the needs of your animal, and not add it to your reef tank. I would again refer you to the two great resources on the web that I listed above (The Cephalopod and NRCC pages), which are absolutely fantastic sources of information about these fascinating animals!

Rob

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