The giant cuttlefish are cephalopods, which are very advanced molluscs. A highly intelligent species the giant cuttlefish is a master of disguise, deception and distraction.
The giant cuttlefish is the most abundant and the largest local species of cuttlefish growing to a maximum length of 1.5 metres and weighing up to 15 kilograms, with males tending to be larger than their female counterparts. They have a broad, flat, oval shaped body with a transparent fin extending around the perimeter and have three skin folds behind the eye. Emerging from the head, the giant cuttlefish has eight sucker-lined prehensile arms, and two extendable tentacles used for prey capture and mating in males. Cuttlefish have a highly developed central nervous system and complex eyes, which focus by changing the shape of the eyeball. Light entering the eye is controlled by the shape of the eye lid. Similarly to other cuttlefish and octopi, the giant cuttlefish can change the texture and colour of their skin in an instant. Specialised skin cells known as chromatophores, create colour pigment in the skin with shades of yellow, red, black and brown, iridophores create an iridescent blue or green sheen in the skin and leucophores produce whiteness in their body patterns. The simultaneous contraction of these cells by the nervous system allows for effective communication and camouflage. Different patterns can be displayed on either side of the body, one designed to lure a potential mate, the other to put off a male competitor. Skin texture changes are common, particularly when threatened. Skin folds, known as papillae, contract to become erect making the cuttlefish appear larger and more aggressive.
The giant cuttlefish possess a broad, internal shell known as cuttlebone beneath their fleshy mantle. Fine chambers within the cuttlebone contain both fluid and gas which can be regulated by the animal to achieve neutral buoyancy at various depths in the water column. When washed ashore cuttlebone can be used to identify cuttlefish species. Giant cuttlefish have the capacity to move extremely fast when threatened, by filling the muscular mantle with water and expelling it through the funnel creating jet propulsion. Like all members of the squid family they use jets of ink to confuse potential predators. Giant cuttlefish are carnivorous feeding on a varied diet of crustaceans and small fish. They are regarded as ambush predators where they shoot out their two tentacles at speed to capture prey. Their sharp internal parrot-like beak kills and cuts up prey, while their rasping teeth (radula), characteristic of all molluscs, grind it up for digestion.
At the onset of winter, male giant cuttlefish duel with other males displaying rapidly changing bright colours, striking pulsing patterns and body posturing, some even mimicking females colouration, for the opportunity to mate. Mating takes place when the male places his spermatophore (packet of sperm) in a pouch beneath the female’s mouth. Once the eggs are fertilised, the female will lay approximately 200 long, white sausage-shaped eggs which she wedges securely into reef crevices or attaches to seagrass. Miniature cuttlefish hatch several months later and immediately hunt for small prey. Adult giant cuttlefish die after a single reproductive season, however they live for two to three years, longer than any other species of cuttlefish, which is perhaps why they grow to such an immense size.
Generally solitary, the giant cuttlefish inhabit rocky reef and seagrass areas to 50 metres depth where they spend up to 95% of their time resting and hiding in crevices from predators. They are native to the southern coast of Australia occurring from Shark Bay, WA to Brisbane, Qld and around Tasmania.
Occurrence at the Busselton Jetty:
The giant cuttlefish is the most common cuttlefish in our temperate waters. They are common under the jetty, and often reside in permanent lairs among the fallen piles, occasionally even seeking protection from the underwater observatory sidling up alongside the windows. Many behaviours have been observed from the observatory, such as releasing a cloud of ink when bumping into the glass, increasing the body’s surface area by inflating papillae and raising arms in an attack position, active feeding, amazing vivid colour and body pattern changes and one individual in particular resting in the same lair everyday over a period of several weeks.
Image by: S. Teede