Earth’s journey around the sun brings a host of new constellations into view with each change of season. During the northern hemisphere’s spring, Leo the lion and Cancer the crab stride and scuttle into view. Around that same time each year, millions of tiny squid crowd the shallows of Japan’s Toyama Bay and put on a light show of their own: undulating constellations of glowing blue stars that change shape with every fin-flutter.
The mollusks, known as firefly squid (Watasenia scintillans), spend the majority of their lives in deeper waters. But each spawning season, between April and late May, females rise to the surface to release their eggs. The squids’ twinkling displays have captured scientists’ attention since the early 20th century, and have helped researchers better understand the chemistry of bioluminescence. Locals are drawn to the show for another reason. They come for the tasty flesh of the hotaru-ika, as the mollusks are known in Japan, scooping them from the bay in abundance during the annual spawning bonanza.
Photographer Solvin Zankl came to capture the spectacle last year, during the brief season when the squids’ orbit intersects with our own. There are few other opportunities to glimpse these unusual creatures, especially in such numbers. For the rest of the year, firefly squid remain out of sight, leaving the details of their physiology, life history and behavior – not to mention the function of their stunning displays – shrouded in mystery.
While the lives of many firefly squid culminate in this dazzling spawning ritual, their origins are far less spectacular: long, gelatinous strings of eggs – each one half the size of a sesame seed – drifting along at the ocean’s surface.
After a week or two, depending on water temperature, the eggs hatch and miniature squid descend to habitats on the ocean floor 200m (650ft) or more beneath the surface. Because this chapter of the squid’s life takes place at great depth and in near-total darkness, scientists rely primarily on what fishers pull up in their nets to glean whatever insights they can about the animals’ activities below.
Researchers analyze stomach contents of the catch to see what creatures prey on firefly squid, as well as how squid diets change as the mollusks grow. From these analyses, they know that very young squid feed on minute, T-shaped crustaceans called copepods. As they grow larger, they add small fish and other squid to their diets. While the young squid pursue their own prey, they must steer clear of predators, which include salmon, sea-run trout and other fish that patrol these depths.
Firefly squid grow quickly but never very large. After just six months, those that have escaped the jaws of predators are fully grown at about 7.5cm (3in) in length. By February, the now-mature squid are ready to breed. Males attach packets of sperm to females, seeding the next generation, and then mysteriously disappear. Although scientists aren’t certain what happens to the males, their subsequent absence from fishing nets suggests that they simply die shortly after breeding.
The females still have a job to do – and if they’re lucky, a few more weeks of life in which to carry it out. Having each collected on average more than a dozen sperm packets over the course of the short breeding season, females rise en masse after sunset and deposit their fertilized eggs at the ocean’s surface. Shortly afterward, their exhausted bodies wash ashore. “That’s basically their last minutes of life,” Zankl says of the spawning frenzy he captured in these images.
Although it’s hard to miss the firefly squid’s profusion of lights, no one knows exactly why the animals glow.
“[An] ecological mystery is why firefly squid emit light”
“Most squids can live without bioluminescence,” says biochemist Katsunori Teranishi of Japan’s Mie University. Teranishi coauthored a 2008 paper on the chemistry of the mollusks’ light organs, called photophores, which are found in various sizes and configurations along their bodies and tentacles. “[An] ecological mystery is why firefly squid emit light,” he says.
One possible explanation is that the species uses bioluminescence to attract prey, Teranishi says. Another hypothesis is that the squid’s glow helps to obscure its outline for predators lurking below – a trait known as counter-illumination found in a number of ocean-dwelling creatures (bioGraphic’s stories “Hide and Glow Seek” and “Invisible Nature: The Glowing Squid” explain how the Hawaiian bobtail squid [Euprymna scolopes] uses light in this way).
A third hypothesis is that the firefly squid uses bioluminescence to secretly signal to rivals or potential mates. Although the light the species produces appears predominantly blue to our eyes, it is made up of longer, greener wavelengths than much of the ambient blue light that filters through from the surface. Because water absorbs longer wavelengths first, green is an unusual color at depth, and many marine creatures are poorly equipped to see it. Scientists think that because most other deep-sea animals are essentially blind to green light, firefly squid may be able to communicate with each other while remaining invisible to predators.
Regardless of the intended recipient, the firefly squid uses its entire body to send signals. Like all squid, the species is equipped with eight arms and two longer tentacles. At the tips of one pair of arms, it has three large photophores that flash with the brightest light, as if the squid is waving burning flares. Five smaller photophores surround each of the squid’s eyes, and hundreds more dot the length of its body.
Although the light display during a firefly squid spawning event is one of the more spectacular of its kind, bioluminescence is by no means uncommon. There are more than 3,500 bioluminescent lifeforms on Earth, and about 80 percent of those live in the ocean. Interestingly, these creatures have evolved a number of different ways to produce light. Even the few species of bioluminescent squid vary in how they achieve their glow. The Hawaiian bobtail squid, for example, houses light-producing symbiotic bacteria inside its tissues, while the deep-sea-dwelling odd bobtail squid (Heteroteuthis dispar) produces a luminous mucus that it spews in place of ordinary ink.
In contrast, the firefly squid generates light within its own cells, using the method most common among bioluminescent organisms. This process, common though it is, involves “a series of complicated chemical reactions,” says Ya-Jun Liu, a biochemist at Beijing Normal University in China whose team described the details of the reactions in 2016. The process hinges on molecules known as luciferins and luciferases. A luciferase is an enzyme that causes oxygen to react with a luciferin. This reaction creates a new, short-lived molecule that produces light. The color of the bioluminescence depends on the exact chemical structures of these molecules, which vary from species to species. That’s why some bioluminescent creatures glow red or orange and others glow blue-green.
Not surprisingly, the firefly squid’s complex life history, secretive nature and short lifespan have made studying the species’ bioluminescence and other aspects of its biology extremely challenging. (One researcher spent 20 years studying the squid’s bioluminescence, because he could only do so in bursts of three to four weeks each spring.) Because of these challenges, some mysteries still remain. For example, it’s still not known where the firefly squid’s luciferin comes from – whether the squid acquire it from animals in their diet or produce it internally. Researchers have also not been able to determine the exact structure of the squid’s final light-emitting molecule.
Despite the hurdles, Liu is undeterred. “Unraveling the mechanism of firefly squid bioluminescence is of paramount importance” for understanding animals with similar light-making chemistry, he says.
The last chapter of the squid’s life story is a harrowing one. Northern fur seals in particular have a taste for firefly squid, and feast on the mature females as they move into the shallows to spawn. When the squid arrive in Toyama Bay, they find humans waiting for them, too. Hotaru-ika is a delicacy for locals, and according to Kotaro Tsuchiya of the Tokyo University of Fisheries, the average annual haul here alone is about 2,000 tons, or 250 million squid.
To photograph the creatures before they ended up on a plate, Zankl staked out the beach at Toyama Bay every night for two weeks. In the end, he successfully photographed the squid on just four nights. That was, in part, because he was competing with local fishers who comb the beach regularly during the spawning season in search of the squid. On most nights when squid did come to shore, he had only an hour or so before word got out and fishers began showing up by the hundreds.
While the atmosphere was friendly, Zankl says, it was also competitive, the human frenzy creating another kind of spectacle. Families with kids waded close to the shore, carrying buckets to scoop squid out of the water. More committed fishers wore tall waders so they could venture farther out, pushing their buckets on floating rafts. Flashlights jostled in the dark, mirroring the flashing photophores of the squid. Farther offshore, fishing boats hauled up nets that looked as if they were filled with stars, as gulls swarmed around them.
As the sun rose on Toyama Bay the next morning, Zankl turned his attention to the female squid that had washed up on the sand. Their spent bodies were glowing no longer, but their eggs were already sailing away on ocean currents. Soon the chains of tiny beads would become the next generation of squid, their lights burning secretly in the deep.
This story originally appeared in bioGraphic, an online magazine about nature and sustainability powered by the California Academy of Sciences.