Cephalopods Here, There, and Everywhere
This year for Cephalopod Week, we’re talking about octopods (or octopuses, but not octopi!) and squids by habitat. Cephalopods live everywhere in the oceans: near the shore in shallow water; far from land in the open ocean; in the ocean’s dark middle depths (the planet’s largest habitat); and in the deep sea. What hardly anyone knows is that octopods occur in each.
Imagine you’re wading through a warm, salty tide pool when you see what you think is a rock moving—but it’s an octopus! On seashores nearly all around the world, octopuses patrol the bottom, searching for food like crabs, snails, or clams. The catch? Animals like fishes and birds are also looking for a meal—of octopus. The octopus must expose itself to danger to satisfy its hunger.
Because they prowl the sunlit, moonlit, or even starlit shallow seafloor, octopuses risk being eaten by predators that hunt by sight. They use the protection of their incredibly changeable skin (both in color and texture), their ability to squeeze through tiny holes, and the smarts to outwit whatever is chasing them. If they have to, they will even squirt an ink cloud at a fish that's closing in. This diversion buys the octopus a few extra moments to change its direction and color, and make an escape (if it’s lucky). Octopuses have to balance their need to go out to catch other animals against the risk of being eaten themselves.
Other cephalopods are not tied to the seafloor. Some not only venture into but can dominate the huge expanse of the open ocean. There, sunlight illuminates the water’s depths in beautiful shades of blue—but it’s not a hospitable environment. The animals that occur far from land are built for speed: tunas, dolphins, and sharks, but only rarely swimming invertebrate animals (jellyfishes just loll around). Without a spot to rest, animals must swim all the time to avoid sinking into the dark depths beneath them. Food can be limited, meaning the animals tend to constantly be on the hunt for something to eat as they move across the surface of the oceans that cover over 67 percent of our planet.
Open ocean squids are sleek—and vicious. It’s been said that all squids in a shoal, or group, are about the same size because when they get hungry, the bigger ones snack on the little ones. The muscular Onychoteuthidae, or hooked squids, and the flying squids of Ommastrephidae do especially well in this realm.
Onychoteuthids have tentacle clubs armed with hooks, modified from the toothed suckers that are common to squids. The hooks help the squids hold onto whatever prey they can grab.
When an ommastrephid squid, sometimes called “flying squid,” is chased by a predator, it can swim so fast to escape that it leaves the water. They’ve even been found on the decks of research vessels in the middle of the night!
Deep below the ocean’s surface are the perpetually dark waters that make up the largest inhabited habitat on our planet: the mid-water depths. Around 200 meters (650 feet) deep, sunlight is nearly undetectable; at 1,000 meters, or 3,280 feet, it is entirely dark. Yet between these depths and the seafloor lives a glorious assortment of animals.
Cephalopods, both squids and odd-looking octopods, are here in abundance. At mid-water depths, animals need to minimize the energy they need, as food is very hard to come by. Some squids do this by incorporating ammonia into their tissues. Ammonia weighs less than seawater, so it gives squids a lift, meaning they don’t have to tread water and can just float.
The most famous of these are the cranchiid squids. They actually keep the ammoniacal fluid in a separate compartment of their bodies and are nearly transparent, except for their eyes and livers—that’s why they’re known as glass squids. Many are able to generate light and have complex light organs. Why? Possibly to attract mates, to attract prey, to scare predators, or to startle their prey with a sudden flash.
Not all squids spend their whole lives at these depths—some undergo what is called vertical ontogenetic descent. They hatch from eggs carried to shallow waters by their mothers. As they feed and grow, they slip deeper in the water column. This type of cross-depth movement explains why seabirds such as albatross can consume deep-water squids.
The deepest part of the seafloor is over 10,000 meters (32,800 feet) deep, where no cephalopods have yet been found. Benthic octopods likely only go to 4,200 meters, a depth record established just in 2016. The deepest-dwelling cephalopods are octopods with fins, reported at 8,000 meters (26,2000 feet) below the surface.
Like all octopods, their eight arms carry muscular suckers (no teeth) and are united by a very deep web. Typical of deep-sea animals, they have very soft bodies that don’t preserve well. We really don’t know a lot about these animals.
They swim, but they don’t use jet propulsion as do their more muscular relatives; they move their fins and open and close their web. This doesn’t sound very efficient—but remember, they live in the dark. If they avoid a predator by three inches, it’s just as good as if they avoided it by half a mile. They don’t have to have the muscular, color-changing bodies of cephalopods that need to avoid predators hunting by sight.
Benthic deep-sea octopuses—those living at the lowest level of the ocean—appear similar to their shallow-water relatives, but they may be unable to change the color or texture of their skin. Like their finned relatives, they live in the dark and have little use for color. Often, they are maroon; one group has a light top and a darker underside. We call this reverse countershading. It’s the opposite of what we see in mammals like deer that have a brown upper side and a white belly. The reverse countershading may help protect the octopus if it attacks a bioluminescent animal. If a bigger predator saw the glow of the prey through the octopus’s web, the octopus would be at risk—so it covers it up!
The places cephalopods live are as diverse—and sometimes elusive—as they are, and there’s still so much to learn about these animals.
Janet Voight, Associate Curator of Zoology, is a specialist in cephalopod mollusks, especially octopuses. In terms of where she works, it is the deep sea. If you work in the deep sea, there are times when you need to be a bit of a generalist, which is how Janet got interested in the wood-boring bivalves, the Xylophagaininae, and their apparent predators, the enigmatic echinoderm, Xyloplax. Discovery reigns supreme in the deep sea, as we know less about it and the animals that live there than we do about the back side of the moon. Janet's work is building the framework that offers to change that situation, that framework of course being based on specimens collected at sea, and available for study at the Field Museum.Deep Sea Biology and Evolution
Cephalopod Evolution & Ecology
Wood-boring bivalves of Xylophagainidae
Taxonomy and Systematics