Bioluminescence: The secrets of animals who glow in the dark
When I was very young, my family visited an aunt in Georgia and I discovered the thrill of chasing fireflies through the sticky, summer night. Like many children, we captured the glowing insects alive and put them in jars together to watch for a short time before releasing them.
Years later I was amazed to see bioluminescent beetles in Ecuador on a field biology course. When the professor netted the insects, we were able to see closer that the beetles lit up different parts of their bodies in different colors. There was still the flash of green that people in North America will associate with fireflies, but there was also a glow of red light that flashed at a different interval from the green.
In the Bahamas, walking along a road at night with the sea smell in my nostrils, I was again enchanted by insects flashing in the dark. I had a hard time believing a friend of mine when he told me a story of seeing fireflies in the Peruvian rainforest. In some parts of the tropics (and even in Great Smokey Mountain National Park) beetles in the family Lampyridae synchronize their flashes. My friend told me of being in the jungle at night and seeing a whole tree light up in a flash and just as quickly disappear into darkness; covered in synchronous fireflies. The phenomena of synchronous fireflies can extend beyond a single tree in places and whole hillsides can light up in an eerily beautiful display.
Caves are a holdout for unique bioluminescent animals as one might expect, although even more simply go without the ability to see and some use echolocation. I travelled to Tasmania and visited a few tour caves situated in Australia’s National Park system to see the spectacular display of glow worms. Watching glow worms high up on the ceiling of a cave in complete darkness is akin to watching the twinkling of stars on a primordial night.
The name ‘glow worm’ is deceptive though, as the insects in question are actually larvae from one of several species of fungus gnat. The larvae create strings of sticky material similar to spider webs that hang below them in the cave. The glowing of the insect attracts other insects (as moths to flames) which become stuck in the sticky strand. When an insect becomes entangled, the glow worm hauls up its line and eats whatever is stuck there, sometimes even adult fungus gnats. The glowing of fungus gnat larvae also increases when a larvae is about to hatch out as an adult, especially in female gnats which attracts male gnats. Male gnats will mate with females cocooned up and about to break free as adults but there is a danger. Especially hungry larval gnats will glow like females close to hatching, so sometimes they catch an adult male fungus gnat which is then cannibalized. Adult gnats have no mouth parts, after mating they starve to death rather quickly. If you can look past the barbarity of the natural history of fungus gnats, the beauty of glow worms is actually enchanting. I’ve heard of other caves in Australia and New Zealand where glow worms are reflected in pools of water, surrounding boaters with specks of light. There are also related glow worms in caves in the southeastern US.
Bioluminescence isn’t only for insects. While I volunteered with sea birds in Alaska, part of our job was to snag dietary samples from rhinoceros auklets. We waited in the dense salmon berry bushes and other vegetation on the hill of an auklet colony. We sat in the lingering twilight before the unsatisfyingly bright night of a northern winter. We could hear the birds quick wing beats as they landed, once on the ground it was quite hard for them to escape our nets though, needing to throw themselves from a cliff to fly free. Mercilessly we collected a few fish the birds had in their mouths. In the shade of the vegetation, I could make out the glow of the fish dimly. In this case, the fish didn’t create the light themselves. The fish glowing in an Alaskan ocean were covered in bacteria or algae that in turn create luciferin, the chemical that glows.
Many marine animals use bioluminescence for their own purposes. The Hawaiian Bobtail Squid has a special organ colonized by bioluminescent bacteria shortly after birth. The squid actually controls the chemical reactions of the bacteria, using the light for her own purpose. Some squid shoot out glowing chemicals to confuse predators.
Lantern fish of course dangle a glowing bait in front of their mouths to attract prey within biting range. Bioluminescence is perhaps best known in the ocean by glowing tides in certain areas, where the waves of the ocean are delicately lit by millions of microscopic animals in the water.
In the Ecuadorian cloud forest, I was thrilled to discover a phenomena I’d long wanted to experience. On dark hiking trails, people sometimes use bioluminescence to their advantage. For utility or fun pieces of wood can sometimes mark trails in a magical way. Fungi that glow at night sometimes colonize rotting wood and appear completely invisible during the day but at night the wood glows faintly. I collected a few of the pieces of this glowing wood and placed it on the edge of the steps of the nature lodge my partner and I were managing.
Bioluminescence has actually evolved separately over 40 different times, which points to how important and useful it can be for the organisms that use it. It’s not always clear why bioluminescence is useful though, and part of the joy of it is the mystery.