There are few marine creatures as fascinating to watch as cephalopods. From their tentacles that each seem to move autonomously, to their unbelievable ability to change color and texture, to their alien feeding habits, these molluscans never cease to amaze us. That wonderment often leads us to observe them closely, and the more we learn about them the more we want to know. This desire for knowledge soon brings us to the curious aquarist’s most typical quandary: “Can I keep it?” While cephalopods are neither easy nor ideal aquarium inhabitants, many of them can in fact be successfully maintained in captive environments. That being said, they are absolutely not suitable animals for a novice aquarist and are challenging regardless of experience level. But before we get into the basic care and keeping of these endlessly amazing creatures, we need to understand a bit about their biology.
Poison ocellate octopus (Octopus mototi) before and after displaying to me that it no longer wants to be bothered by my camera. Photo by Alex Rose.
Although they may look absolutely nothing like clams, cephalopods are considered mollusks and are more closely related to bivalves than any other group of sea creatures. Octopuses, squid, cuttlefish, and nautiluses all belong to the class Cephalopoda, which means “head foot.” Mollusks in general are not known for their intelligence and most don’t even have a proper head, let alone an impressive brain, but cephalopods differ drastically in this regard. They have large brains and well-developed senses and it almost seems as if there’s an evolutionary correlation between the loss of a shell and increased brain function. Perhaps as the hard, constrictive shell shrank, there was more room for a larger brain to develop along with a need for intelligence in the field of self-defense now that their calcified armor was no longer around to protect them. Nautiluses, the most primitive cephalopods still in existence, most closely resemble their shell-dwelling relatives, and can completely withdraw their tentacles and other soft body parts into their shells in case of danger. Cuttlefish have a thick internal shell called a cuttlebone, and squid have an even further reduced structure called a pen, while octopuses lack one entirely.
Flamboyant cuttlefish (Metasepia pfefferi) can “roll” their colors so it looks like their bodies are pulsating in bands of alternating hues. Photo by Alex Rose.
Cepahlopods are known for their startling ability to rapidly change color. They can do this by manipulating extremely high densities of skin pigment cells (chromatophores and leucophores) and reflective cells (iridophores), allowing them to produce many different colors and patterns in rapid succession, in order to communicate with conspecifics, camouflage themselves from predators, or warn intruders of their toxicity. According to an article in National Geographic, this is how chromatophores work: “Each chromatophore is an elastic sac of pigment, surrounded by a starburst of muscles. If the muscles relax, the sac contracts into a small dot that’s hard to see. When the muscles contract, they yank the sac into a wide disc, revealing the color it contains.” It has also recently been discovered that cephalopod skin is loaded with light-sensitive proteins known as opsins, the same ones that render eyes capable of sight. While it is unlikely that these creatures can actually “see” with their skin, it seems probable that these proteins play an important role in light detection independent of their eyes. As explained by vision expert Tom Cronin, “We don’t know if they contribute to camouflage or are just general light sensors for circadian cycling or are driving hormonal changes. They have a job to do but we don’t know what it is.” More research is needed to determine the exact function of opsins in cephalopod skin.
Cuttlefish such as this broadband cuttlefish (Sepia latimanus) can be maintained in captivity, though they are far less readily available in comparison to octopuses. Photo by Alex Rose.
In addition to their state of the art camouflage techniques, cephalopods can also release a cloud of dark ink to confuse their predators long enough to escape. While on the topic of escape, it is important to mention that octopuses in particular can fit into and through unbelievably small spaces, a characteristic that is a valuable survival tool in the wild, but an extremely troublesome one in captivity. The only non-compressible part of an octopus’ body is its beak, so they can essentially fit through anything larger than their mouthparts. This means that one of the major requirements for housing an octopus is an aquarium with a tightly fitting lid; it is also important that there are no holes (plumbing or otherwise) that it can squeeze through.
Of all the cephalopods, octopuses are most commonly kept in captivity. Some adventurous and skilled aquarists have maintained cuttlefish, but squid and nautiluses are rarely cared for outside of public aquariums. The biggest drawback of keeping cephalopods is their short lifespans. Many of them don’t live much longer than a year, and females perish shortly after they lay eggs. There is no way to know if a female has spawned unless she is observed depositing her eggs, so an aquarist could potentially acquire an animal that has little life left to live.
This Atlantic long arm octopus (Octopus defilippi) nabbed a crab for lunch. Photo by Alex Rose.
Another possible issue is the fact that octopuses can be extremely difficult to positively identify when they are young, meaning that an aquarist might purchase an octopus thinking it is a small species that could be housed in a 30g aquarium, when in fact it is a much larger one that will easily outgrow this habitat.
All cephalopods are intense carnivores, and consequently they are poor candidates for anything but a species tank. They cannot be kept in community reef systems because their diet includes fish, invertebrates, and mollusks, and they would likely see nearly all their tankmates as food options.
Bigfin reef squid (Sepioteuthis lessoniana) on a night dive in the Philippines. Squid have a reduced internal shell called a pen. Photo by Alex Rose.
Many cephalopod species spend the majority of their day hiding, waiting for their next meal to arrive, another characteristic that doesn’t make them an ideal aquarium animal. To feel comfortable, they must be provided with ample rockwork and plenty of places to stow away.
Other than the drawbacks just discussed, cephalopods can be incredible animals to keep in home aquaria. Tank size requirements vary greatly and must be evaluated on a species-specific basis. Dim lighting is preferable and promotes activity. They absolutely require good water quality and will not thrive unless provided with it. As mentioned earlier, cephalopods are carnivorous and must be fed a variety of meaty foods; whole animals such as clams and shrimp are preferable. Mealtime is one of the highlights of cephalopod care, and reminds the aquarist exactly why they have a 55-gallon tank for a single animal.
While I wouldn’t recommend cephalopods to the average hobbyist, they are incredible creatures that can be amazing aquatic companions for the right aquarist. Proceed with caution.
Alex Rose holds a B.S. in biology and a M.S. in aquatic biology, and she has a wide variety of experience in the biological sciences, including bioacoustics research, exhibit construction, science writing, teaching, public presentation, and aquatic animal husbandry and breeding. Alex is a professional violinist, photographer, PADI divemaster and lover of all things aquatic. Her driving goal is to find ways to protect our world’s marine habits through diving, writing, education and research. Visit her website at alexroserenaissance.com. You can also read more on the Sustainable Reefkeeping page.