Cephalopods (literally, “head-foot” in Greek) are members of the mollusk class Cephalopoda, characterized by bilateral body symmetry and a modification of the mollusk foot into the form of an arm or tentacles. With only a few exceptions, cephalopods have short lives characterized by very fast growth. In some cases, they can increase their body mass by as much as 12 percent each day. About 800 different species of cephalopods exist in all of the Earth’s oceans. Because of their ability to spray ink, the fishing industry refers to them as “inkfish.” Cephalopods are widely considered the most intelligent of all invertebrates. They have large brains and acute senses of vision and hearing. Moreover, the nervous system of the cephalopod is the most complex among all invertebrates.
Many cephalopods possess chromatophores – colored pigmentation – that they use in several important ways. They can color themselves to blend in with their background as defense against predators. Many cephalopods can also bioluminesce, shinING their light downward so as to avoid casting a shadow for those predators lurking above them. Bioluminescence may also be used to lure prey, and some species use colorful displays to impress potential mates, startle predators or even communicate with one another.
The known species of cephalopods (with two exceptions) have an ink sac. The ink sac is a muscle and is used to expel a cloud of dark ink to disorient predators. When squeezed, the sac inserts the ink into the anus from where it is then expelled in the direction of the predator. The ink can be dispelled by the same water the cephalopod expels for its jet propulsion. The ink itself contains mostly melanin mixed with mucus. The cephalopod has a very sophisticated defense strategy by which it will insert into the ink an amount of mucus which closely approximates the size and shape of the cephalopod itself. In the disorientation of the attack and subsequent dispersing ink cloud, the predator then attacks the “fake” cephalopod, called the pseudomorph.
The reproduction of cephalopods begins with the transfer of spermatophores by the penis – a long and muscular end of the gonoduct – to the cephalopod’s “arm,” called a hectocotylus. Deep water squid have the greatest known penis length relative to body size of all mobile animals, second in the entire animal kingdom only to certain species of barnacles. In mating, using this hectocotylus arm, the cephalopod transfers the spermatophores directly to the female.
In courtship, since cephalopods have no distinguishing external sexual characteristics, a male cephalopod will use bright, rippling displays of body coloras a means to attract a female. The other cephalopod, if female and available, will become pale in response, and mating will occur. If, however, the other cephalopod remains brightly colored despite the male’s color display, that is taken as a sign that the courting male should move along quickly, and no mating will occur. Some cephalopods mate head to head, and the male transfers the spermatophores directly to the female via the hectocotylus arm. Others may even detach the sperm-carrying arm and leave it attached to the female.
The post-hatching strategies of cephalopods vary: Some may be brooded, others place their eggs under rocks and aerate them with their tentacles until they are hatched. Often, eggs are left to their own devices, hidden in crevices or on the floor of the sea.
Cephalopod eggs span a large range of sizes, from 1 millimeter to 30 millimeters in diameter. The length of time before hatching varies greatly; smaller eggs in warmer waters are the fastest to hatch, and newborns can emerge after as little as a few days. Larger eggs in colder waters can develop for more than a year before hatching.
How does a cephalopod’s nervous system work?
Characterized by an elongated symmetrical body and a prominent head, cephalopods also have a modified mollusk foot and a muscular hydrostat in the form of tentacles. The number of tentacles varies between eight and 90 depending upon species. These tentacles attach in a crown-like fashion and are greatly effective when used to grasp prey. Cephalopods’ mouths are hard and beak-shaped like that of a parrot and can tear into prey easily while using the tentacles to hold the prey steady. Many cephalopods have the ability to squirt ink as a defense mechanism, which has earned them the name inkfish in the fishing industry. The ink is harmless but has a blinding effect under water.
Because of the complex nature of the cephalopod nervous system, cephalopods rank closer to vertebrates than other invertebrates in comparing their nervous system and behavior. With large brains, larger than gastropods (snails and slugs), and with keen senses, cephalopods’ brain and body size ratio ranks them somewhere between cold-blooded and warm-blooded vertebrates. Having three hearts pumping bluish-colored blood throughout their bodies, the cephalopods’ circulatory system is virtually closed.
Most invertebrates have cell clusters known as ganglia. These ganglia operate in unison as a simple brain. In cephalopods, the ganglia are fused together and concentrate to form an actual brain similar to that of a vertebrate. Their brain works from different sections to control varied areas of the body. As an example, the closing of its suckers and forward swimming is controlled by the cerebral ganglia.
Cephalopods have highly acute senses including vision, with the eyes of the giant squid being the largest within the entire animal kingdom. With the ability to sense light, their vision can form images and detect brightness and size. They can even tell the horizontal and vertical orientation of objects. This acute sense of sight is highly conducive to the cephalopods’ success while hunting. Cephalopods are highly aggressive carnivores, with the largest of their class, the giant squid, documented attacking sperm whales.
Skilled in the art of defense, cephalopods will use their ability to squirt sepia, an ink-like black substance that clouds the water and temporarily blinds predators. Cephalopods can also fend off predators with their tentacles. Another method of defense is the chromataphores in their cell. These allow them to change color and camouflage themselves.
Cephalopods move by ambling along using their fins and tentacles to gently propel through the water. However, when fleeing a predator or attacking prey, the cephalopod is capable of a form of jet propulsion by filling its mantle with water and forcing it out with a powerful squeeze. This allows its missile-shaped body to propel at great speed through the water. This is highly energy draining for the cephalopod, and it not used as a typical means of traveling.
In the end, the complexity of these creatures has led to long-term ongoing studies. The large fibers that form the nerves of the cephalopod mantle have even been used in experimentation by neurophysiologists because of a lack of myelination.