Deep below the ocean—3,000 feet beneath the surface, where sunlight cannot seep—the sea is smothered in a cold, inky darkness. But a bobbing bright light sometimes breaks through the abyss. The glimmering shades of blue and green mesmerize curious fish and squid, tempting them closer. They likely think that they’ve stumbled upon a prized floating morsel in a relatively resource-deprived habitat. Little do they know that they’re the ones about to become a meal. The last thing they’ll see is a spiny, globe-shaped shadow emerging from behind the glowing orb—then they’ll be ensnared in the toothy maw of an anglerfish.
Researchers have known for some time that this dazzling, deadly display is a typical feeding tactic of all anglerfish species, but a study on the rare Pacific footballfish sheds new light on how they get their sparkle.
According to results published in the Journal of Fish Biology, this particular species doesn’t just emit a glow: it also converts its shining colors into “an amazing disco ball of light,” says study author Todd Clardy, the ichthyology collections manager at the Natural History Museum of Los Angeles County (NHM).
“There’s a lot of different anglerfishes, and they all look pretty different and come in different sizes,” says Bill Ludt, a coauthor on the paper and the museum’s ichthyology curator. That level of variety extends to the glowing lure they use to attract mates and prey.
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Some anglerfish species have very simple lures—which are also known as escas—with just one little dot of light at the tip. Others have longer, more complex lures, or even multiple glowing, dangling appendages. But each shares the same source of illumination: bioluminescent bacteria. These microbes, which can be found in many sea creatures, such as the Hawaiian bobtail squid, can be used to draw in prey, create camouflage, distract predators, and communicate with members of the same species. In anglerfish, these photobacteria live within the fleshy esca and act like little light generators, producing a soft, electric blue hue. It is the only source of light found below 650 feet in the ocean.
But as the new study demonstrates, the Pacific footballfish (Himantolophus sagamius) uses an additional trick to create its light shows: Biofluorescence. While the two qualities can be often confused, bioluminescent organisms produce light, while organisms with biofluorescence are able to change light. “You could think of it as a manipulation—the light comes in one color [or wavelength] and then gets emitted in a different color,” says Ludt.
“Biofluorescence is pretty widespread actually in shallow water marine environments, but the fact that we’re seeing this in the deep sea where there is no ambient light makes this really fascinating,” he adds.
In the absence of sunlight, the study authors suggest, the anglerfish has instead evolved the ability to harness and transform the light of its shining microbes. Fluorescence adds extra flair to their flashy trap.
The new insights stem from Ludt and Clardy’s examination of a female Pacific footballfish that washed up to shore in Newport Beach, California and went viral on social media in May 2021. Nicknamed “Spiny Babycakes” on Twitter, the specimen was about a foot long and probably lived between 1,000 and 4,000 feet deep. Only females grow to this size, as most males of anglerfish species are small, and often even latch on to females parasitically. It’s a rare find, with only around 30 other anglerfish specimens in collections worldwide, says Ludt. The team received the footballfish frozen and thawed her, with plans to inject the precious sample with formalin for preservation. But Clardy pumped the brakes—why not examine her in fluorescent light first?
To their surprise, blue wavelengths of light revealed a green glow on the fish’s lure. Upon closer examination, they found small, biofluorescent crystalline structures on the illicium, or the stalk that holds the lure—where there isn’t any bioluminescent bacteria. Ludt and Clardy reason that the bioluminescent lure provides the necessary light source for the crystal structures to fluoresce.
“Already this species has a very complicated, elaborate display that it can create with its lure, and this fluorescence adds on top of it,” says Ludt. The hanging esca bulb is noticeably larger than some other species, and is flanked by numerous tentacle-like appendages that each have a silvery, light-emitting tip. While the researchers can’t determine exactly what its function is until footballfish are observed in the wild, they suspect that this multi-color array could act as a way to attract mates or confuse prey.
“I mean, imagine being in a pitch black cave or something and all you see in the distance is little dapples of light, like bright blues here and some tiny shades of green there,” Ludt says. “It just complicates the optical display overall for anything that’s viewing underwater.”
There are a handful of other deep sea creatures known to use this combo of biofluorescence and bioluminescence, including a crystal jellyfish and some siphonophores. One bottom-dweller called the stoplight loosejaw dragonfish uses biofluorescence to alter bioluminescent blue light into red, which acts like a flashlight that most of its prey are physically unable to see.
The new findings suggest that biofluorescence might be more common among species of anglerfish and other deep sea organisms than previously thought.
“This was surprising to me, particularly because biofluorescence requires a light source, and light is not something we typically attribute to the deep sea,” says Christopher Martinez, a fish biologist and an assistant professor in the department of ecology and evolution at UC Irvine. “In a region of the ocean with so many ‘strange looking’ organisms, anglerfishes stand out in their wildly divergent and extreme morphologies. Discoveries like this one are great because they bring to light new dimensions of diversity along with new ways to think about how organisms in the deep sea survive and evolve.”
“We are just starting to learn about biofluorescence in fishes and its possible functions,” says Martinez. “However, the truth is that we really don’t know how biofluorescence is used in fishes, and that is exactly why it’s so exciting.”
Ludt and Clardy are working with colleagues at NHM to survey the diversity of fishes in the museum’s collection. This latest finding highlights the elaborate—and extreme—adaptations that creatures have evolved in order to dwell in the murky depths.
“The deep sea is not a very hospitable place—it’s cold, dark, there’s a lot of pressure,” Ludt says. “I think this [study] just really showcases how complicated these adaptations are, and how remarkable they are as well.”