Cuttlefish

Cuttlefish are marine animals of the order Sepiida belonging to the Cephalopoda class (which also includes squid, octopuses and nautiluses). Despite their common name, cuttlefish are not fish, but molluscs.

Eyes

Cuttlefish eyes are among the most developed in the animal kingdom. The organogenesis of cephalopod eyes differs fundamentally from that of vertebrates like humans. Superficial similarities between cephalopod and vertebrate eyes are examples of convergent evolution. The cuttlefish pupil is a smoothly-curving W shape. Although they cannot see color, they can perceive the polarization of light, which essentially enhances their perception of 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. Also, cuttlefish eyes do not have a blind spot such as that in vertebrate eyes.

Blood

The blood of a cuttlefish is an unusual shade of green-blue because it uses the copper-containing protein hemocyanin to carry oxygen instead of the red iron-containing protein haemoglobin that is found in mammals. The blood is pumped by three separate hearts, two of which 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. A cuttlefish’s heart must pump a higher blood flow than most other animals because hemocyanin is substantially less capable of carrying oxygen than haemoglobin.

Changing color

Cuttlefish are sometimes called the chameleon of the sea because of their remarkable ability to rapidly alter their skin colour at will. Their skin flashes a fast-changing pattern as communication to 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, with up to 200 of these specialized 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. Yellow chromatophores (xanthophores) are closest to the surface of the skin, red and orange are below (erythrophores), and brown or black are just above the iridophore layer (melanophores). The iridophores reflect blue and green light. Iridophores are plates of chitin or protein, which can reflect the environment around a cuttlefish. They are responsible for the metallic blues, greens, golds, and silvers often seen on cuttlefish.

(from Wikipedia)