Marine animals placed in the order Sepiida are cuttlefish. Cephalopods are they. Octopus, nautilus and squid are close relatives of cuttlefish. As the word fish is attached, one might get confused by the term, but they are molluscs and not fish. Latest research indicates that they are the smartest invertebrates that live today. Of all invertebrates, they also have the greatest ratio of brain to body size. The name Cuttlefish was derived from an old English word. There is an internal shell. They have pupils in the W-shape, eight arms and two tentacles. The arms are provided with denticulated suckers that function as successful prey capture organs. They can reach a length of 15-25 cm, while the largest species is 50 cm long. It can achieve a body weight of up to 10.5 kg. Molluscs, shrimps, octopuses, crabs, fish, worms, and other cuttlefish are being devoured. Dolphins, whales, sharks and other cuttlefish are their potent predators. The average life span is 1-2 years or so. check it out for more details.
They have a well-developed structure that is porous and composed of aragonite, known as cuttlebone. This cuttlebone supplies the animal with buoyancy. By altering the gas-to-liquid ratio in the chambered cuttlebone via the ventral siphuncle, buoyancy can be controlled. Every species has a particular cuttlebone shape, size and texture. The cuttlebone is unique to these species and separates them from the squids known as their nearest relatives. Jewellers and silversmiths use cuttlebone as moulds to cast small objects, but their best use is to serve as an additional source of calcium, such as parakeets, for caged birds. They are often referred to as the chamaeleons of the sea because they can change the colour of their skin at will. In order to communicate with other seedlings and camouflage themselves from predators, they are able to alter the colour of the skin as well as the polarity of light. Red, yellow, brown and black chromatophores, present above the reflective layer of iridophores and leucophores, perform the colour-changing role. They are pigmented cells that are highly formed and correspond to around 359 DPI. For storing the pigment, the pigmented chromatophores have a sac and are covered with a membrane that is withdrawn when folded. For successful activity of the pigment sac, there are 6-20 muscle cells. Yellow chromatophores, accompanied by red and orange, lie very close to the surface of the skin. Above the iridophore layer are the brown and black chromatophores present. It is understood that iridophores reflect blue and green light. In fact, they are small plates that are both made up of chitin or protein and are known to represent the seedling climate. These plates are responsible for displaying on cuttlefish the colours of metallic blues, silver, green and blue. By the collective action of various chromatophores such as the red and yellow chromatophores, different colour combinations are also possible, responsible for producing orange colour.
In all species, eyes are very formed. Cephalopod eye organogenesis is somewhat different from that of human eye development. Convergent evolution is due to superficial similarities between the eye of the cephalopod and the vertebrate eye. Even though they do not identify colour, they have well-developed powers of sensing light polarisation. They have two spots on their retina with focused sensor cells, one of which is used to look forward and the other to look backward. As the optic nerve behind the retina is present, there is no blind spot. At the early stage of their development, scientists say that the eyes are completely formed in cuttlefish and they begin to observe things even inside the egg. As it is filled with copper containing haemocyanin dye, the blood is greenish blue in colour. In these species, Haemocyanin is an excellent oxygen carrier. For circulation, there are three hearts that serve. The gills are supplied with blood by two hearts, and one pumps blood throughout the rest of the body. They have inks that are used for warding off predators, much like squid and octopuses. Their saliva is rich in neurotoxins that are bacterially produced.