Sepia apama, also known as the Australian giant cuttlefish, is the world's largest cuttlefish species, growing to 50 cm 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 colour 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.
Life cycle and reproduction
Sepia apama live from two to three years. Breeding takes place with the onset of the southern winter. Males, which outnumber females 11 to 1, abandon their normal cryptic colouring and set out to dazzle the females by adopting rapidly changing bright colours and striking patterns. Devious males mimic female colouring and form in order to gain access to females protected by dominant males which are extremely territorial. 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 underside of rocks where they will hatch within three to five months. Sepia apama are semelparous and death follows shortly after a single mating and laying of eggs that will spawn the next generation.
Upper Spencer Gulf population
Discovered by divers in the late 1990s, the upper Spencer Gulf population is the world's only known mass cuttlefish spawning aggregation and the Sepia apama congregate around Point Lowly near Whyalla between May and August. With densities of one cuttlefish per square metre, the sheer number of Sepia apama makes this breeding aggregation unique in the world. As the cuttlefish are oblivious to divers while spawning, they are now a major attraction for divers from around the world.
The Sepia apama upper Spencer Gulf population displays two alternative life cycles in both sexes (growth pattern polymorphism). The first involves rapid growth with maturity reached in seven to eight months with small adults returning to spawn in the first year. The second involves slow growth with maturity reached in two years with large adults returning to spawn in the second year.
Prior to the mid-1990s, the population was fished for snapper bait. Between 1995 and 1997 commercial fishing of the spawning grounds harvested around 450 tonnes leading to 50% of the grounds being closed to commercial fishing in 1998. In 2005 the closure was expanded to the entire spawning grounds.
In 2011 the cuttlefish failed to return to breed. Beginning in May, the cuttlefish leave deep water and migrate along coastal reefs to reach their spawning grounds. Local fishermen claim that a small "finger of land" near Point Lowly extends outside the exclusion zone and that commercial fishers have been targeting the area, intercepting the Sepia apama before they can reach the spawning grounds. Being semelparous breeders, ecologist Bronwyn Gillanders believes the cuttlefish to be in danger, stating that it is hard to determine whether this a natural phenomenon or something else and that the cause requires more research.
Effect of local industrialisation
In 1984, before the spawning grounds were discovered, Santos built a hydrocarbon processing plant at Port Bonython. The plant appears to have had no effect on cuttlefish spawning despite being located in the middle of the grounds. Santos now provides funding for cuttlefish research.
BHP Billiton has plans to build a desalination plant at Point Lowly to supply fresh water to Roxby Downs. The plant, located within 200 metres (660 ft) of the breeding grounds, will dump around 120 megalitres (32,000,000 USgal) of brine (46–60 ppt) into the area each day. As cuttlefish embryos die off as salinity levels rise (optimal range 28–38 ppt, 100% mortality at 50 ppt), there has been considerable public opposition to the proposed plant because of the expected environmental impact.
Due to its proximity to the ore deposits of the Middleback Ranges, several mining companies have indicated they want to build petrochemical and diesel refining facilities at Point Lowly. A second jetty for the loading of iron ore, and possibly copper and uranium, is also planned. The Cuttlefish Coast Coalition has been formed to fight these proposed developments.
As a result of the above threats to the population, in 2010 an application was made to list Sepia apama in the list of threatened species. On February 2, 2011, the Threatened Species Scientific Committee ruled the species was not eligible for listing as the affected population was not taxonomically distinct from the rest of Sepia apama for the purposes of the Act.
Physiology and biochemistry
Genetic studies have shown that there is little if any interbreeding between Sepia apama populations. While there is some genetic divergence, the various populations are not considered taxonomically distinct and are commonly referred to by their location, e.g. Sepia apama upper Spencer Gulf population.
Sepia apama is a neritic demersal species. 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 polarization 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 polarization of light. While the Mantis shrimp is the only known creature to have true polarization vision, it is believed that cephalopods may also. Because the optic lobes of cuttlefish are larger than any other region of the brain and their skin produces polarized reflective patterns, it has been postulated that they may communicate through this visual system. By raising elaborate papillae on their skin, S. apama can change the shape and the texture of their skin to imitate rock, sand or seaweed.
A recent energetics study found that Sepia apama are primarily diurnal and have a small home range (90–550 meters) 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% during the 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.
Role in ecosystem
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.
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- Silvery fish reflect polarised light; they reflect the same amount of light in the same direction as the light they are viewed against, making them almost invisible in water. If the polarisation is reduced, the fish become easily visible. Cuttlefish will ignore fish with reduced polarisation and preferentially attack fish emitting polarised light.
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