WORK IN PROGRESS
Complex systems and behaviour are commonly associated with vertebrates, in particular mammalian, even though among invertebrates one taxon stands out for its exceptional features: the Cephalopods.
Cephalopods (from the Greek words kephalos, head, and podos, foot: “head-footed”) form a class in the mollusc phylum, closely related to snails, chitons and bivalves, and includes species commonly known as octopuses, cuttlefishes, squids, vampire squids and nautiluses.
They are exclusively marine animals, with a bilateral body symmetry, a prominent head, a set of arms and tentacles with chemosensitive suckers, three hearts and a parrot-like beak. Almost all of them share the common ability to squirt ink, which is a useful tool for distracting predators. Despite their colour blindness, they are able to change colour and body pattern to match the natural background with incredible rapidity. This achievement is accomplished using a set of specialized organs, which include the chromatophores, which are neuromuscular organs containing pigments. The dynamic patterns produced by the chromatophores have an important role in camouflage, but also in intra- and inter-specific visual signalling.
Cephalopods come in a variety of size and life style. They occupy all the oceans of the world, from the interdal pools to the perpetual darkness of the abyss, from the hot tropical waters to the cold polar waters. Pygmy squids, represented by the family Idiosepiida, are the smallest cephalopods, and male can mature at less than 10 mm long. This is approximately the same length as the egg of giant squids of the family Architeuthida which can grow up to 18 m long, including tentacles, and are easily the largest invertebrates.
Although cephalopod and vertebrate lineages separated more than 500 millions years ago, when protostomes and deuterostomes split, the two groups achieved to evolve, independently to each other, exceptional analogies. From a purely functional point of view, cephalopods are fish: body form, locomotion, vision, vascular and nervous systems, the similarities are striking and non incidental. Evidence shows that this evolutionary convergence is not merely due to physical demands of the same aquatic environment, but by response to dynamic interaction between the two groups which shared the same adaptive zone since the Palaeozoic. The cephalopod ring-type brain is extremely large for invertebrate standards and the ratio brain-body weight is comparable to most vertebrates. For all those reasons, cephalopods are ideal comparative models to vertebrates for understanding the complexity of their high-evolved, both novel and convergent, systems.