The class Cephalopoda includes octopus, cuttlefish and squid, which belong to the phylum Mollusca. They are soft bodied animals, with a head, a body containing the guts (and shell in some species), and 8-10 tentacles, very mobile and equipped with suckers. The mouth located in the middle of the arms, contains a hard beack and a toothed tongue (radula). In many species the beak is connected to a venom gland. All cephalopods are predatory, mainly living on crustacea and fish, that they catch using the tentacles and kill using a venomous bite.
In the order Octopodia (octopus) the shell is missing. The body is soft, allowing it to enter the smallest holes. Squid and cuttlefish are good swimmers, whilst octopus are mostly benthic. Behind the eye a funnel can be seen, used to drive the water inside and outside a gills chamber, and is also used for movement, in a sort of jet propulsion. Cephalopods are very efficient animals. They can manipulate objects thanks to their mobile tentacles and to the many suckers. They have good eyesight. The cephalopod eye, is similar to a vertebrate eye and very effective.
There are cephalopods in all the marine environments. Cuttlefishes (Sepia officinalis) move by swimming rather than crawling, and can disappear by burying themselves completely in a sandy bottom. The shell (cuttlebone) allows a buoyancy control, that makes the animal neutrally buoyant (for swimming) or negative (for hiding in the bottom).
Squid, like the Costa Brava species, Loligo vulgaris, are open sea animals only seen close to shore occassionally on a night dive or when they come to lay their eggs in shallower areas. Many cephalopods live in very deep water, and their lifestyles remain a mystery.
When mating, the male uses a modified tentacle (ectocotile) to transfer the sperms to the female. In many species the eggs give birth to a young animal, very similar to the adults. In other species a transparent larva lives phases of planktonic life, before a metamorphysis into the adult form.
Cephalopods’ ability to change colour and shape provides a distinct evolutionary advantage. Their skin contains pigmented cells (cromatophores), each one connected to muscular fibres, that can extend or contract the cell, changing the colour very quickly. Nerves transmit the stymulus to each cell, allowing the animal to change colour and to assume complicated colour patterns in an instant. Other muscular fibres can smoothe the skin or give it a rough texture, to better conceal the animal on the bottom. Colour changes are used for camouflage and also to intimidate and confuse predators. Another system to generate confusion in predators is the ink secretion, that can be quickly expelled while the animal changes direction. Due to its density, it becomes a false target attracting the predator and allowing the mollusc to make its escape.