Sepia apama

Sepia apama, also known as the giant cuttlefish and Australian giant cuttlefish, is the world's largest cuttlefish species, growing to 50 cm (20 in) in mantle length and over 10.5 kg (23 lb) in weight. Using cells known as chromatophores, the cuttlefish can put on spectacular displays, changing color in an instant. S. apama is native to the southern coast of Australia, from Brisbane in Queensland to Shark Bay in Western Australia. It occurs on rocky reefs, seagrass beds, and sand and mud seafloor to a depth of 100 m. S. apama live 1–2 years. Breeding takes place with the onset of the southern winter. Males abandon their normal cryptic coloring and set out to dazzle the females by adopting rapidly changing bright colours and striking patterns. Females are polyandrous, and collaborative research indicates the tendency for females to reproduce using male genetic material deposited in spermatangia more favorably than in sperm receptacles directly. Females then attach their eggs to the undersides of rocks in caves or crevices, where they hatch within three to five months. S. apama is semelparous, and death follows shortly after a single mating cycle and laying of eggs that will spawn the next generation. S. apama has poor anaerobic capability compared to most aquatic invertebrates and a lack of food leads to catabolism. Stomach-content analysis indicates fasting during the breeding season, and as S. apama can catabolise no more than 50% of its body weight, it slowly loses physical condition as the season progresses and eventually dies. Throughout their range, these cephalopods breed in pairs or small groups, laying eggs in suitable caves or rock crevices. Loose spawning aggregations can form, but rarely exceed 10 animals in any one location, with one known exception: hundreds of thousands aggregate along rockey reefs between Whyalla and Point Lowly in the Upper Spencer Gulf. While surveys suggest that juveniles leave these spawning grounds after hatching, nothing is known of their subsequent movement or lifestyle strategies as a juvenile. Adults return to the aggregation site the following winter, or delay their return by an additional year. Genetic studies have shown that little if any interbreeding occurs between S. apama populations. While some genetic divergence is seen, the various populations are not considered taxonomically distinct and are commonly referred to by their location, e.g. Sepia apama upper Spencer Gulf population. The upper Spencer Gulf population is unique in that a permanent salinity gradient in the Spencer Gulf may physiologically exclude other populations from the zone occupied by the upper Spencer Gulf population. The upper Spencer Gulf population may in fact be a separate species, as it does show some hallmarks, such as genetic separation, differences in morphology, and different patterns of sexual dimorphism from adjacent populations. S. apama is a neritic demersal species. They are carnivorous, opportunistic and voracious predators who feed predominantly on crustaceans and fish. Using neurally controlled cells known as chromatophore organs (red to yellow), iridophores (iridescent: spans the entire visible spectrum from blue to near-IR) and leucophores (white), the cuttlefish can put on spectacular displays, changing colour and patterns in a fraction of a second. Located in three layers under the skin, leucophores make up the bottom layer, with chromatophores the outermost. By selective blocking, the three layers work together to produce polarised patterns. Unlike those in most animals, cuttlefish iridophores are physiologically active; they can change their reflectivity, and the degree of polarisation can also be controlled. Cuttlefish are colourblind; however, the photoreceptors of cuttlefish eyes are arranged in a way which gives them the ability to see the linear polarisation of light. While the mantis shrimp is the only known creature to have true polarisation vision, cephalopods may also. Because the optic lobes of cuttlefish are larger than any other region of the brain and their skin produces polarised reflective patterns, they may communicate through this visual system. By raising elaborate papillae on their skin, S. apama squid can change the shape and the texture of their skin to imitate rock, sand, or seaweed. A bioenergetics study found that S. apama is primarily diurnal and has a small home range (90–550 m or 300–1,800 ft) over short recording periods while travelling large distances to breed. They are able to channel most of their energy directly into growth because they spend 95% of the day resting, suggesting bioenergetics more like that of an octopus than a squid. Very little time is spent foraging (3.7% during the day and 2.1% at night); most of their time is spent resting and hiding in crevices from predators. The exception to this behavioral routine is the mass spawning aggregation, where cuttlefish are far more active during the days or weeks that they spend there. The Australian giant cuttlefish is eaten by Indo-Pacific bottlenose dolphins, which have been observed (in South Australia's Spencer Gulf) to have developed a technique for removing the ink and cuttlebone from a cuttlefish before eating it. They are also eaten by New Zealand fur seals. Discovered by divers in the late 1990s, the upper Spencer Gulf population is the world's only known mass cuttlefish spawning aggregation, with hundreds of thousands of S. apama squid congregating on subtidal reefs around Point Lowly near Whyalla between May and August. While outside of the breeding season, the sex ratio is one to one, Spencer Gulf males outnumber females by up to 11 to one in the spawning aggregation. If this is due to fewer females taking part or to males breeding for a longer period of time than females is not known. With densities of one cuttlefish per square metre, covering about 61 hectares (150 acres), the sheer numbers of S. apama make this breeding aggregation unique in the world. As the cuttlefish are oblivious to divers while spawning, they are now a major regional tourist attraction for divers from around the world. Professor Roger Hanlon of the Woods Hole Oceanographic Institution has called the breeding aggregation 'the premier marine attraction on the planet.'