Anatomy of a Cuttlefish

Both the arm (top) and the tentacle (bottom) are lined with suckers.

Arms and Tentacles
Unlike the octopus's arms, which that animal often uses to move and carry objects, the cuttlefish's eight arms are specialized for grasping prey after the cuttlefish captures it with its two elongated tentacles. When potential food sources such as fish or shrimp swim near, the cuttlefish can alter the color of its skin while waving its arms in a mesmerizing display. This lures potential prey to within reach of the cuttlefish's tentacles, which can then shoot rapidly from a pocket at the base of the arms to grab the prey. The arms are also important for a defensive display in which the cuttlefish sucks water into its mantle cavity and spreads its arms in order to appear larger to its potential opponent.

The dark area seen here is part of the cuttlefish's strong, sharp beak, the rest of which lies behind the buccal (cheek) mass.

Beak
The cuttlefish's beak looks much like a parrot's beak, but it is hard to see because it lies buried at the base of the animal's eight arms. The cuttlefish can use its beak to help subdue prey and to defend itself against predators and rivals by biting. Like cuttlebones, beaks differ among species, and their remains enable scientists to identify which cuttlefish species have lived and died in certain areas.

Unlike in mammals, the cuttlefish's optic lobes are located outside of its cartilage brain casing. Above, a transverse cross-section of the cuttlefish brain.

Brain
The cuttlefish has one of the largest brain-to-body size ratios of any invertebrate, perhaps even larger than that of the octopus. The cuttlefish brain can handle input from a variety of senses, including sight, smell, and even "sound" (in the form of pressure waves). According to some scientists studying cephalopod learning, the cuttlefish can use visual clues to solve mazes, making it as intelligent as the octopus or land animals like the pigeon.

The rigid cuttlebone allows the cuttlefish to keep a constant internal volume, unlike a fish's swim bladder, which expands and contracts with depth.

Cuttlebone
A defining characteristic of the cuttlefish, the cuttlebone is a porous internal shell that helps control buoyancy, making it functionally similar to swim bladders in fish. Cuttlebones have both gas-filled forward chambers and water-filled rear chambers. Although it can take hours for the cuttlefish to change its density through its cuttlebone alone, the animal can control its positioning in the water with the aid of its specialized fins and mantle. The cuttlebone is rich in calcium and is often sold in pet stores as a nutritional supplement for birds.

A cuttlefish looks on through its large eye. Note the clear "W" shape of its pupil.

Eye
Although color-blind, the cuttlefish has two of the most highly developed eyes in the animal kingdom. It can see well in low light and can also detect polarized light, enhancing its perception of contrast. While we humans reshape our lenses in order to focus on specific objects, the cuttlefish moves its lenses by reshaping its entire eye. Also, the cuttlefish's eyes are very large in proportion to its body and may increase image magnification upon the retina, while the distinct "W"-shaped pupil helps control the intensity of light entering the eye.

The cuttlefish's undulating fins can move more freely than fish fins because they lack both bony and cartilaginous supports.

Fin
While the cuttlefish uses its mantle cavity for jet propulsion, it relies on its specialized fins for basic mobility and maintaining consistent speeds. Resembling a short, flouncy skirt, the muscular fin can maneuver the cuttlefish in nearly any direction: backward, forward, even in circles, with such movement being more energetically efficient than jetting. The movement and positioning of the fins also come into play when smaller males in certain species mimic the opposite sex in order to swim past larger males and gain access to females.

The cuttlefish's pair of orange gills (one appears above) filter oxygen from seawater and deliver it to the bloodstream.

Gills, Hearts, and Blood
The cuttlefish has three hearts, with two pumping blood to its large gills and one circulating the oxygenated blood to the rest of its body. The blood itself is blue-green in color because it possesses hemocyanin, a copper-containing protein typical in cephalopods—cuttlefish, octopuses, and squids—that transports oxygen throughout their bodies. (Mammals' red blood uses the iron-rich protein hemoglobin to do the same thing.)

The dark ink sac can be seen clearly in this image of part of the mantle cavity.

Ink Sac
Like its close relatives, the squid and octopus, the cuttlefish is equipped with an ink sac that can help it make a last-ditch escape from predators that hunt by sight. The cuttlefish can eject its ink in two ways. One way creates a smoke screen behind which the animal can escape perceived danger. In the other, the released ink takes the form of "pseudomorphs," or bubbles of ink surrounded by mucus that are roughly the size of the cuttlefish and can act as decoys. The ink, which contains dopamine and L-DOPA, a precursor to dopamine, may also temporarily paralyze the sense of smell in predators that hunt by scent.

In this scanning electron microscope image, "L1" and "L2" mark the lateral lines, while "A" indicates the cuttlefish's arms.

Lateral Lines
Although the cuttlefish can't hear, it can detect sound in the form of pressure waves using its lateral epidermal lines. Seen here via a scanning electron microscope, these lines consist of thousands of hair cells. The cells seem to be especially sensitive to sounds ranging between 75 and 100 Hz, with 100 Hz being similar in frequency to a typical automobile engine running at maximum speed. One physiological study showed that in total darkness, healthy cuttlefish could capture about 50 percent of available prey, whereas cuttlefish with compromised epidermal lines could capture only about 30 percent. The hair cells can also be used in defense, allowing cuttlefish to detect the movement of possible predators.

In this view of the inside of an adult cuttlefish's mantle, the orange gills and dark ink sac are clearly visible.

Mantle
The multifunctional mantle cavity is important for cuttlefish locomotion, giving the animal its characteristic jet propulsion ability. To jet away from a predator, the cuttlefish sucks water into the cavity and then uses its strong mantle muscles to expel the liquid with great force, driving the cuttlefish in the opposite direction. Water exits through a movable part called the funnel, which controls the angle of the spray. The mantle cavity also aids in respiration by bringing water to the animal's gills, which in turn filter oxygen into its bloodstream.

Males and females face each other and embrace while mating.

Reproductive Organs
During mating, the male uses a modified arm to transfer his genetic material into the female's buccal area. This is the part of the female's mouth that stores the male's spermatophores (sperm packaged in special containers) until she is ready to use them to fertilize her eggs. Because the female often accepts more than one mate, the male sometimes sprays water through his mantle funnel into the female's buccal area to wash out other males' spermatophores. When she is ready to deposit her eggs in safe locations such as under rocks or in discarded shells, the female uses her arms to wipe the stored spermatophores onto each egg.

Stripes ripple across a cuttlefish's skin. To see another example, go to "Quick Change Artists" elsewhere on this Web site.

Skin
When it comes to changing one's skin color, the cuttlefish outshines even the chameleon, in both degree and kind. Its skin possesses up to 200 chromatophores (pigment cells) per square millimeter, allowing the animal to pattern itself with a variety of colors. When vying for a mate, for example, some male cuttlefish will showcase "intense zebra displays" (see left). The cuttlefish can also use muscles in its dermis to change its skin texture from smooth to rough, enabling it to hide easily among rocks on the seafloor, for instance.