An Eight-armed Wonder: The Octopus

Completely motionless, the killer waited in the nighttime shadows as its unsuspecting victim neared. Watching the scene unfold in front of me from only a few feet away, I knew the strike could come at any instant. My heart began pounding in my chest, but I, too, tried to remain as still as I possibly could. Then, ever so slowly, the perpetrator extended an arm right into the victim’s anticipated path. The trap was set, and it appeared that the quarry failed to notice as it moved ever closer.

The hunter, a red octopus, was stalking a tuna crab, a creature also known as both a pelagic red crab and squat lobster. In other geographical areas the octopus and crab might have been different species, but the encounter would likely play out in similar fashion. The setting was the sandy sea floor at La Jolla Shores, a popular San Diego dive site.

When the crab got to within only a few inches of the octopus’ sucker-laden arm, the hunter’s body appeared to “shiver” slightly, perhaps in anticipation of the upcoming attack and an ensuing meal. I thought that movement might cost the hunter a meal, but once again the crab did not seem to notice. Seconds later, the octopus quickly grabbed the crab with its arm and pulled the flailing crustacean under its mantle. With the crab under control, the octopus quickly bit the crab, injected it with venom and sucked the meat out of the crustacean’s shell. From first contact to discarded shell took only a few seconds. No doubt about it, the tuna crab had been “sucker-punched.”

Apparently, this morsel was a mere appetizer, as the octopus moved away to set another trap. Within no more than 10 minutes the octopus made short work of four tuna crabs, and then the clever mollusk slipped away into a crevice by moving through an opening that was no more than one-third as large as the animal’s head.

The Amazing Octopus

In oceans all around the world, similar scenes are played out every day of the year just as they have been for eons. There is little question that octopuses are high on the list of favorite creatures for many divers to encounter. Close relatives of the squids, cuttlefishes and nautiluses, octopuses are mollusks that are described in the class Cephalopoda. The term cephalopod is derived from the Greek words meaning “head” and “foot,” so it should not come as a surprise that the heads and feet of these mollusks play important roles in their lives. Cephalopods have prominent heads and are considered by many specialists to be among the most highly intelligent of all invertebrates. The foot in these mollusks has been modified over the course of evolutionary time into sucker-bearing arms projecting from the head. These arms assist in snaring and holding prey as well as in sensory interpretation of the animals’ surroundings. As a general rule, squids and cuttlefishes live in the water column where they prey upon a wide variety of fishes, while most octopuses are primarily bottom dwellers that usually feed on worms, crustaceans and other mollusks.

The phylum Mollusca includes nudibranchs, sea slugs, snails, sea hares, clams, scallops, oysters and mussels in addition to the cephalopods. Most mollusks have hard shells and many have external shells that can easily be seen. But the cephalopods are unusual mollusks in that most species lack a hard shell. The chambered nautilus is the only cephalopod that possesses a complete shell. In squids and cuttlefishes, the shell is small and internal, while octopuses have given up their shell altogether and developed other means of defense. Of the roughly 600 species of cephalopod mollusks, about 200 are octopuses. They range in size from “small enough to hold in the palm of your hand” to the Giant Pacific octopus that attains proportions of 600 pounds (270 kg) and 12 feet (3.6 m) from arm tip to arm tip. In general, the larger species inhabit colder waters.

Most octopuses live in shallow coastal waters, inside dens or small caves on the ocean bottom. These species tend to live rather solitary lives. Some will even block the entrance to their den with rocks to keep intruders away. If no natural hiding places are readily accessible, octopuses seem to be quite content taking up residency inside of a discarded tire, clay pot, glass jar or other man-made debris.

Sophisticated Sensory Systems

Compared with members of many other invertebrate phyla, mollusks are highly developed animals. Octopuses, like all mollusks, are equipped with distinct organ systems and they have highly sophisticated sensory systems. The bodies of all mollusks display bilateral symmetry, with the bodies being organized fore and aft and the right and left sides being mirror images of each other. All species have a brain, or central bundle of nerves, toward one end of the body, and all possess an organ called the mantle, a thin flap of skin that surrounds the body and in many species is responsible for secreting and maintaining a hard shell. All mollusks also possess a musclelike foot usually used to anchor the animal to the substrate and to assist in locomotion.

The senses of vision, smell and touch are also well-developed in octopuses and other cephalopods. As a result, octopuses can respond quickly when exposed to a variety of stimuli. Capable of sharp focus and creating an accurate image, the eyes of octopuses are elaborate mechanisms that in many respects are quite similar to the eyes of humans and other vertebrates.

The body organs such as the heart, stomach and gills of mollusks are enclosed in the mantle. The mantle of octopuses looks like a wrinkled leather sack. The muscular mantle is thick, yet very flexible, and strong.

The eight tentacles (arms) of octopuses are equipped with one or more rows of suckerlike discs, hooks and other adhesive structures that help them ensnare prey. The arms of octopuses may have as many as 240 suction cups on the underside, in two rows. If an arm is bitten off by a predator, in many instances the missing appendage will grow back.

Octopuses have rather sophisticated chemoreceptors on the end of each of their tentacles. These receptors help octopuses interpret their surroundings and avoid danger; they also aid in detecting and directing prey toward their mouths. This chemosensory system is so effective that specialists often claim that octopuses are equipped with eight noses. While some species of octopuses do hunt during the day, most are nocturnal predators that concentrate their efforts toward crabs, shrimps, lobsters and a variety of mollusks. When hunting at night they depend greatly upon the ability of their chemosensory system. In fact, nighttime is usually the best time for octopus encounters. However, there are exceptions such as the Hawaiian Day Octopus.

Spineless and Squishy Bodies

Slinking, sauntering, crawling and flowing, octopuses sometimes seem to melt into their surroundings as they “evaporate” into any hole that is wide enough for them to squeeze their head into. Being invertebrates, and therefore lacking a spine, the bodies of octopuses are like a fluid muscle. The largest hard part of their body is their beaklike radula, and as long as their radula fits through the hole, an octopus can probably figure out a way to easily slide the rest of its body through an opening.

Defense Mechanisms

Given their excellent vision, it’s no wonder that these often reclusive mollusks retreat when they see a diver even when that diver is far away. Often when octopuses retreat into their dens or into crevices and other hiding places, they position their eyes so that the animals can look out at the world around them.

Occasionally, when caught unaware of an approaching diver, or fish that might be a predator, octopuses extend their arms, flatten their bodies and spread out on the bottom. Simultaneously, they change the color and texture of their skin. Often they do so in an attempt to blend in, or “disappear into” their surroundings, but sometimes they “flash,” or display, bright colors that are believed to be a “get away from me” warning.

Many octopuses display a strong sense of curiosity about divers and once they see you they commonly peer back from the protection of a lair or other hiding place. Patience and slow movements are often rewarded with a prolonged close encounter filled with a “color change show” or an extended tentacle in an act of cautious curiosity.

If a diver reaches into the dens, which are often marked by a pile of discarded shells known as a midden heap, octopuses will sometimes squirt a jet of water at the intruder. In most cases, once they feel threatened, the octopods are likely to try to remain hidden in the safety of their den. In many instances, when they feel like their best option is to flee the den, it is likely that the octopuses will opt to build a new home and not return to their abandoned domicile.

Although there are times when octopuses seem to prefer to stand out prominently against their background, as a rule they alter the color and texture of their skin to match their surroundings. If an octopus changes its location, the animal is usually quick to alter the hue and tone of its skin to match the brightness and color of the new setting. Specialized pigment cells called chromatophores that are primarily controlled by the animal’s muscular and vascular systems enable octopuses to quickly change color. This remarkable ability enables octopuses to move about the sea floor as if they are almost invisible.

When camouflaging doesn’t work, octopuses have evolved a “cloak” to disappear behind. They can emit clouds of ink produced by specialized glands. The ink clouds are used as smoke screens to confuse predators such as moray eels and a variety of fishes, seals and sea lions. But even more importantly, because of the nature of the chemical makeup of the ink, it is also used to dull the olfactory receptors used in the sense of smell by these predators.

Form Follows Function

Similar to squids, octopuses create thrust by forcing a powerful stream of water through a tubelike, highly directable opening known as the excurrent siphon. This natural “jet engine” system works by first extending the mantle, causing it to fill with water. Then powerful muscles rapidly contract the mantle, forcing the water out under pressure through the excurrent siphon, thus providing thrust.

In some instances, octopuses form their bodies into “wing shapes” while they glide through the water column. As is the case in air, the use of a wing that provides lift like the wing of an aircraft is extremely efficient in water. Being energy-efficient is important to all animals, but it is especially so with octopuses. When the animals swim, as opposed to glide, they become quickly exhausted. Even when at rest octopuses metabolize extremely high percentages of the oxygen they extract from the water. As a result, they tire easily as they can’t create much of an energy reserve.

The mouth and beak are in the center of the “underside” of the parachute. The beak, which is used for feeding, looks similar to a parrot’s beak, and can be surprisingly large. The beak is used to crush the shells of prey items. Once the beak has pierced the prey, a poison gland will sometimes inject a potent substance into the body of the prey. The poison paralyzes the victim and softens the meat so that the octopus can suck the “liquefied” flesh into its small mouth. In this way, the hunter devours everything but the shell.

Octo Courtship

On some occasions, the bodies of octopuses ripple in spectacular displays of continuously repeated color changes. The “rippling” is believed to be a form of intraspecies communication and is especially common during courtship seasons.

As is the case with most mollusks, the sexes are separate in octopuses. Males of each species possess a tentacle known as a hectocotyle arm that is used to transfer packets of sperm to females. The females lay as many as 45,000 fertilized eggs in grapelike clusters in the dens. In most species the females constantly guard, clean and oxygenate the eggs for several months before the young hatch. As a rule, the females are extremely diligent in their duties, and once the eggs are laid the mothers neither feed nor leave their den. The attendant ladies may lose more than 50 percent of their body weight before the young hatch, and in many species the females die shortly after hatching occurs.

While all of this information helps understand octopuses in a scientific sense, there is an entirely different level of appreciation of these mollusks that can only come from encounters in the wild. Any diver who has had anything more than a brief encounter with an octopus can not help but realize that octopuses are truly amazing creatures.

One Octopus. Two Octopi?

One is an octopus, but what are two? If the specimens are the same species, the correct plural is octopus. If they are different species, both “octopuses” and “octopods” are acceptable.

Story and photos by Marty Snyderman