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Deep-sea anglerfish illuminate their pitch-black home using a lure that contains a colony of bioluminescent symbiotic bacteria, but now researchers have discovered that one anglerfish species creates a glow in a second, unexpected way.

The Pacific footballfish (Himantolophus sagamius) can also biofluoresce, which means it absorbs one wavelength of light and reflects it as a different colour. It is an ability seldom documented in the ocean鈥檚 inky depths, and has never been seen in anglerfish before.

In May 2021, a dead female footballfish washed up on the shore of Crystal Cove State Park in California. Park staff transferred the fish to the Natural History Museum of Los Angeles County, much to the excitement of , an ichthyologist at the museum.

鈥淭his is an extremely rare species,鈥� says Ludt. 鈥淭here鈥檚 only around 30 or so large adult females in museum collections worldwide.鈥�

Photos of the anglerfish were circulated in news coverage and across social media, thanks to widespread captivation with the animal鈥檚 strange, spherical proportions and thorny, black skin, so Ludt could already see the specimen was in 鈥渋mmaculate condition鈥�, he says.

Ludt and his colleagues had been assessing other fishes鈥� relative biofluorescent capabilities. They wondered if this anglerfish might reflect light under certain wavelengths too.

鈥淚t was a long shot,鈥� says Ludt. 鈥淔or something to fluoresce, it usually needs a source of light, and this is a deep-sea fish that lives in an area of the ocean that has no natural light.鈥�

When the researchers put the anglerfish under a particular shade of blue light, the front of the lure above the fish鈥檚 head glowed with faint, green speckles, indicating it was fluorescing.

The colour of light emitted by the Pacific footballfish鈥檚 bioluminescent lure has never been documented. However, if it is in the blue range 鈥� as is common in deep-sea fishes 鈥� this species might use light from its own lure to cause the same organ to fluoresce a green colour.

The delicate spattering of green may be a way to signal would-be mates in the intense darkness. But Ludt thinks it is more likely that the biofluorescence helps enhance the lure鈥檚 attractiveness, encouraging prey to swim near the fish鈥檚 fanged maw.

鈥淚t is an interesting and exciting finding in a rarely collected group of fishes,鈥� says at the University of Kansas in Lawrence.

Smith says that valuable future insights could come from living footballfish, and that it would be interesting to investigate the origins of the lure鈥檚 fluorescent properties.

There are only a handful of other deep-sea animals known to both bioluminesce and biofluoresce, including a jellyfish and the . But biofluorescence may be more widespread than long assumed, says Ludt. He suggests that researchers look for fluorescence more broadly in deep-sea animals, especially in those that produce bioluminescent light.

For Ludt, the discovery highlights the importance of museums in the collection of animals and plants and in the study of natural history. 鈥淥ur planet is changing so fast, and yet there are still so many species out there that we know almost nothing about,鈥� he says.

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