Scanning the austere coastline surrounding South Australia’s Spencer Gulf, you would have little reason to suspect that anything extraordinary ever happens in this place. A monotonous landscape of sand, rocks, and shrubs is interrupted only by the occasional gas refinery or lead smelter. The name of the area’s most prominent landmark, Point Lowly, practically promises obscurity. But dipping beneath the surface of the Gulf’s chilly mid-winter waters reveals a different world. Here, aggression, deception, and sex rule the day.
For reasons no one completely understands, Point Lowly has become the spot where the world’s largest cuttlefish—a type of free-swimming mollusk most closely related to octopuses and squids—comes to spawn en masse. In a patch of ocean roughly the size of 10 soccer fields, as many as a quarter of a million giant Australian cuttlefish (Sepia apama) emerge from their otherwise solitary existence and gather to compete for mates and find suitable places to lay their fertilized eggs before they die.
This underwater spectacle remained relatively unknown until the late 1990s. That’s when a few intrepid divers strapped on scuba masks and splashed into the 10 °C Gulf waters. What they discovered just offshore turned out to be the only known dense spawning aggregation of any species of cuttlefish anywhere in the world. Ever since, this annual phenomenon has provided priceless opportunities for scientists to study the behavior and biology—from mate selection and deception to color change and camouflage—of this otherwise elusive sea creature.
At Point Lowly, researchers needn’t venture far to find their study subjects. According to acclaimed underwater photographer Justin Gilligan, who captured these images, giant cuttlefish can be found spawning in water as shallow as knee deep, just steps away from the point’s historic lighthouse—and then as far as the eye can see. The sheer abundance of animals and activity makes for an interesting predicament, says Gilligan. Typically, his challenge as a wildlife photographer is finding interesting behaviors to capture. During the giant cuttlefish spawning aggregation, it’s the opposite. With so many remarkable maneuvers and displays happening all around, the hardest part is knowing which direction to aim his camera.
As overwhelming as this scene might be for an underwater photographer, it’s a pretty unusual circumstance for a cuttlefish, too. These creatures spend the vast majority of their short lives in isolation—camouflaged and hiding in crevasses or amidst patches of seaweed. Although they’re voracious and opportunistic predators, consuming nearly any fish or crustacean they can catch, giant Australian cuttlefish actually spend as much as 95 percent of their time resting. This enables them to channel most of their energy toward growth. And they grow fast. In just 12 to 18 months—the average lifespan of the species—a male cuttlefish may reach a length of 1 meter and a weight of 16 kilograms.
Although the giant cuttlefish is found only in Australia, its range is vast—extending along the continent’s entire southern coastline—and encompasses a wide variety of habitats, from coral and rocky reefs to seagrass beds and sandy and muddy seafloors. A key to the species’ adaptability is camouflage. Like many other cephalopods, the class of organisms that includes octopuses and squids, cuttlefish possess a remarkable capacity to vary the color, pattern, and texture of their skin—and to change these characteristics moment to moment, flawlessly matching their surroundings and blending in as they move across the seafloor.
Despite the cuttlefish’s ability to survive and thrive in a wide range of habitats, reproduction presents a particular set of challenges. First, there’s the task of finding a mate, always tricky for a solitary creature that spends most of its life hidden away. Second, there’s the need to find suitable conditions in which to lay eggs. The spawning aggregation at Spencer Gulf solves both of these problems. Here, the water is teeming with potential mates, and the habitat—different than elsewhere along the coast, with large, flat rocks scattered along the seafloor—provides nearly endless platforms under which females can tuck their eggs. Scientists think that cuttlefish embryos may also be adapted to the somewhat higher levels of salinity in the Gulf than those found in the open ocean.
With abundance comes competition. In the middle of a spawning aggregation, the task of locating a potential mate couldn’t be simpler. But attracting a mate is another story—particularly when surrounded by dozens of lookalikes (at least to our eyes). Male cuttlefish in Spencer Gulf experience this competitive pressure far more acutely than females. At any given time, males on average outnumber females 4 to 1, and sometimes by as much as 11 to 1. As a result, females can afford to be choosy—and they are. Studies show they reject up to 70 percent of all mating attempts.
To improve their chances of reproductive success, male giant cuttlefish compete with one another in a variety of ways. Size and might are two of the more obvious means of winning and keeping a mate. Large males physically guard females and nesting sites, and warn off would-be competitors with rhythmic, wave-like patterns of pulsating color generated by the same system that makes these animals masters of camouflage. If the warning goes unheeded, males escalate their defensive tactics, chasing off rivals, and, if necessary, engaging in physical battles waged with grasping tentacles and slashing beaks.
Brawn is not the only way for a cuttlefish to achieve reproductive success, however. Rather than attempting to beat dominant males at their own game, smaller males have found ways to sneak past their larger foes, either by slipping in when their competition is busy fending off other rivals, or by donning a clever disguise. In the world of the cuttlefish, what you see is not always what you get. By taking on the mottled skin pattern typical of females and hiding their fourth arm (an appendage that females lack), smaller males can slink in and copulate with a female under the watchful gaze of a guard. It’s a risky strategy, but one that can pay off handsomely in a smaller male’s ability to pass its genes on to the next generation. Scientists estimate that small, sub-dominant males are involved in more than a third of all successful mating attempts.
Decades after it was first discovered, Spencer Gulf’s reproductive spectacle continues to be a draw for many scientists and recreational divers, who flock to these otherwise unremarkable waters to observe and document one of the most astounding cephalopod conventions on Earth. But for those who know this place best and have witnessed the marked decline in cuttlefish numbers here since the late 1990s, fascination has turned to concern about the vulnerability of this unique population, and the need to better understand the threats it faces.
Some of these threats are easy to identify. It doesn’t take long to see that Spencer Gulf, which reaches deep into South Australia’s heartland, is anything but pristine wilderness. Industries of various types dot its shoreline. And with each new factory and mine comes an increase in shipping traffic, noise pollution, and the potential for toxic spills.
Then there is the spawning aggregation itself. For all its importance to the longevity of the giant Australian cuttlefish, concentrating so many breeding individuals in a single location—whether it’s surrounded by industry or not—is a risky proposition. One obvious threat is overfishing, and indeed, fishermen hauled cuttlefish out of Spencer Gulf by the hundreds of thousands in the late 1990s before restrictions were put in place. Then there are the hypothetical scenarios. One toxic spill, one natural disaster, at just the wrong time could be catastrophic. And these types of risks are only heightened by the cuttlefish’s short life span. The fact that individuals live less than two years means that there are simply fewer generations in reserve to repopulate after a decline.
Exactly which factors have been responsible for the most recent declines, no one knows for sure. What’s clear is that the breeding population here declined dramatically—from nearly 200,000 individuals in 1999 to fewer than 14,000 in 2013. Scientists point to such factors as industrial pollution, fishing pressure, natural predation, and global-warming-related increases in water temperature and CO2 levels as possible contributors. But they also point out that the recent cuttlefish declines may have been part of a natural cycle. One promising piece of news is that—since its low point four years ago—the population has begun to rebound, with numbers once again exceeding 100,000 in 2016.
For now, scientists remain hopeful that this fragile population will continue to thrive in Spencer Gulf, putting on a Technicolor display each year that would undoubtedly captivate anyone bold enough to brave the frigid water. “It’s as if their bodies are glowing, and this is happening all over the yellow seaweed-covered seafloor,” says Gilligan, as he describes the impact of looking out at dozens of giant cuttlefish, their bodies rapidly pulsating with waves of color. “It’s quite a dramatic sight.”
This story originally appeared in bioGraphic, an online magazine about nature and sustainability.
Steven Bedard is a science writer and editor-in-chief of bioGraphic.
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