Fireflies are unusual animals, but not because their butts light up;
light-up body parts are actually really common in the animal world. Instead, what makes fireflies so
weird is that they don’t live in the ocean. I’m Cameron, and this is MinuteEarth. To us
humans, light-up creatures seem really rare. That's because only a handful of the animals
we regularly see - those on land, in the air, and in lakes and rivers - are bioluminescent - which
means they make their own light: there are fireflies, some beetles, glow worms,
millipedes, and a single species of freshwater snail. But the vast majority of animals in the
ocean – around 75 percent of them – make light in one way or another, which means lighting up
in the ocean is the rule, not an exception. We don’t really know why bioluminescence is
so common in the ocean but so scarce everywhere else. Perhaps one reason for this is darkness.
Much of the ocean is dark or dim, so lighting up there might be a good way to startle potential
predators. It’s also useful for connecting with mates, or even to lure prey to a toothy doom.
But there are non-ocean animals that live in places where it’s dark: The poles, for example,
where it’s dark for months at a time, or in caves, where it’s dark pretty much all the time. Also,
let’s not forget the animals that are only active at night. In all cases, these animals
have the same needs as ocean dwellers, and could conceivably benefit from light up parts as well.
So darkness alone doesn't seem to explain why so few animals outside the ocean light up.
Perhaps non ocean animals just haven’t had the time to evolve bioluminescence. Ocean
animals did have a ~200 million year head start on animals that live on land and in freshwater,
so they have had tons of extra time to develop light-making mechanisms,
and maybe in 200 million more years non-ocean animals will have caught up.
But ray-finned fishes have independently evolved light-making machinery at least
27 times in the last 150 million years, which suggests that bioluminescence actually evolves
pretty quickly. So non-sea creatures should have had plenty of time to evolve light-up parts too.
So maybe it’s because of obstacles, or a lack thereof. Non-ocean environments, like forests,
are too full of light-blocking obstructions that might make lighting up there not as useful as it
is in the open ocean. Or maybe bioluminescence is just easier in water. Light-making chemicals
might build up to toxic levels if an animal can’t process them fast enough, and water
might be great at washing those chemicals away before they become deadly. But neither
of those ideas can explain why freshwater lakes – especially ones like Lake Baikal, which are super
deep and dark - have no bioluminescent species. Maybe salt in seawater plays a role in making
light, but experiments haven’t supported that idea…or really any of the others, for that matter.
Which… is unsatisfying, but I think you get the point – we are still trying to figure out
why bioluminescence is only a bright idea in the ocean - for now, we’re still in the dark.
Despite being stuck in the dark, there are things we can do to make our world a little brighter,
like reducing our negative impacts on the climate. And thanks to Wren,
all I have to do is go to their website and answer a few simple questions about my lifestyle so they
can generate a personalized report estimating the amount of carbon my activities release. I
already try to minimalize my impact, but Wren takes this a step further by helping
me to offset my emissions by contributing to carbon-reducing projects like mineral
weathering and cleaner cooking fuels. I receive monthly updates from projects I support that let
me see every tree planted in my name and every reforested acre those trees live in. Wren can
help you offset your impact as well – And for the first one-hundred of you who sign up for
Wren using the link in the description, Wren will plant ten extra trees in your name! Thanks Wren!
Also, our merch store is getting brighter with beautiful new products that tell cool
science stories, including a holographic sticker sheet with awesome bioluminescent
animals! This limited print run will sell out fast so go grab yours at DFTBA.com/minuteearth