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.^[1]

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.^[2]

Life cycle and reproduction

Sepia apama from Fairy Bower, New South Wales

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.^[3]

Upper Spencer Gulf population

Sepia apama from Whyalla, South Australia

The upper Spencer Gulf population is the world's only known large cuttlefish spawning aggregation and the Sepia apama congregate 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 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.^[3]

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.^[3]

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.^[4]

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.^[3]

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.^[5][6]

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.^[7]

See also

• Cephalopod size

References

• Kassahn, K.S., S.C. Donnellan, A.J. Fowler, K.C. Hall, M. Adams & P.W. Shaw 2003. Molecular and morphological analyses of the cuttlefish Sepia apama indicate a complex population structure. Marine Biology 143(5): 947–962. doi:10.1007/s00227-003-1141-5