Why Is (Almost) All Bioluminescence in the Ocean?

far below the ocean's surface life has adapted to a world of Darkness strange creatures haunt the crushing depths some of them completely blind others with eyes that point upwards to help them avoid Predators attacking from higher levels others still have eyes so enormous you begin to wonder if it's so dark at these depths what are these eyes designed to see no sunlight penetrates the deep water of the ocean but this ecosystem is by no means devoid of light it instead shines with one of the most widespread adaptations in the ocean bioluminescence many creatures have evolved

the ability to glow in the dark either by harnessing bioluminescent bacteria or initiating a chemical reaction within their own bodies these unique adaptations have intrigued scientists for decades but more recently researchers have realized bioluminescence isn't limited to the darkest parts of the ocean in fact the proportion of animals that glow is similar at all levels of the ocean from the surface down to the sea floor bioluminescence is less a special ability than a widespread adaptation in the sea more than 75 percent of organisms in the ocean can produce their own light from the smallest bacteria

all the way up to large sharks fishes and Rays have evolved to this feature at least 29 different times making it an impressive example of convergent evolution and if you consider that 71 of the earth's surface is water that means bioluminescence is actually one of the most widespread traits to have ever evolved but that also raises the question why is bioluminescence so common in the ocean and so rare on land the only terrestrial organisms with the ability to produce their own light are a few species of fungi arthropods like fireflies and a handful of larvae

like glowworms and while the ocean is certainly a dark place our terrestrial world is bathed in darkness half the time too if bioluminescence isn't in fact limited to the pitch black depths of the ocean why does it seem to be constrained to barely exist on land what is it about the world of water that makes it so much more suited to bioluminescence when 18th and 19th century Travelers wrote about glowing water on the ocean their accounts were dismissed as fantasy Jules Verne described a Milky light coming from the ocean in his popular novel 20 000

Leagues Under the Sea but it wasn't until the 21st century that we were able to understand that the phenomenon of glowing really can illuminate huge patches of the ocean today we know that bioluminescence has emerged independently more than 94 times within hundreds of species organisms that use bioluminescence have two options for producing their own light they can use intrinsic bioluminescence in which they mix chemicals in their own bodies to produce photons of various colors and intensities or they can use symbiotic bioluminescence which requires cultivating colonies of glowing bacteria in specialized organs the chemical process involves

a molecule called luciferin that generates light when mixed with oxygen and a luciferase enzyme luciferin and luciferase are generic catch-all terms in reality there are many different molecules that can be combined to catalyze this light emitting reaction this process likely originated with bacteria and some researchers hypothesize its incredibly ancient the first bioluminescence may have appeared between 2 billion and 500 million years ago however the two different types of bioluminescence that exist today have very different evolutionary histories researchers found that intrinsic bioluminescence only evolved eight times in fishes but that symbiotic bioluminescence has happened at least

17 times always with the same family of bacteria on land the trait seems to have taken longer to appear a fossilized ancestor of fireflies from a hundred million years ago was discovered to have a light producing organ suggesting that this has long been an adaptation among that group and it probably originated as a form of Defense to warn off Predators another group of researchers have suggested that other bioluminescent terrestrial insects evolved from a single ancestor that lived 65 million years ago around the time that dinosaurs went extinct and mammals took over but that doesn't explain

fungi since they belong to a whole other branch on the Tree of Life researchers analyzed the Genome of living fungi to trace the roots of glowing mushrooms back to 160 million years ago they theorized that this bioluminescent ability evolved from one common ancestor that then was either lost retained or regained across time and many different species it's possible that the effect makes spores more attractive to Insects who might spread them but bioluminescence might also be a byproduct of other chemical reactions which would explain why it's disappeared multiple times through the evolutionary history of fungi if

it was serving no secondary purpose it wouldn't need to be retained but the fact that bioluminescence has evolved separately on land and has been retained in several species means that it is a useful land-based adaptation so why is it still so rare what about the ocean makes bioluminescence so ubiquitous living in the ocean is not at all like living on land the most obvious difference between marine and terrestrial environments is the medium water versus Air both present vastly different challenges and opportunities to life forms who live in them but a less obvious difference is the

fact that in most of the ocean there's nowhere to hide rodents and other small animals on land have trees shrubs rocks and Burrows that help them stay safe from predators meanwhile for most aquatic life there's nothing in the environment to keep them safe Reef systems are the exception rather than the rule they only occur in about one percent of the ocean and only some species live on the ocean's floor nearly everywhere else in the ocean is an open expanse that means many species have had to find other ways of keeping themselves safe from predators and

as unexpected as it might be light is one of the best methods jellyfish and siphonophores are some of the best examples of creatures that use light to evade capture nearly all of them have some form of bioluminescence comb jellies flash brightly to startle Predators while siphonophores can produce a constellation of small lights mimicking Plankton to confuse predators some jellies even release a glowing slime that sticks to their Predator a strategy also employed by tiny Crustaceans called ostrocods or sea fireflies when fish try to swallow them they release one of the brightest lights in the ocean

momentarily blinding the fish and smearing it with light by Illuminating themselves or their Predator these organisms might also be calling over an even larger Predator the ones who might eat the smaller organisms would be predator consider dinoflagellates those single-celled Plankton that can produce such wonderfully glowing waves there are many species of dinoflagellates and those that are capable of bioluminescence are less likely to be eaten this could be because their bright flashes act like a burglar alarm essentially announcing to the nearby ocean dwellers hey this crustacean is trying to eat me you want to come eat

him the last way swimmers use light as a source of Defense sounds the most counter-intuitive light as camouflage the deeper you go in the ocean the less light filters down through all that water between 200 meters and a thousand meters lies the mesopelagic also known as The Twilight Zone because a certain amount of light does come from above a fish swimming above a predator would have its dark silhouette outlined against the light to keep this from happening they use counter illumination little spots of light on their Underside break up their silhouette confusing potential Predators it's

such a well-honed trick that creatures like the Hawaiian bobtail squid can change the amount of light it emits depending on the time of day like its own glowing circadian rhythm the story on land is a little more complicated since lighting up in the darkness is more likely to draw attention to you than to obscure your outline but this glow can still be a defensive strategy for some organisms by advertising that they are poisonous it's like a nighttime version of the poison dart Frog's vibrant skin color this strategy is used by some worms as well as

millipedes but of course it's not only prey who are using light to keep themselves alive Predators have many of their own tricks to lure in their meals members of the anglerfish family might be the best known deep sea dwellers to entrance their prey with glowing lures but the strategy also seems to be used by giant squid who have been observed flashing Bright Lights while hunting either as a way of stunning their prey or getting them to come closer and this hunting technique is something shared among many of the bioluminescent insects that glow on land consider

the glow worms that have Enchanted people for centuries in Australia and New Zealand these cave dwellers fill the ceilings of their abodes with Starry blue constellations but that light is actually a creepy hunting strategy the worms spin dripping Nets of glowing spit into which other insects fly the larvae of Click beetles operate very similarly hiding in holes and emitting a soft yellow green glow whenever insects approach to investigate the Beetles pincers snap shut around its meal just like in the sea organisms on land have been able to turn bioluminescence to their advantage when it comes

to hunting but perhaps the most mysterious use of bioluminescence is light for mating and communication scientists recently discovered that the Humboldt squid a voracious Hunter uses a number of different light patterns depending on the context it's in these changes might reflect messages that they're communicating to other squid whether it's dominance displays or claiming certain prey and then there's also the case of lantern fishes which include more than 250 species swimming around the mesopelagic in addition to having lights on their stomachs for camouflage purposes they also have glowing spots on the sides of their heads researchers

have hypothesized that different species have subtly different glowing signals they use to find mates if it's a strategy that sounds familiar it might be because so many of us have watched lightning bugs dance through the darkness on summer nights these flying beetles also use bioluminescence for mating purposes with different species flashing at different intervals to attract mates but still light up displays like Firefly dances are the exception not the rule for life on land despite its clear utility for some creatures researchers think that it might be harder for terrestrial life to develop bioluminescence because the

chemicals involved can be quite toxic and might be harder to cleanse away when the creatures aren't always swimming through water but this toxicity is unlikely to be the full story as to why bioluminescence isn't more common on land since bioluminescence is also basically non-existent in fresh water where toxicity could be removed as well as in the ocean the answer perhaps has to do with something simpler time life hasn't existed on land and in fresh water as long as it has in the ocean and evolution is a slow process perhaps it's not that land-based organisms don't

have bioluminescence it's that they don't have bioluminescence yet it's clear that we have a lot of work yet to do to understand the past and future of bioluminescence but if we ever make it far enough into the universe to discover life on other planets scientists say we should be on the lookout for bioluminescence because where there's water and air there's a pretty good chance we'll find lots of glowing life as millions of tiny vulnerable fish navigate the dark ocean water so too do millions of tiny vulnerable humans navigate the abyss of the Internet it's all

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