The cephalopods are an amazing group. The octopus, squid, cuttlefish and nautilus are intelligent, well adapted, and behave in ways that humans can relate to, and yet they are each of a form that makes them seem utterly unlike us, to the extent that they figure in trashy horror stories as villains.
The cephalopods are mollusks, so they all have shells. In the octopus the shell is hidden, a vestigial bit of bone where muscles are attached. The nautilus has a famous many chambered shell where the animal can take shelter. Argonauts are almost octopuses; the major difference is that the argonaut has an external shell. In the squid, the shell has evolved into a long horny structure that runs the length of the body. Cuttlefish have a porous internal shell.
Cephalopods all have multiple arms. The octopus and argonauts have eight. Squid and cuttlefish have ten. The Nautilus may have different numbers, up to sixty depending on species. Nautilus arms are arranged in two concentric circles and have no suckers. Other cephalopods have suckers on their arms, tough circles that can act like suction cups to help them grasp their prey. The suckers of the octopus and argonaut lie flat against the arms, while the suckers of the squid and cuttlefish are raised and hardened. Cuttlefish and squid have eight arms plus two essentially prehensile tentacles.
The eye of the cephalopod and the mammal work in very similar ways, yet took radically different evolutionary paths to reach their present function. Both forms have a single lens, and a pupil that admits light. The cephalopod pupil is squarish though, and can resemble a cursive w when contracted. This pupil admits light which is focused on a retina, but the lens does not change shape to focus near or far the way the human eye does. Rather, the cephalopod eye itself changes shape, drawing the lens closer to the retina or farther away to focus the captured image that will be sent to the brain.
Cephalopods are photochromic. They can change their coloration in a flash, and they have this ability from birth. They use this amazing trait to signal other cephalopods, or for camouflage. Because these flashing changes are so rapid, and because the animals are capable of so many color variations, it is easy to believe that the cephalopod is using language.
The secret of their color changes lies in the composition of their many-layered skin. The bottom layer is leucophores, which produce a shiny white color. Above that are iridophores, which can appear green or blue or give cephalopods a metallic or iridescent sheen under some circumstances. Above that layer are three layers of chromatophores, a bottom layer of pigment cells colored brown or black, then a layer of red, and then one of yellow. Each chromatophore layer is full of sacs of pigment with tiny muscles connected to each. These muscles can pull each sac out flat, so its color is displayed, or contract it to hide the color in a blink. Cephalopods can also change the texture of their skin, to better blend into, or stand out from, their surroundings.
The camouflage of the nautilus is a bit different. The curved shell is a rippled beige and bone color on top, to look like the ocean bottom. From underneath, the whitish underside of the nautilus resembles the shiny roof of the ocean surface. These mollusks live on the lower slopes of coral reefs, only in the Indian Ocean or the Pacific, and only within 30 degrees of the equator. They live about 300 meters down, but rise to about 100 meters to feed, mate, or deposit their strings of eggs. They mate once a year, and may live twenty years. The male has a spadix, which is a modification of four of its tentacles, which it uses to deposit sperm under the female's mantle. She leaves strings of eggs attached to rocks, where they will hatch in 8 to12 months into juveniles about 30 mm long. The adult usually reaches about 16 cm, but there are nautilus off Western Australia that reach 27 cm.
The nautilus begins life in small pearly chambers, which it soon outgrows. It builds a new, larger chamber onto the original shell, seals up the old one and moves into the new. It repeats this process throughout its life, gradually creating a spiral shell of up to 30 chambers.
The nautilus is the only cephalopod that does not have an ink sac. All the others are capable of releasing a cloud of ink into the sea, which helps blind predators as well as confuse or paralyze their sense of smell. The long bodied squid secretes ink that is sepia, a rich brown, not black like the ink other cephalopods release. Because the ink was once used in drawing, painting, and writing, the Greeks called the squid by a name that meant sepia producer. Like the octopus, the squid has a trunk covered by a mantle, from which its arms extend. The squid's trunk is generally more oblong than that of the octopus though.
The mantle covers the gills and the viscera, including the siphon. The siphon is an organ cephalopods use for locomotion and respiration. It consists of a funnel that draws in water along the edges of the mantle and then squirts it out through a tube that hangs below the mantle. This forces a continual supply of oxygen rich water over the cephalopod's gills. There has to be a high volume of oxygen because the blood of these beings does not carry oxygen as efficiently as hemoglobin based blood does. Cephalopod blood is blue or green because it uses copper in its hemocyanin where hemoglobin uses iron. Oxidized copper is green. The siphon can also serve as jet propulsion when the animal is in a hurry.
Cephalopods eat mollusks, crab, fish and other cephalopods, while sharks, dolphins, fish and other cephalopods prey on them.
Cuttlefish may live one to two years. The cuttlebone that is harvested to hang in bird cages helps the cuttlefish manage its buoyancy. It is porous and the animal fills it with gas or liquid to change its density and move up and down in the water. A submarine works in a similar way, becoming heavier or lighter to rise and fall in the water.
The octopus lives until it mates, in the case of the male, or until the eggs hatch, in the case of the female. This may mean a lifespan as short as a year or two. The female tends the eggs carefully, keeping them free of fungus and trying to protect them from predation until the young are born.
Squid may be as small as 1 inches, or may grow to 55 feet or more. They are the largest of invertebrates. The lifespan of he average squid is believed to be 1 to 2 years, and the giant squid may live five. However, we don't really know. The ocean is vast and deep.
There is little doubt that cephalopods are intelligent. Researchers have seen them act purposefully and carry out plans. Humboldt squid are seen to cooperate in hunts. There is certainly enough brain mass in larger species to support associative areas, areas where information can be stored and thinking can take place. Yet their short solitary lives are so unlike ours. The question is really: what is their intelligence like?