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Surprising History of Glowing Fish

Black Dragonfish (Idiacanthus).
Black Dragonfish (Idiacanthus).
Photograph by Matt Davis

During the Cretaceous period, while flowers and tyrant dinosaurs were spreading over the land, and pterosaurs and birds were taking over the skies, in the oceans, fish were starting to glow.

Today, some 1,500 fish species are bioluminescent—able to make their own light. They have luminous fishing lures coming out of their heads, glowing stripes on their flanks, bright goatees dangling from their chins, flashing headlamps beneath their eyes, or radiant bellies that cancel out their silhouettes to predators watching from below.

They evolved their glow in a variety of ways. Some came to generate it on their own, through chemical reactions within their own cells. Others formed partnerships with luminous bacteria, developing organs for housing these microscopic beacons.

Despite the obvious diversity of bioluminescent fish, no one knew exactly how often these animals evolved their self-made light. “We thought it might be a dozen or so just by eyeballing a list,” says Matthew Davis from St. Cloud State University, “but the actual number was considerably higher.”

Together with John Sparks and Leo Smith, Davis built a family tree of ray-finned fish—the group that includes some 99 percent of fish species. By marking out the bioluminescent lineages, they report Wednesday in the journal PLOS ONE that these animals independently evolved their own light at least 27 times.

Illuminated Netdevil (Linophryne arborifera). Credit: Leo Smith
Illuminated Netdevil (Linophryne arborifera). Credit: Leo Smith

Of those 27 origins, 17 involve partnerships with glowing bacteria, which the fish took up from the surrounding water. Deep-sea anglerfishes housed the microbes in their back fins, which they transformed into complex lures. Ponyfishes kept the microbes in their throats, and controlled the light they produced by evolving muscular shutters and translucent windows.

But to Steven Haddock from the Monterery Bay Aquarium Research Institute, these partnerships are fundamentally different from cases where fish evolved their own intrinsic light. “If one species of bacteria evolves the ability to glow, then is eaten and proliferates in the guts of four different fishes, you could argue that bioluminescence evolved once in the bacterium,” he says. “To me, this is much less interesting than fishes that have their own chemical and genetic machinery.”

Those intrinsic light-producers have come to dominate the open oceans. There are around 420 species of dragonfishes, most of which have long bodies and nightmarish faces armed with sharp teeth. They include the bristlemouth, the most common back-boned animal on the planet; hundred of trillions of them lurk in the deep ocean. The lanternfishes are similarly prolific; the 250 or so species account for around 65 percent of the fish in the deep sea by weight. “They’re among the most abundant vertebrates on the planet in terms of mass, but the average person doesn’t know anything about them,” says Davis.

Silver Hatchetfish (Argyropelecus). Credit: Leo Smith
Silver Hatchetfish (Argyropelecus). Credit: Leo Smith

These groups aren’t just diverse, but unexpectedly so. In the relatively short time they’ve been around, they’ve accumulated far more species than is normal, and far more so than lineages that got together with glowing bacteria. Why?

Davis thinks it’s because they can exert greater control over their light. While ponyfishes have to rely on body parts that obscure the continuous glow of their microbes, lanternfishes and dragonfishes can turn their glows on or off, using nerves that feed into their light organs. That means they can flash and pulse. They can use their light not just to lure prey or hide from predators, but to communicate with each other.

Many scientists think that deep-sea fish could use bioluminescence as badges of identity, allowing them to recognises others of their own kind and to mate with partners of the right species. This might also explain why these fish became so extraordinarily diverse, in an open world with no obvious features like mountains or rivers to separate them.

“Biodiversity in the deep sea used to be viewed as somewhat of a paradox given the apparent lack of genetic barriers,” says Edie Widder from the Ocean Research and Conservation Association. Davis’s study hints at an answer. After evolving their own light, some fish may have effectively built luminous towers of Babel—different flashing dialects that split single communities into many factions. (Something similar may have happened among electric fish in the rivers of Africa.)

The same story applies to sharks. They’ve evolved bioluminescent at least twice, and these luminous species account for 12 percent of the 550 or so species of shark. And the groups whose light organs allow them to communicate with each other seem to be exceptionally diverse. As Julien Claes from the Catholic University of Louvain told me last year, “They’re some of the most successful groups of sharks. We discover new ones every couple of years.”

So forget great whites and makos, salmon and tuna, clownfish and angelfish. The most common and diverse fish in the world are the obscure ones that you probably haven’t heard about, swimming somewhere in the open ocean, basking in their own glow.


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How An Icon of Evolution Turned to the Dark Side

A mutation giving rise to the black form of peppered moths has been discovered and is estimated to have occurred around 1820.
A mutation giving rise to the black form of peppered moths has been discovered and is estimated to have occurred around 1820.
Ilik Saccheri

In the early 19th century, coal-fired factories and mills belched a miasma of soot over the English countryside, blackening trees between London and Manchester. The pollution was bad news for the peppered moth. This insect, whose pale speckled body blended perfectly against the barks of normal trees, suddenly became conspicuous—a white beacon against blackened bark, and an easy target for birds.

As the decades ticked by, black peppered moths started appearing. These mutants belonged to the same species, but they had traded in their typical colours for a dark look that once again concealed their bodies against the trees. By the end of the century, almost all the moths in Manchester were black.

As British air became cleaner and trees lighter in colour, the black moths faded back into obscurity. But in their brief reign they became icons of evolution. As geneticist Sewall Wright put it, they are “the clearest case in which a conspicuous evolutionary process has actually been observed.”

The story has endured a fair amount of controversy. Creationists asserted that the blackening of the moth was just a case of shifting gene frequencies rather than an outright change from one organism into another, ignoring that the former is the very definition of evolution. They also seized onto technical disputes between scientists themselves, over whether the moths’ colours really made any difference to their vulnerability to birds. The latter dispute was resolved through some groundbreaking experiments by the late Michael Majerus.

And now, Arjen van’t Hof and Pascal Campagne from the University of Liverpool have strengthened the peppered moth’s iconic status even further by identifying the gene behind its classic adaptation. And in a wonderful twist, the gene turns out to be a jumping gene—a selfish bit of DNA with the power to hop around its native genome.

I’ve written about jumping genes many times before (as has fellow Phenomena blogger Carl Zimmer). These mobile stretches of DNA can cause havoc by disrupting other genes and increasing the risk of cancers or developmental disorders. Or, they can provide opportunities by creating variation that evolution can act upon. Such genes have driven the evolution of mammalian pregnancies and our immune systems. And now, it seems that one of them helped the peppered moth to cope with the Industrial Revolution.

Back in 2011, the Liverpool team, led by Ilik Saccheri, bred and compared dark and light moths to identify the gene or genes responsible for the shadowy look. They narrowed their search down to a small section of the insect’s 17th chromosome—one containing 13 possible genes. Since then, having studied more moths, they’ve homed in on one particular gene called cortex.

Patterns on a Heliconius melpomene butterfly wing are made of tiles of overlapping coloured scales.
Patterns on a Heliconius melpomene butterfly wing are made of tiles of overlapping coloured scales.
Nicola Nadeau

In almost all the dark moths, the cortex gene contains a unique stretch of DNA that’s missing from all the light individuals. It has all the hallmarks of a jumping gene, including the ability to make an enzyme that cuts it out of its original location and pastes it elsewhere. The moth’s genome contains up to 255 copies of this gene, which the team calls carbonaria. It clearly gets around a bit.

And on one particular jump, it landed in the middle of cortex. This fateful event, which nestled one gene (carbonaria) within another (cortex), is what darkened the moth’s body. Van’t Hof and Campagne estimate that it probably happened somewhere around 1819—a couple of decades before entomologists first saw the dark moths in the wild.

The timing fits, but other details are less clear. For example, how exactly did carbonaria cause the dark colours? Genes encode instructions for building proteins—tiny biological machines that perform various jobs around an animal’s cell. You might guess that carbonaria changed the instructions in the cortex gene, leading to the production of a different protein with new capabilities. But not so—the jumping gene actually landed in a part of cortex  that gets discarded, and never contributes to building proteins.

Rather than changing what the cortex gene built, the team suspects that carbonaria changed when and where it is activated. Indeed, with the jumping gene in place, cortex switches on very strongly at the point in the larval moth’s life when it starts producing its adult wings. It’s unclear why that happens, or how it leads to dark wings, but for now, it seems that cortex affects the development of wings and that carbonaria changed how it did its job.

Indeed, in a separate study, Nicole Nadeau and Chris Jiggins from the University of Cambridge showed that cortex controls the patterns of the beautiful Heliconius butterflies, probably by influencing the development of their wing scales. By fiddling with this gene, natural selection has repeatedly tweaked the palettes and patterns of insects.