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Don't Fear the Raptor - An Octopus in the Home Aquarium

<< Cephalopod Articles | By Dr. James B. Wood

Are you ready to keep a new pet: something different, challenging, intelligent, and cunning? A creature that has entirely too many plural noun names: octopuses, octopi, octopods, octopoids, octopodes, octopussies (Lane)—and you thought ozone kinetics was confusing! A creature that has a reputation and a place in folk lore. Octopuses, or devil fish, have battled at least a few epic heroes to the death and have allegedly attacked and sank ships in order to dine on sailors. Whether in fiction or fact, octopuses are amazing creatures; and with preparation and knowledge, they can be successfully maintained in a home aquarium.

Octopuses are classified in the phylum Mollusca which includes gastropods (snails, nudibranchs, etc.), polyplacophorans (chitons), bivalves (clams, oysters, and mussels), scaphopods (tusk shells), and some others. Octopuses are placed in the class Cephalopoda which translates as 'head-foot.' This most advanced molluskan class also includes nautili, cuttlefish, and squid. Of all invertebrates which compose over 95 per cent of the Animal Kingdom, octopuses may be the most intelligent.

Some of the octopus's remarkable abilities include the capability to expel ink and to change skin texture and apparent size. They are able to jettison water through their siphon by contracting their mantel to provide quick thrust. They have well developed eyes and an efficient closed circulatory system. Octopuses have surprisingly strong but sensitive suckers on their eight arms. Some octopuses make 'speculative attacks' on rocks hoping to flush out prey that might be hiding beneath. Others use their bodies as cast nets and 'parachute' onto prey to trap them. There are reports of octopuses dropping rocks in between the open shells of bivalves, preventing the shell from closing and enabling an easy kill (Lane).

Octopuses can adapt to new situations and solve problems; they appear to be curious about their surroundings ('hmmm, I wonder if I could get out of this tank' or 'hmmm, where does that tube go?') They have the intelligence and ability to get into and out of fishermen's crab and fish traps, and certainly to get beneath an undergravel filter plate. Octopuses have very little hard material in their bodies, only a beak and a radula in most, enabling them to squeeze into or through amazingly small spaces.

Additionally, octopuses can change color using their chromatophores, which contain red, orange, or yellow pigment sacs, and melanophores, with brown or black pigment sacs. When the sacs are dilated, the pigment spreads out and color is seen. Iridocytes, which differentially reflect light, are responsible for the beautiful blue coloration of the blue-ringed octopus as well as the eye spots of some other species (Nesis). With chromatophores, iridocytes, and a well-developed nervous system, octopuses can quickly camouflage themselves to distract or scare a predator. Another amazing characteristic is that octopuses change their color seemingly to reflect their mood: usually strong red indicates 'anger' and white denotes 'fear.' There are exceptions to this. For example, the beautiful brown and white defensive coloration of Octopus horridus is believed to mimic the poisonous lionfish. My first octopus, Houdina (O. briareus), would rapidly pulsate color seemingly to indicate excitement as in 'I might get fed soon—oh boy!'

One would think that the lack of a hard defensive shell would make an octopus an easy meal for predators. However, the aforementioned adaptations are often used together to effectively evade danger. An octopus could darken, expel a cloud or decoy of dark ink, immediately jet away while turning white, and then disappear under a rock. This trick has fooled me more than once, even when I knew it was coming. Perhaps the nickname 'primates of the sea' better describes them than the name 'devil fish.'

Despite their strong reputation, octopuses are usually shy and retiring. Most pose little, if any, threat to man. Max Gene Nohl, a diving expert, summed it up by saying 'In my opinion the chance of a diver being attacked by an octopus is as remote as the possibility of a hunter in the woods being attacked by a rabbit.' (Lane) Octopuses have a hard parrot-like beak and can bite although they rarely do so. Generally, their salivary toxins do not affect man. An exception to the above is the beautiful blue-ringed octopus. While it definitely won't viciously 'attack' a person, if it feels threatened, it may defend itself and deliver a venom that can kill in minutes (Kerstitch).

One major drawback to keeping these fascinating creatures is that they are short-lived. This does not have to be caused by our lack of proper care for them—octopuses do not live very long in the wild either, perhaps two years for O. vulgaris, one year for O. maya and O. briareus, and even less for O. joubini (Boyle 1983). These life spans are from hatching—in other words they are unfortunately not from the time of purchase or capture. Enjoy them while you can! It seems strange to us that in most cases a female octopus will not accept food even after her eggs have hatched. Almost always she will waste away and die. The males (of at least some species) seem to have more or less genetically programmed life spans also as they do not live any longer than the females. At least in some species, a female that has not been impregnated will still lay (infertile) eggs, brood them, and starve—this happened to Houdina. On the positive side, it is possible for the advanced aquarist to hatch the eggs and raise the young of certain species of 'large-egged' octopuses.

Both of the Atlantic octopuses (Octopus briareus) that I have kept were caught in the Florida Keys. Houdina, was found in her lair under a rock while I was snorkeling in shallow water off Big Pine Key. Legs was captured in a tidepool off Missouri Key at 5:00 A.M. during a low tide. Gilliat (undescribed species) was also captured early, around 5:00 A.M. in less than three feet of water in Kaneohe Bay, Hawaii. All three were captured with handheld nets. Glass jars and zip lock bags can also be used to catch them. If you live near the ocean, you may be able to collect your own specimen as well as live food. Be sure to check local collecting regulations first.

Octopuses are escape artists and care must be taken to prevent losing one once you have captured it. Immediately after being caught, Legs discovered a small hole in the seam of my new dive net, went through it, plopped in the water, and disappeared from sight... all in about two seconds! Luckily, at least for me, I was able to relocate her. Once back at the Hall of Justice, (o.k., o.k., hotel room) Legs was placed in a five-gallon bucket with its lid slightly ajar to provide an air line to circulate the water. Later, that same morning, a distinctive 'splop' awoke us, somehow penetrating our half asleep brains. That we heard it was no small miracle as we were no doubt still traumatized from waking up at 3:00 A.M. to go look for octopuses. We rushed into the bathroom of the hotel room and there was Legs, obviously upset, scooting around the floor. We managed to catch her and return her to the bucket (yes, the feeling of octopus suckers on your skin is weird at best). Besides keeping the octopus safe, a well-sealed collecting bucket and/or octoaquarium helps to allay a loved one's fear that 'that thing is going to crawl out in the middle of the night and suck my brains out through my nose'.

That wondrous but unsightly invention, duct tape, can be an octoaquarists best friend for securing a loose lid on an octopus' tank or collecting bucket. Don't forget to cover all intakes to power filters; a whisper bio-bag or old nylon stocking secured to the intake tube with a rubber band has also worked well (so far). I have also successfully used plastic-coated (they are lead after all) dive weights to eternally warp my plastic aquarium hoods... I mean weight them down. As a side note, certain individual octopuses seem to get great personal satisfaction out of repeatedly submerging floating items such as hydrometers and then releasing them. Children will play.

For the most part, octopuses need a tank to themselves. They will view most fish, crustaceans, and mollusks (possibly including smaller octopi) as food. Echinoderms such as sea stars, brittlestars, and sea urchins seem to be safe tankmates (Haywood). Those of you with reef tanks shouldn't have to worry about corals, sponges, tunicates, etc. unless you don't approve of your octopuses interior design tastes (the larger heavy rocks should be safest).

A 15-30 gallon tank should do for a 'normal'-sized octopus. Keep in mind that there are many different species and that size at maturity varies. Also remember that a young, healthy, well fed octopus may grow considerably. As is always true, no harm can be done (except to your wallet) by having a tank that is too large. During a power outage, a large tank will be depleted of oxygen much slower and a large volume of water may effectively dilute ink.

One important factor that will help ease ol' eight legs' nerves is to provide plenty of hiding places in the tank. This can easily be accomplished with any or all of the following: limestone or other aquarium safe rocks, shells, pvc pipe, and glass jars. I have seen octopuses in tanks with little cover 'so that they wouldn't hide all the time and could be seen,' and I personally view this as cruel. Despite their aggressive reputation, octopuses are somewhat shy and retiring. If you wish to see something out and moving all of the time, an octopus is not a good choice; perhaps goldfish and/or those annoying little airpowered toys would be better.

Once the octopus settles in and feels secure in it's new environment, it is likely that it will come out hoping to be fed when you are near the tank. After Legs's first few weeks in the 'rocky tide pool tank', I did not have much of a problem with being able to view her on a nightly basis. She would come out and flash colors, perhaps to get my attention or in anticipation of food. I attempted to feed her at the same time every night; after a few weeks she would come out and be waiting near the top of the tank. Several people have told me that their octopuses jet water out of the tank at them in order to get fed. Who says an octopus can't beg?

Another important aspect that is often ignored and can lead to dire consequences is that an octopus tank should be escapeproof (read 'welded shut.') These intelligent creatures are quite capable of turning into invertebrate jelly and escaping or disappearing into your aquarium equipment. It is not unusual to hear stories of an octopus leaving its tank to raid the refrigerator errr... feeder tank or museum crustacean display. I was recently told an especially amusing antidotal story and will repeat it here. Apparently two college students were sharing an apartment, and one of them had an octopus which he fed feeder goldfish. The feeder goldfish were disappearing much more often than the roommate who owned the octopus was sacrificing them. Apparently he was short on beer money at the end of the week as the feeder goldfish were disappearing faster than he cared to replace them. As always seems to be the case in such situations, the other roommate was blamed. Soon a cold war started between the two. It got worse from there. This lasted for about two months until, on a morning, a wet trail between the feeder tank and the octopus's tank was noticed on the wall.

Almost all aquarium books that I have read state that octopuses demand optimal water quality and that they can not endure any water pollution (Haywood, etc). Most of us immediately associate the words 'optimal water quality' with the words like 'wet/dry', 'protein skimmer', 'ozone', and 'considerable financial undertaking.' In stark contrast, Dr. Hanlon has published some surprisingly forgiving water quality parameters. For example he states that at least for the five species that he dealt with, no reduction growth or feeding was noted at pH's as low as 7.5, salinities in the range of 32-38 ppt, and both ammonia and nitritein concentrations of 0.2 ppm on a long term basis. Similarly, he reports that nitrate concentrations up to 500 ppm did not seem to affect growth or feeding much, if any. However, he mentioned that nitrate concentrations above 100 ppm may affect reproduction. One water quality fact that is crystal clear, is that excessive heavy metals, especially copper, are deadly.

I believe one reason that water quality is emphasized in aquarium manuals is because octopuses are very sensitive to low dissolved oxygen concentrations. The common octopus (O. vulgaris) will die if the oxygen concentration falls below 2.5 ml/L (Nesis). For reference, 100% oxygen saturated seawater would contain approximately 5-7 ml/L of oxygen in a 'normal' aquarium. The decomposition of organics, the oxidation of ammonia to nitrite, and the oxidation of nitrite to nitrate will lower the dissolved oxygen level in a closed system. While excessive amounts of these waste products may not be directly toxic, their breakdown may have a detrimental effect on respiration. Perhaps another reason that water quality is overstressed is that many aquarists don't realize that octopuses have a relatively short life span—'Water quality' seems to be an easy target for unexplained death.

In light of the above, I believe that octopuses need an established tank with fairly good water quality and sufficient oxygenation. My 'rocky tide pool tank' had an over-sized wet/dry filter beneath it and a small skimmer in the sump. However, this setup is overkill. Some hobbyists have had success with undergravel filters with powerheads to move the water (McHenry). Personally I don't advise the use of these filters as the octopus may chose to reside under the undergravel filter plate. Johnston and Forsythe (1993) suggest siliconing riser tubes and caps to the filter plate, placing screen guards over the openings to the riser tubes, and using a small amount of silicone to glue the filter to the tank bottom. An octo-proof tank with a hang-on-the-back type filter and an extra powerhead or airstone for increased water circulation should also work well, as would a canister filter. In any octopus tank, the water does not need to be blasting around, but it should be noticeably circulating. The use of an inexpensive skimmer or activated carbon would be a good insurance policy.

Octopuses do not need any specialized spectrum or high intensity lighting, a definite advantage for those of us who are on a budget or are still confused by the myriad lighting possibilities and their ramifications. If given a choice, an octopus would probably choose little or no lighting for its tank. But, of course, we humans do like to view our pets. A compromise of one or two standard, cheap, white florescent bulbs ought to do the trick. I prefer to place the lights on an inexpensive timer in order to add one more constant to the system.

It is my understanding that many octopuses die during shipping because during this stressful period they release ink into their water. John Forsythe, the Senior Research Associate at the University of Texas Medical Branch at Galveston, stated that although octopus ink is not directly toxic 'if an octopus inks in a small volume of water, the ink can mechanically coat the gill surface causing asphyxiation. 'Octopuses can control the amount of ink they release. The ink is believed to provide a smokescreen for escape. In addition, it is widely believed that the ink chemically suppresses a predator's sense of smell. Since inking is a defensive mechanism, it would be a good idea to tell your friends (who will no doubt be in total awe of your fascinating pet) that rapid or sudden movements are not a good idea. If an octopus should release an appreciable amount of ink into a small tank do your next water change early. Johnston and Forsythe (1993), however, state that the use of carbon eliminates ink as well as the need to do a water change.

Octopuses are carnivorous and primarily eat crustaceans, fish, and mollusks in the wild (Boyle, 1987). In my home aquarium, I have had success feeding them live fish (including the damsel that was in the tank to cycle it, which Houdina ate the first night! whoops), live crabs, and live shrimp. In addition, I have had success occasionally feeding thawed out frozen crustaceans. However, I have been told that some octopuses will not accept any thing but live food (McHenery). Others have had success rearing some species of adult octopuses exclusively on frozen marine shrimp for over two months (DeRusha). Once acclimated an octopus eats a surprising amount of food. Mine was fed at least every other day and definitely preferred live food, especially crustaceans.

Octopuses are very responsive to their environment. If you have had an octopus for a few weeks, something is seriously wrong if it is hiding all the time, especially at night, and still not eating. Prolonged whitish coloration, especially if the octopus is not eating, is not a good sign.

Keep in mind that most species of octopuses are nocturnal (if you are a college student or a vampire you can take this as a plus). If you are a night owl, I advise feeding at night; if society forces you to wake up early, try feeding in the morning. In my experience, feeding at a regular time almost guarantees that you will see your pet at least once a day.

It should be noted that the recommendations in this article are what I view as the best way to maintain an octopus. Just because others have kept octopuses in tanks that do not have lids ('Gee, why do the tropical in the neighboring tank always disappear? or 'Hey, who put the octopus in the crustacean tank... and where are the crustaceans!) does not mean that you should. And then there is a friend who told me of an octopus that would use the cap on the undergravel filter as a door, always shutting it after going in or out (cute huh?). As many behavioral experiments have shown, octopuses are individuals and often react to the same situation differently. While an individual octopus' mistakes are sometimes fatal in the short run, their adaptive plasticity helps to ensure that the species will survive in the long run.

If you have had some experience with marine aquaria and are interested in a different and fascinating marine invertebrate that can truly be called a pet, perhaps an octopus is for you.

I am indebted to Kate Cullison, John Cigliano, John Forsythe, Miss. and Mrs. Goldstein, Jim McHenry, Stewart Reed, Dr. Richard Young, and Dr. Wolff for their patience and advice.

References

Barnes, Robert D. 1987. Invertebrate Zoology. 5th ed. Saunders College Publishing, Fort Worth. 893pp.
Boyle, P.R. Ed. 1983. Cephalopod Life Cycles. Vol 1. Academic Press, London. 475pp.
Boyle, P.R. Ed. 1987. Cephalopod Life Cycles. Vol 2. Academic Press, London. 441pp.
DeRusha, R. H., J. W. Forsythe, F. T. DiMarco and R. T. Hanlon. 1989. Alternative Diets for Maintaining and Rearing Cephalopods in Captivity. Laboratory Animal Science.
Forsythe, John. Senior Research Associate at The University of Texas Medical Branch at Galveston. Personal Correspondence
Haywood, Martyn and Sue Wells. 1989. The Manual of Marine Invertebrates. Tetra Press, Morris Plains, NJ. 208pp.
Hanlon, Roger T., and John W. Forsythe. 1985. Advances in the Laboratory Culture of Octopuses for Biomedical Research. Laboratory Animal Science.
Johnston, Liz., and John Forsythe. 1993. An Octopus in Your House? - Part II. Aquarium Fish Magazine. Vol. 5., No. 12.
Kerstitch, Alex. 1990. Molluscan Touch' in Seascope. Vol 7 (Summer).
Lane, Frank W. 1960. Kingdom of the Octopi. Pyramid Publications, New York. 287pp.
McHenry, Jim aka Laptop Bandit. Personal correspondence.
Nesis, Kir N. 1982. Cephalopods of the World. T.F.H. Publications, Neptune City, NJ. 351pp.


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The Cephalopod Page (TCP), © Copyright 1995-2014, was created and is maintained by Dr. James B. Wood, Associate Director of the Waikiki Aquarium which is part of the University of Hawaii. Please see the FAQs page for cephalopod questions, Marine Invertebrates of Bermuda for information on other invertebrates, and MarineBio.org and the Census of Marine Life for general information on marine biology.