The giant Australian cuttlefish calls the southern waters of Australia home and they are often found in the shallow reefs along the coast. Weighing up to 10.5kg and reaching up to one metre in length, they live up to their ‘giant’ name.
Big Animals in Unexpected Places
It could be argued that the giant Australian cuttlefish doesn’t quite fit the mould of a ‘big animal’. Because after all, compared to true ocean giants like humpback whales or apex predators such as tiger and great white sharks, they may seem diminutive, insignificant even…
But I personally rate them as big animals because most encounters with large creatures occur as part of some form of annual migration or aggregation. And of all of those events I have experienced in the last ten years or so, the annual gathering of Australian giant cuttlefish in Whyalla is up there in the top five! What makes that event so extraordinary is the unparalleled intimacy it offers. Nowhere else I have been to is it possible to get so close to the animals and yet be effectively ignored as they get on with the business in hand – reproduction, and the continuity of the species!
The undisputed stars of this underwater drama are the mature bull males. These vibrant giants are at the apex of their brief lifecycle, but are on borrowed time and will be dead by the end of the season. Whether or not they know that is anyone’s guess, but when it comes to securing their share of the reproductive action, they sure act like they do!
Their interaction with the wanton females and the many young male challengers is simply amazing to observe and the fact this is all happening on such a grand scale, just off the beach to the north of Whyalla, is incredible!
Meet The Giant Australian Cuttlefish
Sepia apama is the world’s largest cuttlefish and can reach up to half a metre in length and weigh in at up to 11kg. Solitary animals, they are found all around the coastline of southern Australia. They are truly fascinating creatures with an amazing ability to instantaneously change their colour and skin texture.
A skill they use to great effect as camouflage when they are hunting or being hunted, to communicate with other cuttlefish and as part of the amazing displays during the mating season. Giant Australian cuttlefish are also remarkably intelligent and are said to have the largest brains of all marine invertebrates.
Life Cycle and Polymorphism Explained
Both male and female cuttlefish have relatively short life cycles of between one and two years and are polymorphic… Which means they have two alternate development cycles. With the first cycle using a ‘growth spurt’ over their initial seven to eight months to reach maturity by the end of their first summer and be ready to mate at the start of winter.
The second cycle involves much slower growth where they don’t reach maturity and mate until they are in their second and final year.
Outside of the winter mating season the ratio of males to females is around 1:1. Why it changes so dramatically when they do mate is not known. The correct term for cuttlefish mating is polyandry – which means that each female cuttlefish will have multiple male partners to ensure better genetic variability of the species. All of which makes sense from a somewhat dry scientific perspective. But when observed in practice with so many cuttlefish gathered together and the females outnumbered by as much 11 to 1, it takes on a completely different dynamic.
‘Spectacularly promiscuous’ would probably better describe the behaviour of the female cuttlefish!
Mating Behaviour: Bulls, Females and Sneakers
The basic strategy of the mature bulls is to impress the females with their size and ‘displays’. If it works, the bull will try and keep her out of site from all the other males, and when successful, makes the pair look like a bit like a happily married couple content in their own company.
But when other males do come calling, the bull has to judge the situation and respond in a way that intimidates the challenger and sends him on his way. That usually leads to both males engaging in quite spectacular ‘face-off ’ displays of aggression, with rapid changes of colour and body movements that are believed to send specific messages.
These bouts can go on for up to 20 minutes or so, but while they appear hostile and violent, there is rarely any real physical contact. It seems to be more like a cross between shadow boxing with a bit of mild wrestling, and I sometimes wonder if the real purpose is to impress the female!
Sneakers…
Probably the most-intriguing strategy is the one deployed by the young males – who have matured enough to indulge in the mating, but are too small to challenge the mature bulls.
Apart from the obvious differentiator of the bull’s size, it is almost impossible to identify a male cuttlefish from a female one – the cuttlefish themselves cannot tell the difference, and males display a subtle zebra pattern on their sides to signal their sex. So, the young males take advantage of their similar size to the females and use the Cephalopod equivalent of cross-dressing by changing their displays to look like a female. Often they deploy this strategy when the resident bull is distracted by a challenger and is too busy to do his normal ‘due diligence’…
Instead, he will shove the young male down into his little harem in the sea grass, intending to return as soon as the challenger has been dealt with. This is the young male’s chance to mate and usually the female will accommodate him – much to the annoyance of the bull male when he realizes what is happening.
Where and How to Dive with Them in Whyalla
The annual aggregation is concentrated along a 6km long stretch of the eastern Spencer Gulf, generally called Point Lowly. However, because over a third of that area is occupied by a gas treatment plant, the 3km between Black Point and Stony Point is where all the encounters take place.
The easiest access point is at Stony Point, which is usually referred to as the Fence Line because of the boundary fence to the gas plant.
The nearest town is Whyalla, which is a 25-minute drive to the south and has lots of options for food and accommodation. The local dive shop, Whyalla Diving Services, is very well equipped for tank rental and fills, plus anything else you might need. And the owner, Tony Bramley, has done a tremendous amount of work to raise the overall awareness of just how special the annual giant Australian cuttlefish aggregation is.
Don Silcock
Don is Scuba Diver’s Senior Travel Editor and is based from Bali in Indonesia. His website has extensive location guides, articles and images on some of the best diving locations in the Indo-Pacific region and ‘big animal’ experiences globally.
Frequently Asked Questions
Where can you see giant Australian cuttlefish in the wild?
The best place to see them is the Whyalla region of South Australia, where thousands gather each winter for their annual breeding aggregation along Point Lowly.
How big do giant Australian cuttlefish grow?
They can reach around one metre in length and weigh up to 10–11kg, making them the largest cuttlefish species on the planet.
Why is the Whyalla cuttlefish aggregation so famous?
It’s the only place in the world where cuttlefish gather in such numbers and allow divers extremely close, uninhibited encounters during their breeding season.
What behaviour can divers expect to see during the aggregation?
Expect dramatic colour changes, combat-style displays between rival males, mating attempts, and clever “sneaker male” tactics designed to outwit dominant bulls.
When is the best time to visit Whyalla for the cuttlefish?
Peak aggregation typically occurs in winter (May–August), when mature bulls, females, and young males converge on the shallow reefs to mate.
This article was originally published in Scuba Diver Magazine
Subscribe today with promo code DIVE1 — enjoy 12 months for just £1!
