Cephalopods: Octopuses, Squids, Cuttlefish and Nautiluses

Cephalopods: Octopuses, Squids, Cuttlefish and Nautiluses

The class of mollusks known as cephalopods is comprised of more than 600 species of octopuses, squids, cuttlefish and nautiluses. Collectively speaking, the members of this class are considered to be the most intelligent of all invertebrates. The hard shell so characteristic of other mollusks is often missing or greatly reduced in most cephalopods. In fact, in squids and cuttlefish the shell is small and internal. In octopuses the protective shell is entirely missing as they have evolved other means of defense. In contrast, the shells of nautiluses completely cover their bodies.

The word cephalopod is derived from the Greek words for “head” and “foot.” The “head foot” reference is made in acknowledgment of the two most prominent characteristics of cephalopods, the extremely enlarged head and foot-like arms. The foot has been modified into sucker-bearing arms that are used for mobility, ensnaring prey and manipulation.

With the exception of nautiluses, the arms of cephalopods are covered with one or more row of suckers. Each sucker contains small hooks or other adhesive structures that are used for ensnaring prey. While octopuses have eight arms, squids and cuttlefish have eight arms and two tentacular-clubbed arms making a total of ten appendages, and nautiluses have as many as 90 arms. While squids and cuttlefish primarily use these arms and suckers for capturing fish and other mid-water animals, while octopuses use their arms to trap and pin prey against solid substrate as they primarily feed on bottom-dwelling invertebrates such as other mollusks, arthropods, and worms. In addition to the use of sucker-covered arms for immobilizing prey, most cephalopods possess a hard parrot-like beak. Most beaks contain an immobilizing toxin that is released into the body of the prey once it has been bitten.

The senses of smell, sight, and touch are extremely well developed in cephalopods. These senses aid them in escaping predation as well as hunting for prey. At the end of each arm octopuses are equipped with well-developed chemoreceptors that aid them in detecting prey and directing it toward their mouths. In fact, it is often said that octopuses possess eight “noses.” The eyes of cephalopods contain spherical lenses similar to the eyes of humans that are designed to see through air, a characteristic that has long intrigued scientists. On the other hand, fish have flattened lenses and their vision is not as well developed.

Because of their soft bodies, most cephalopods are extremely vulnerable. In order to cope with this vulnerability, they have developed advanced forms of defense. Their defensive adaptations include their propulsion-like movement and ability to squirt clouds of anesthetizing ink, their ability to squeeze their soft bodies through tiny holes and into small crevices where predators cannot gain access, and their ability to quickly alter their color to match their surroundings.

In many cephalopods, the art of camouflage often includes the ability to control and alter the texture of theirskin in addition to its color. Vision plays a key role camouflage. Specialized pigment cells controlled by the muscular and vascular systems and that are known as chromatophores enable cephalopods to rapidly change their color. The ability to change color is also used as a means of communication. At times, the bodies ofcephalopods ripple and flash in spectacular color displays. Color flashing is most often utilized during elaborate courtship rituals and when competing males challenge each other as they pursue females. For example, some squids gather in large aggregations and flash red and white to signal their readiness to mate.

Cephalopods propel themselves through the water column by the means of a natural, jet-propulsion system. These mollusks create thrust by forcing a powerful jet of water through their excurrent siphon. This propulsion system first expands the mantle and causes it to fill with water. The powerful muscles of the mantle then quickly contract and force water out through the excurrent siphon under a lot of pressure. The force of the water causes the animals to be propelled through the water with their tentacles trailing behind. This sometimes gives the illusion that the animals are moving backward.

An organ known as a siphuncle that passes gasses and fluids through the opening into each of the numerous internal chambers in their shell is unique to nautiluses. The siphuncle forces gasses into and out of their chamber to aid the animals in moving up and down in the water column.

The sexes are separate. Fertilization is internal and females lay their fertilized eggs individually or in clusters inside of egg casings that are attached to the substrate. Males have a specialized arm known as a hectocotyle arm that transfers packets of sperm to females. Eggs are typically translucent and slender and appear in a variety of colors. While squids lay their long egg casings on the ocean floor and leave them, females octopuses and cuttlefish lay their fertilized eggs in grape-like clusters that hang from the ceiling of their dens. Octopuses and cuttlefish guard, aerate and clean their eggs for several weeks until they hatch. These females do not leave their dens during this time and consequently, do not eat. Often these females lose more than 50 percent of their body weight and die soon after the young hatch from the eggs.