For the male pillow octopus, being dwarfed by your mate is simply a way of life. Full-grown male pillow octopuses are only a couple of centimeters long and weigh less than a gram. Meanwhile, the females can reach more than six feet (2 meters) and weigh 100 pounds (45.4 kilograms). That means a female is 40,000 times heavier than a male [source: Pickrell].
In human terms, that would be like a man asking out a woman four times the size of the Statue of Liberty or a woman flirtatiously giving her phone number to a man the size of a beetle. Luckily for the male pillow octopus, however, mating isn't a full contact sport. Males of this species have a modified arm that contains long rows of sperm. He simply tears off this arm and politely hands it to his mammoth female mate, who then saves it for later use.
The pillow octopus is just one fascinating example of the octopus order (octopoda), which is as varied as the different plural forms of its name. Octopi, octopods, octopuses and other variations are all used by scientists. Ranging in size from the one-fourth gram male pillow octopus to the record 600 pound (272 kilograms) plus giant Pacific octopus, this most advanced of invertebrates has flabbergasted and amused researchers for years.
In this article, you'll find out why new camera lenses are patterned after the octopus's eye and how a Seattle octopus earned herself the name Lucretia McEvil. But first, let's take a look at the octopus's unique body style.
Octopus Body Plan
The octopus belongs to the phylum mollusca, where you will also find its slimy next of kin, the clams, snails and slugs. But octopuses are separated from the mollusks into the class cephalopoda, which includes the most advanced animals of the phylum. Squid, cuttlefish and nautilus belong to this class as well. The octopus has evolved most since the cephalopods originated more than 600 million years ago. While the other cephalopods sport some form of inner or outer shell like their relatives the mollusks, the octopus has none.
The word cephalopod literally means "head-footed" and refers to the fact that these animals' arms branch directly off of their heads. Some cephalopods have tentacles as well as arms, but the octopus manages just fine with the eight arms it uses for practically everything: eating, moving, hunting, tasting and mating.
Behind the octopus's head, directly opposite the arms, is its mantle. The mantle is a highly muscled structure that houses all of the animal's organs. Its gills, hearts, digestive system and reproductive glands are all crammed into this one space. The strong muscles in the mantle protect the organs and help with respiration and contraction. The octopus also has a funnel, sometimes called a siphon, which is a tubular opening that serves as a pathway for water. You'll find out the purpose for this funnel later in the article.
In place of the protective shell found in many other mollusks, the octopus brandishes a startling array of defense mechanisms. One researcher who calls octopuses "the wizards of camouflage" goes so far as to say that chameleons are humdrum by comparison [source: Stewart]. Masters of disguise, octopuses can alter their skin to take on a diversity of colors and textures to blend in with their surroundings.
On the next page, find out more about the octopus's magical ability to change colors in less than a second.
Octopus Defenses: Putting Houdini to Shame
The secret sauce for the octopus's color-changing talent is its chromatophores. These pigmented cells contain three sacs of color each, and tens of thousands of them cover the octopus's skin. Each chromatophore is surrounded by muscles that change how the pigment is displayed by relaxing or contracting. A publication from the Smithsonian institute likens the phenomenon to a drop of ink on a rubber band: when the band is loose, the color is concentrated in one spot and not readily visible. But when the band stretches, the color spreads out [source: Smithsonian National Zoological Park].
Each chromatophore is independently controlled by the nervous system, which enables a high degree of complexity in the display of colors. The high level of control by the nervous system also means the color-change splashes the skin in a flash -- an octopus can change its appearance in less than a second. Reflective cells on the skin, called iridophores, further accentuate the dramatic color changes by mirroring the surrounding environment's colors. Projections on the skin called papillae add to the disguise by changing textures to blend more readily with substances like coral or sand.
In addition to confusing predators through color and texture changes, the octopus has a secret weapon: ink. An ink sac is located near its digestive system, and when necessary, the octopus can eject ink out of the sac along with a burst of water from the funnel. The combination creates a black cloud. The octopus can shoot the ink out in little blobs that serve as decoys, or it can shoot it out in one big mass to obscure a quick getaway. To top it off, the ink contains tyrosinase, a compound that impairs smell and taste, which further confuses the predator.
It takes a big brain and a sophisticated nervous system to keep track of thousands of chromatophores and bursts of ink. Read on to learn about the systems that make it all possible.
Octopus Intelligence: The Complex Nervous System
As you might expect based on its skills in camouflage and avoiding predators, the octopus possesses a well-developed nervous system as well as complex sensory organs. Its eyes, for example, are similar to ours and possess an iris, pupil, lens and retina. The octopus is excellent at spotting prey and reaching to just the right spot to snag it.
In addition, an octopus's brain is proportionally as large as some birds' and mammals' brains. It displays a high level of organization in order to do things like coordinate all of the chromataphores' color changes. The brain is only part of the story though. Three-fifths of the octopus's nerves are distributed throughout its eight arms [source: Mather].
Octopus arms are incredibly strong and flexible. Made almost entirely of muscle, the arms possess the strength to wrestle sharks and to break through Plexiglas. And without those pesky bones and joints (like ours) to limit movement, the arms have an almost infinite range of motion. And yet the octopus can even mimic a human arm by making its arms semi-rigid and bending them in precise places.
In addition, recent research suggests those arms may have minds of their own. Studies indicate that octopus arms each have their own independent nervous system [source: Mayell]. It turns out the brain may simply delegate orders, while the arm is responsible for deciding exactly how to execute the order. Essentially this means that the brain can give a quick assignment to the arm and then not have to think about it anymore. Scientists tested this by severing the nerves in the arms from other nerves in the body and brain and then tickling the arms. Amazingly, the arms responded to the tickling just as they would in a healthy octopus [source: Pickrell].
Octopuses use their limbs for everything from hunting to mating. And, did you know that the seemingly lethargic octopus can actually reach speeds of 25 mph (40 kph)? We'll explore these fascinating facts in the next sections.
The octopus spends much of its solitary life in a den, leaving at night to hunt. For reasons not clearly understood, it generally likes to search for new real estate every week or two. Octopus dens are usually under a rock or in a crevice, and the animal has even been known to take up residence inside an old, discarded bottle on the sea floor.
When it does venture out of its den, the octopus uses one of several methods to get around. The preferred method of locomotion for many octopuses is a form of walking. Rows of suckers on the underside of each arm enable the octopus to move itself forward along the sea floor. Because its arms are highly sensitive -- each sucker has up to 10,000 neurons -- the octopus can also learn a lot about its surroundings this way, and it may even chance upon a tasty meal.
But the octopus can jet into much higher speeds if needed. When it wants to make a quick escape, it takes water in through its mantle and then closes it off to seal in the water. Next, it expels the trapped water forcefully through its funnel, which propels the octopus in the opposite direction at speeds of up to 25 mph (40 kph). Using this method, which is a lot like filling up a balloon with air and then letting it go, the octopus can change its direction by pointing its funnel a different way.
The rarer, more primitive and less researched finned octopus (cirratte) can also use its fins for swimming. It often combines the use of its fins with the propulsion method. Little is known about the cirrate, however, because it lives in very deep water. Most articles, like this one, deal with the more common, non-finned octopus (incirrate).
If an octopus isn't busy preventing itself from becoming a meal, odds are it's out looking to make a meal out of something else. The octopus uses a variety of techniques to capture and consume prey. Its long, flexible arms are ideal for reaching into crevices after tasty crabs and crayfish, and its soft bodies are able to squeeze into tiny spaces after small fish or clams.
And although the octopus does not have any teeth in the standard sense, it has several other just as effective methods of cracking into crustaceans and mollusks. The octopus has a veritable Swiss Army knife of tools located inside its mouth to pry open the shells it can't open with its tentacles. Directly inside its mouth, it has a hard retractable beak similar to a parrot's. This beak is useful for breaking open clam shells and tearing apart flesh. Next to the beak is the radula, a barbed tongue the octopus uses to scrape an animal out of its shell once the shell is opened. And if these tools don't do the trick, it also has a tooth-covered organ called the salivary papilla that it can use to drill into shells. The papilla's bodily secretion also erodes the shell and then weakens the prey so it can be consumed.
One of the octopus's preferred methods for capturing prey while swimming is to envelop it in the web of skin between its tentacles, as though capturing it with a net. Then it devours the prey with its beak.
In spite of the effort it can take for an octopus to make a meal, most species grow and gain weight quickly. Next, we'll look at why this happens -- and how adults manage to mate without all their legs getting in the way.
Octopus Reproduction: Birth, Life, Death, and Birth Again.
A juvenile octopus grows at a rapid rate, perhaps because of its short life span. Extremely effective at turning the food it eats into body mass, a young octopus increases its weight by 5 percent each day. By the end of its life, an octopus will weigh one-third as much as all the food it has eaten [source: The Economist]. The common octopus only lives an average of three to five years, though, so it doesn't have much time to waste.
Once the octopus reaches adulthood, it will eventually get the urge to mate. As with most creatures, the octopus's main purpose in life is to reproduce. However, if it knew just what was waiting for it soon after, it might think twice. Both the male and female octopuses die soon after mating. The male dies a few months afterwards, while female dies shortly after the eggs hatch. For octopuses, mating is a pretty subdued affair. A few species have flashy mating rituals, but many octopuses seem like they're just conducting business.
The male octopus has a modified arm called the hectocotylus, which is about a meter long and holds rows of sperm. Depending on the species, he will either approach a receptive female and insert the arm into her oviduct or take off the arm and give it to her to store in her mantle for later. In the latter scenario, the female keeps the arm until she lays her eggs, at which time she takes the arm out and spreads the sperm over her eggs to fertilize them.
The female meticulously cares for her eggs until they hatch, forgoing food the entire time. She blows currents across the eggs to keep them clean and protects them from predators. The eggs might incubate anywhere from two to 10 months, depending on the species and the water temperature. Once they hatch, they're on their own -- one source cites an estimated 1 percent survival rate for the giant Pacific octopus from hatchling to 10 millimeters. Depending on the species, some octopuses begin life as miniscule specks floating on the ocean's surface that drift down upon reaching a larger size, while some start out a bit bigger on the ocean's bottom. Little else is known about the early lives of octopuses [source: Scheel].
A substantial amount of research has been done, however, on octopuses' intelligence. On the next page, you'll meet an octopus named Lucretia McEvil and find out if cephalopods live up to their reputation as the brainiacs of the invertebrate world.
Octopus Personality and Lucretia McEvil
Octopuses in captivity present researchers with an excellent opportunity to learn more about their personas. Research on the octopus is still relatively new due to the animal's reclusive nature. It's fairly well-accepted among researchers that the octopus is an intelligent creature -- stories abound of those octopuses that manage to escape from their aquariums to pry open bins of food, and those that learn to solve mazes or pick up red balls as opposed to white ones [source: Stewart]. But could they have personalities?
After she heard that some interesting octopus characters at the Seattle Aquarium were given names -- a practice generally reserved only for the most advanced species -- marine biologist Jennifer A. Mather wanted to find out the answer. Thus, Leisure Suit Larry, a particularly touchy-feely octopus; Emily Dickinson, a recluse; and Lucretia McEvil, who destroyed the interior of her tank, became the starting point for a revealing study.
To be fair, personality is a hard thing to prove, but Mather and marine biologist Roland C. Anderson pinned down a method for the study: They would identify a number of personality traits and then rate each animal against each of those traits. They exposed 44 red octopuses to three different situations seven times, each for a period of two weeks. Did the octopuses show personality? The answer was a resounding yes. In all, the test octopuses displayed 19 distinct behaviors, which the researchers categorized into three buckets -- activity, avoidance and reactivity. Emily Dickinson, for example, would have been rated low on activity, high on avoidance and low on reactivity because she liked to stay in her den no matter what. You can find more about the personality study in the February 2007 issue of Natural History.
The scientific community's obvious interest in the octopus underscores the creature's fascinating qualities. Be sure to click on the links on the following page to see some incredible footage of octopuses in action and learn more about related HowStuffWorks topics.
Octopuses have blue blood unlike the red blood of mammals. Find out why scientists think octopus blood is blue at HowStuffWorks.
Related HowStuffWorks Articles
More Great Links
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