  
There are about 120 species of cuttlefish in the world. The Giant Australian Cuttlefish, Sepia apama, is the largest. These remarkable creatures have colour-changing skin, panoramic vision, three hearts, and blue blood!
Their home is in the shallow reefs of the southern waters of Australia. These cuttlefish can weigh as much as 10 kg and be up to a metre long, although most are not that big. They have 8 arms and two feeding tentacles. The tentacles normally are retracted into sockets above the arms, beside each eye, but can shoot out almost instantaneously to capture its prey, which include fishes, crabs and other crustaceans. The Giant Australian Cuttlefish isn't a fish at all, but a mollusc, although unlike most molluscs, it has no shell, just a remnant called a cuttlebone inside their mantle, the main body cavity that contains their internal organs, like a squid. Interestingly, cuttlebones are used in birdcages as a calcium-rich dietary supplement for birds.  The Giant Australian Cuttlefish can change the colour of its skin dramatically to camouflage itself. It does this using tiny pigment-containing and light-reflecting sacs containing cells called chromatophores, iridophores, and lecuophores. When muscles contract, these sacs widen and the colour is exposed. Chromatophores produce colours such as orange, yellow, red, brown, and black. Patches of these little cells are connected by nerves that can change in a split second. These cuttlefish have more chromatophores per square centimetre of their skin than any squid or octopus. Underneath the chromatophores lies another thin layer of colour-changing cells called iridophores, which reflect light at different wavelengths resulting in blues, greens, silvers, and pinks. Lecuophores are also light-reflecting cells, but these scatter light so that it appears white. This can provide the cuttlefish with a blank backdrop, helping to increase the intensity of its chromatophores.  Being able to imitate rocks, sand, or seaweed makes it challenging for predators to find the cuttlefish. In addition, the muscle tissue of cuttlefish is highly poisonous, as lethal as that of blue-ringed octopuses, which also helps protect them from predators. Its flashy colour displays can also be used to communicate with other cuttlefish, especially during mating season. Cuttlefish spend 95% of their time resting. Because of their short lifespan, only about two years, they put all their energy into growing and getting ready for mating. Cuttlefish are semelparous animals, which means they only reproduce once during their lifetime. Similar to the octopus, females lay their lemon-shaped eggs on the underside of rocks. Around 100 to 300 eggs are left with no one to guard or care for them. They hatch 3 to 5 months later. Soon after mating and egg-laying, the adults die. Watch an intriguing video about cuttlefish here. 
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