Cuttlebones are among the most common sights on the beaches of southern Australia. Yet despite their distinctive shape and extraordinary abundance, they remain an enigma to most beach-goers. Why are they there? Where do they come from? Parents often suggest to their children that the streamlined white shapes are part of an animal that sheds them in differing sizes during their life (they aren’t) and that they’re useful for budgies and parrots to work their beaks on (they are). Beyond that, we take them as part of the coastal landscape. But researchers in Adelaide are revealing strange and wonderful details of the lives of Giant Australian Cuttlefish (Sepia apama) that might cause us all to rethink the fragility of these reclusive creatures.
Professor Steve Donnellan is an evolutionary biologist at the Museum of South Australia. In 1997, he was among an international group of researchers who began studying the rise in commercial fishing at the Giant Australian Cuttlefish breeding site off Whyalla in the Spencer Gulf. Sepia apama occur right across southern Australia’s coastline. They are the world’s largest cuttlefish (growing a mantle of up to fifty centimetres and a total body length of a metre) and Whyalla is the world’s best place to study them. The breeding aggregation at Whyalla is massive: they congregate in a density of up to one animal per square metre.
There has, for many years, been a very low-level commercial take of South Australian cuttlefish as bait for snapper. But the cuttlefish are an easy catch due to their concentration in the water: as the fishery rapidly gained pace, Primary Industries and Regions SA (PIRSA) imposed a moratorium on the take until they could figure out the cuttlefish’s population dynamics. The issue for the researchers was whether the cuttlefish at Whyalla are part of an Australia-wide population, or a distinct, localised grouping. The answer to that question would determine whether they could be protected under the federal Environment Protection and Biodiversity Conservation Act.
The answer, discovered by Steve Donnellan’s team about seven years ago, is that they are a distinct grouping, and furthermore, the local population in the Upper Spencer Gulf might be a separate species of its own. The Spencer Gulf is an ‘inverse estuary,’ he told GOQ: normally the fresher water’s upstream, but the Spencer Gulf is saltier as you go up. ‘So possibly this population’s adapted to high salinity.’
Once these things were known, a permanent ban was imposed on all harvesting of the breeding aggregation. Despite that, numbers at the breeding site have dropped dramatically in recent years, and nobody seems to know why.
Researchers have spent many hours underwater with the cuttlefish during the mating aggregation. Their voyeurism has uncovered a startling variety of mating strategies: in perhaps the best example, small males that can’t compete for a mate against larger males ‘cross-dress’ their camouflage to resemble females. The large male that has paired up with a female allows this extra ‘female’ to get quite close. When he is distracted, the cross-dressing male unmasks himself ‘ aha! ‘ mates with the female, and quickly swims away from the unsuspecting large male without a potentially deadly confrontation. The multiple approaches to mating, along with their remarkable talents for camouflage, might be responses to the extraordinary numbers of cuttlefish at the breeding aggregation. Giant Australian Cuttlefish live for less than a year, and only get to breed once. Giant squid, by comparison, live for more than a decade, but all other species of cephalopod are very short-lived. So the whole population’s investment in eggs lies exposed on the reef at the Whyalla breeding site at the one time.
The female cuttlefish need rocky reefs to lay their eggs on. They aggregate in the winter months, when sexually mature, to lay them on the undersides of rocky surfaces. They don’t guard the eggs, which can be destroyed by urchins moving over the surface and dislodging them. The hatchlings that survive are the size of a human thumbnail, and are fully-formed, miniature adults ‘ cuttlefish have no larval phase. Dolphins prey on the adults at the aggregation site when they are senescing (that is, after mating but before death), although this is unlikely to be a population pressure, as the business of mating is done by then.
But habitat loss is another thing: there has been a protection zone in place around the breeding area since 1998, although some cuttlefish live and breed outside the zone. More recently, three proposed developments have arisen with the potential to affect the egg-laying sites at Whyalla: a fertiliser and chemical factory (which has been withdrawn); a proposed desalination plant (currently on hold) that BHP Billiton want to use to service their Roxby Downs mine site; and an ore-loading jetty for Cape-class ore vessels that would cross the egg-laying area. The South Australian EPA has studied the effects of conveyor-belt dust emissions, asking how they might affect water clarity and visibility, given cuttlefish are highly visual in their mating behaviours. The current view is that the dust might not
These concerns are vital from multiple perspectives. The Whyalla cuttlefish population has become an important source of eco-tourism in South Australia. The breeding activity occurs in water as shallow as one metre. As Professor Donnellan says, ‘You can flop in off the rocky shoreline with a mask and snorkel and be surrounded by breeding pairs’. World-renowned cephalopod experts come to Whyalla to study: ‘It’s so much easier for them,’ according to Steve, ‘because they’re scarcer in other places. The Spencer Gulf is the only known mass breeding aggregation in the world.’
Professor Bronwyn Gillanders is an aquatic ecologist, working at the School of Earth and Environmental Sciences at the University of Adelaide. She’s been responsible for a tagging program that’s provided further insights into the Whyalla cuttlefish aggregation. ‘We’ve tagged animals on the breeding aggregation, and monitored them via signals emitted,’ she told GOQ. ‘We get a tag and put it up through the mantle, just for a short time. It emits a signal, and receivers are set up offshore, tracking them.’
The research effort is hampered by a lack of comparative data. ‘We’ve only got good estimates of abundance back to the late 1980s,’ she says, ‘so we don’t know if the current numbers are a peak or a trough over the long term. There’s some limited data from newspapers back around 1910, but that’s it.’
What’s known, she says, is that ‘the key area where they breed is limited to the Whyalla Peninsula at Point Lowly and a bit north of there. And they’re temporally limited between late April and August or September.’ Because of their declining abundance at the aggregation,there’s a whole-of-government working group, including Bronwyn Gillanders, devoted to watching the population.
The University of Adelaide tagging program revealed that the males are on the breeding aggregation longer than females, which explains the highly male-skewed sex ratio. According to Bronwyn Gillanders, ‘they breed, then they die.’ That’s why beach-goers see huge numbers of cuttlebones on the shoreline, particularly in spring. The white flecks along our surf beaches are the only physical remainder of an entire generation of the species. There is, as Gillanders points out, ‘no storage in the population. Each generation depends critically on the previous one.’
STORY: JOCK SERONG. PHOTOGRAPHY: PAUL MACDONALD