Cuttlefish are most closely related to the squids (order Teuthida), and while they are similar to squid in many ways, they are also totally unique and amazing in their own right. Instead of tails, cuttlefish have a continuous rippling fin surrounding their mantle, a muscular yet highly flexible construction that allows cuttlefish speed and maneuverability; with their fin, cuttlefish can move backwards, forwards, even flip over. Cuttlefish ink contains dopamine and dopamine chemical precursors that may suppress predators' sense of of smell. The pigments in the cuttlefish's ink were the original source for the shade of brown known as sepia and were once used in the art world for paints, inks and dyes. Cuttlefish possess aptly-named cuttlebones, calcareous internalized shells with tiny layered chambers connected by a strand of tissue called a siphuncle that regulates the cuttlebone's gas-to-liquid ratio (and, in doing so, the animal's buoyancy) through osmoregulation (this is really very similar to the nautilus's camerae system). Cuttlefish have venom in their suckers that can poison or kill prey, and one species – Pfeffer's Flamboyant Cuttlefish, an intelligent and aggressive cuttlefish that is almost constantly dramatically modifying its body structure, skin texture and color – is one of the few toxic cephalopods, with toxins as lethal as a Blue-Ringed Octopus's. Their eyes, with mustache-shaped pupils, allow them high-resolution perception of the angles at which light is reflected and polarized. And then, of course, there's the color-changing thing:
In addition to their psychedelic color-changing skills, cuttlefish can also change the texture of their skin to match the surrounding substrate. They use circular concentric muscles to force up liquid beneath the mantle into spikes or nodes, varying in size. These folds of skin, known as papillae, allow cuttlefish to blend in with the ocean bottom. Many species, including the Giant Australian Cuttlefish, use their papillae not only in camouflage but in communication.
This provides a fascinating insight into cephalopod sexual selection. While the animal kingdom is rife with examples of females choosing showy, colorful, ostentatious mates with well-developed secondary sexual characteristics – these organisms most likely have higher relative fitness and would pass those genes on to her offspring – the female Giant Australian Cuttlefish, like organisms in other species with advanced intelligence, are equally willing to mate with physically well-developed indidviduals and individuals that have exhibited intelligence.
Giant Australian Cuttlefish have no osmoregulatory mechanisms and are overall very senstitive to changes in temperature, salinity, and other aspects of water chemistry. As they have have 100% mortality at 50 ppt salinity (28-38 ppt is their preferred range), recent plans to construction desalination facilities that would discharge 120 megalitres (32 million US gallons) of brine into the water every day have been met with opposition from scientists and conservationists. S. apama populations have also been affected by hydrocarbon processing and other industrial activities along Australia's southern coast.