A Pacific footballfish, a rare and unusual species with a globular body, fang-like teeth, and a long filament extending from its head with a bulbous, lighted tip, washed up on a San Diego beach in early 2023. People took pictures of it and shared them online, and the response was predictable: fascination combined with something that deep-sea fish sometimes create but is difficult to pinpoint. The animal resembles something put together from disparate parts rather than a fish since it is designed for a setting so dissimilar from the surface world. Most people initially spotted the light at the end of its lure, which had been blazing in the deep water hours before the animal showed up dead.
Despite being one of the most researched light-producing systems in marine biology, anglerfish bioluminescence is still one of the least understood. The same fundamental luciferin-luciferase chemical reaction that powers fireflies, deep-sea jellyfish, and numerous other bioluminescent organisms is used by symbiotic bacteria, microorganisms that reside inside the esca, the fleshy tip of the anglerfish’s modified dorsal fin ray, to produce light. The light is not produced by the anglerfish. It cultivates the bacteria that do, giving them a safe haven and managing the organ’s blood flow to manage the glow. In turn, the bacteria are the only source of light in an area that is never exposed to sunlight.
Understanding the evolutionary process that led to this arrangement and the true function of bioluminescence in the deep sea’s ecological setting is what marine researchers have been focusing on more recently. The symbiosis appears to have been established multiple times independently in different anglerfish lineages, according to a 2022 study published in Nature that mapped the relationship between anglerfish species and their bioluminescent bacterial symbionts over roughly 200 million years of evolutionary history. This is a remarkable example of evolutionary convergence driven by the same environmental pressure. A lure was needed for the deep ocean. Light was necessary for the lure. Bacteria were necessary for the light. This answer was repeatedly discovered via evolution.
The biofluorescence dimension is relatively less well-characterized and more recent. Over the past ten years, biofluorescence—the absorption of light at one wavelength and its re-emission at a different, longer wavelength—has been reported in a number of deep-sea fish species, including several relatives of anglerfish. These animals emit green or red fluorescent patterns when exposed to blue light, which are undetectable in white light.
Theoretically, other species with suitable visual receptors could see these fluorescent patterns in the deep ocean, where blue light predominates in the surrounding environment. This would result in a signaling system that functions in a different spectral range from the bioluminescent esca. Research is ongoing to determine whether it is the true situation or if the fluorescence is a byproduct of skin chemistry with other main purposes.
The public’s perception of anglerfish, which leans toward horror-adjacent interest due in large part to the female’s striking physical appearance and the much smaller male’s parasitic mating method, differs significantly from what the biology actually entails. The lure is more than just a stick with a light on it. Luring prey in complete darkness is a behavior that neither organism could accomplish on its own. It is the result of millions of years of co-evolution between two very different living forms. Research is currently ongoing to determine the control systems that enable the anglerfish to alter the brightness and possibly the pattern of the lure. Another unanswered concern is whether the esca’s light is adjusted to the sensitivity range of the prey species it is intended to attract.
