The giant cuttlefish’s technicolour mating display is globally unique. The SA algal bloom could kill them all

Every year off the South Australian coast, giant Australian cuttlefish come together in huge numbers to breed. They put on a technicolour display of blue, purple, green, red and gold, changing hues as they mate and lay eggs.

This dynamic, dreamlike display takes place in the upper Spencer Gulf, near Whyalla. This short strip of coastline is the only place in the world to host this spectacular event.

But South Australia’s killer algal bloom is advancing towards this natural wonder. If the algae reach the breeding site in the coming weeks or months, they could wipe the cuttlefish population out.

Now, scientists may have a chance to get there first, take some eggs and raise an insurance population in captivity. This rescue operation would be a world first.

Why are the cuttlefish so vulnerable?

The giant Australian cuttlefish congregate to mate in waters off Whyalla every winter, in a gathering known as a “breeding aggregation”. The sanctuary area received National Heritage status in 2023.

The displays of movement and colour take place as abundant males vie for the attention of a female. Each year it attracts tourists, photographers and marine life enthusiasts. To witness it, all you need is a thick wetsuit, mask and snorkel.

Cuttlefish are cephalopods, alongside octopus and squid. While cephalopods are adaptable to environmental change, their generations don’t overlap. This means the parents die before the offspring are born, and so the population cannot be replenished by the parents if the offspring are wiped out.

By now, in upper Spencer Gulf, most adult cuttlefish will be breeding and naturally dying off, leaving the eggs behind. They will incubate for about three months, then hatch and swim away.

What if the algal bloom reaches the cuttlefish?

The harmful microalgal bloom of Karenia mikimotoi first appeared in March this year on two surf beaches outside Gulf St Vincent, about an hour south of Adelaide. It is thought to have been triggered by a persistent marine heatwave coupled with prolonged calm weather, and possibly excess nutrients from the 2022–23 Murray River flooding event.

It has since spread to many corners of South Australia, and has now reached the lower to middle reaches of Spencer Gulf. Preliminary modelling revealed last week shows the bloom could spread through Spencer Gulf, up to Whyalla and across to Port Pirie.

The disaster has already affected about 400 types of fish and marine animals. And we know this algal species can rapidly dispatch cephalopods, both large and small. In other parts of South Australia already affected by the algal bloom, dead octopus and cuttlefish have been extensively photographed and recorded.

If the latest batch of eggs dies in the algal bloom, their parents will no longer be around to rebreed and restore the population next year. This means the population could go extinct.

Could we lose a species?

More than 100 cuttlefish species exist worldwide. The giant Australian cuttlefish is found throughout southern Australia, from Moreton Bay in Queensland to Ningaloo Reef in Western Australia.

However, the breeding aggregation is genetically distinct from even its closest cuttlefish neighbours in southern Spencer Gulf, about 200 kilometres away. And genetic evidence suggests the upper Spencer Gulf population could well be its own species, although scientists haven’t confirmed this yet.

Regardless, this cuttlefish population is truly unique. It is the only population of giant Australian cuttlefish, and the only population of cuttlefish worldwide, to breed en masse in such a spectacular fashion.

That’s why saving it from the algal bloom is so important.

Can we save this natural wonder?

Today I’ll be meeting with fellow marine and cephalopod experts at an emergency meeting convened by the South Australian government. There, we will discuss the feasibility of collecting an insurance population of eggs from the cuttlefish population.

Timing is everything. Two or three months from now, the eggs could be too developed to collect safely, because moving can trigger premature hatching. Even later, the eggs will have hatched and the hatchlings will have swum away.

Ironically, while the mass gathering of cuttlefish makes the species vulnerable to a permanent wipeout, it also makes them easier to rescue.

Collecting, transporting and raising eggs in tanks is a relatively straightforward process at a smaller scale. It has been done successfully for research purposes in South Australia.

Raising hatchlings is harder and more labour intensive. Then there is the question of what to do with them once they hatch. But the three-month incubation period would buy us time.