Cuttlefish are sometimes called the chameleon of the sea because they are able to change their skin color. Their skin can flash a colorful pattern in order to communicate with other cuttlefish and to camouflage them from predators. This color-changing function is produced by groups of red, yellow, brown, and black pigmented chromatophores above a layer of reflective iridophores and leucophores. These are all in the cuttlefish's skin, and work together to change its colour. There are up to 200 of these special pigment cells per square millimeter.
The pigmented chromatophores have a sac of pigment and a large membrane that is folded when retracted. There are 6-20 small muscle cells on the sides which can contract to squash the elastic sac into a disc against the skin.
Iridophores are plates of chitin or protein, which can reflect light. They are responsible for the metallic blues, greens, golds, and silvers often seen on cuttlefish. All of these cells can be used in combinations. As well as being able to influence the colour of the light that reflects off their skin, cuttlefish can also affect the light's polarization, which can be used to signal to other animals which sense polarization.
Cuttlefish eyes are among the most developed in the animal kingdom. The way cephalopod eyes develop is fundamentally different from that of vertebrates like humans, but the way they work is rather similar. The similarity between cephalopod and vertebrate eyes is an examples of convergent evolution. Although they cannot see color, they can perceive the polarization of light, which improves their ability to see contrast. They have two spots of concentrated sensor cells on their retina (known as fovea), one to look more forward, and one to look more backwards. The lenses, instead of being reshaped as they are in humans, are instead pulled around by reshaping the entire eye in order to change focus.
The blood of a cuttlefish is an unusual shade of green-blue because it uses the copper-containing protein haemocyanin to carry oxygen instead of the red iron-containing protein haemoglobin that is found in vertebrates. This is similar to the blood of arthropods. Haemocyanin is not so good at carrying oxygen as is haemoglobin. The blood is pumped by three separate 'hearts'. Two of these are used for pumping blood to the cuttlefish's pair of gills (one heart for each gill), and the third for pumping blood around the rest of the body.
Cuttlefish have inks like squid and octopuses. As with squid, the ink is a defensive tool, squirted out to cover its retreat in the case of danger. This ink was formerly an important dye, called sepia. Today artificial (not natural) dyes have replaced natural sepia.
Cuttlefish are caught for food in Mediterranean, and East Asian cultures. Although squid is more popular as a restaurant dish all over the world, in East Asia dried cuttlefish is highly popular.
Cuttlefish is popular in Italy and Croatia, where it is used in "Risotto Nero". "Nero" means 'black', which is the colour the rice turns because of the cuttlefish ink. Spanish cuisine often uses cuttlefish and squid ink for cooking due to the marine flavor and smoothness that it provides to the meals and it is included in dishes such as rice, pasta and fish stews.
Cuttlefish made their most important literary appearance in the title of Eugenio Montale's collection of poetry called Cuttlefish Bones ("Ossi di seppia"), published in Turin in 1925. Montale was awarded the Nobel Prize for Literature in 1975.
There are over 120 species of living cuttlefish, in 5 genera. Sepiadariidae contains seven species and two genera; all the rest are in the Sepiidae.
Nautilus
Nautilus is the common name for cephalopods of the family Nautilidae. They are the only living members of the subclass Nautiloidea. In appearance, they have not changed much in millions of years. The term "chambered nautilus" is also used for any species of the Nautilidae.
Many biologists see them as 'living fossils', because they are the only living descendents of the group which gave rise to the ammonites in the Palaeozoic era.
Today, there are six living species in two genera. The species "Nautilus pompilius", from Western Australia is the largest and best known. It get up to 27 cm in diameter. However, most other nautiluses never exceed 20 cm. The Bellybutton nautilus, "Nautilus macromphalus", is the smallest species, usually measuring only 16 cm.
The radula is wide and has exactly nine teeth. There are two pairs of gills.
Nautiluses are the only cephalopods who have an external shell. This is an ancestral or basal, feature. The animal can withdraw completely into its shell. It can the close the opening with a leathery hood formed from two specially folded tentacles. The shell is coiled and calcareous, with mother-of-pearl lining. It is pressure resistant, but will implode at a depth of about 800 m. The nautilus shell is made of two layers: the outer layer is a dull white, while the inner layer is a striking white with iridescence. The innermost portion of the shell is pearlescent, blue-gray. A osmena pearl is not a pearl, but a jewellery product got from this part of the shell.
The shell is internally divided into chambers. All the walls between chambers ('septa') are pierced in the middle by a duct, the siphuncle. Through this the animal can change the air pressure in the chambers. As the nautilus matures its body moves forward, sealing each chamber in turn with a new "septum" (wall). The last fully open chamber, the largest one, is used as the living chamber. The number of chambers increases from around four at the moment of hatching to thirty or more in adults.
The shell colour keeps the animal hidden in the water. When seen from the top, the shell is darker in color and marked with irregular stripes, which makes it blend into the darkness of the water below. The underside is almost completely white, making the animal indistinguishable from brighter waters near the ocean surface. This mode of camouflage is called "countershading".
The nautilus shell is one of the finest natural examples of a logarithmic spiral.
In order to swim, the nautilus draws water into and out of the living chamber and makes use of jet propulsion. When water is inside the chamber, the siphuncle extracts salt from it and diffuses it into the blood. When water is pumped out, the animal adjusts its buoyancy with the gas contained in the chamber. Buoyancy can be controlled by the pumping of gas and fluid into or from the chambers using the siphuncle. The control of buoyancy like this limits the nautilus; they cannot operate under extreme pressure.
In the wild, nautilus usually lives in depths of about 300 m, rising to around 100 m at night for feeding, mating and egg laying. The shell of the nautilus cannot withstand depths greater than approximately 800 m.
Nautiluses are predators and feed mainly on shrimp, small fish and crustaceans, which are captured by the tentacles. However, due to the very little energy they devote to swimming, they need only eat once a month. Unlike other cephalopods, they do not have good vision; their eye structure is highly developed but lacks a solid lens. They have a simple "pinhole" lens through which water can pass. Instead of vision, the animal is thought to use olfaction as the primary sensory means during foraging, locating or identifying potential mates.