Figure 1: Zooplankton collected from Station ALOHA using a MOCNESS net. The community of zooplankton emit bioluminescence as they are disturbed in a tube. The likely emitters are various copepods from the genus Pleuromamma.
When you close your eyes and think of bioluminescence what do you see? People often see the twinkle of fireflies on a warm summer night, or the ethereal blue glow of bioluminescent bays. I see a glowing blue trail of dinoflagellates behind my dog as he chases a stick off the coast of Vancouver Island. Although these cases are striking, there is much more to bioluminescence than meets the (human) eye. Bioluminescence has evolved more than 40 times across living organisms and is found in creatures as different as bacteria, mushrooms, shrimp, fish, and squid. It is used to attack prey, defend from predators, and find a mate. Many tricks humans have devised through technology and cunning have been invented by evolution long before we arrived on the scene.
Researchers studying animal behaviour want to do so without distracting the animals with lights, sounds, smells. We can watch animals in the dark with cameras equipped with infrared light and sensors. This allows us to see natural behaviours without blinding or surprising the animals. We aren’t the only ones with night vision though, stomiid fish have a trick up their sleeve. Most animal eyes in the deep sea are sensitive to blue light only. Stomiids have the unique ability to detect red light and they also produce red light from bioluminescent patches under their eyes. As they swim through the deep, dark water they shine their red flashlight. This secret red channel, not visible to most other animals, can be used by stomiids like night vision goggles to search for and capture unsuspecting prey.
To be seen or not to be seen? Hunters wear patterns of green, brown, and black to hide from deer in the forest. The camouflage patterns or materials we have developed are quite sophisticated. It may seem counter intuitive, but rather than color or pattern, many ocean dwelling animals actually shine light to hide! The ocean gets dark as you dive deeper, but up to about 1000 meters deep there is still faint light filtered from the sun. In these conditions animals swimming through the water will cast a shadow visible to predators below. Some shrimp, fish, and squid have bioluminescent organs on their underside and, based on the light above them, will produce light just as bright to hide from predators below. From a hungry predator’s perspective, the camouflaging animal above doesn’t look like a meal but faint sunlight shining down.
Even the best camouflage in the world can’t keep you hidden forever. If by chance or by the ingenuity, unwelcomed guests can find even the most hidden hovels. People have developed a number of ways to alert ourselves to unwanted guests and call for help when needed. The burglar alarm does just that. Dinoflagellates, tiny unicellular protists, also use a burglar alarm system in response to unwanted attention. If a predator, say a small shrimp, comes nibbling, dinoflagellates will defensively emit light. When done en masse this creates enough light to draw the attention of large fish in the area. Dinoflagellates are not on the fish’s menu, and so the fish will come to the rescue and slurp up the unwanted shrimp now illuminated by the bright burglar alarm.
Defense mechanisms come with different costs. If you were eating a steak in the woods and found yourself face-to-face with a hungry wolf, your first instinct may be to throw the steak at the wolf allowing you to escape! You would go hungry and have wasted hard won pay on a gourmet dog treat, but you live to dine another day. Brittle stars go a grisly step further when confronted with a similar situation. When brittle stars are being attacked by a predator, a crab for instance, they forcibly remove one of their appendages. The removed arm begins to writhe and bioluminesce to draw the crab’s attention while the brittle star, perhaps a little distressed but still alive, quietly escapes.
There are countless more examples of different forms and functions of bioluminescence in nature. One of the reasons I am participating in the Chief Scientist Workshop is to learn how to lead a successful research cruise at sea. I am fascinated by vision and bioluminescence, how it is made, and how animals use it. In some areas of the ocean more than three quarters of all animals are capable of bioluminescence. It is a major ecological trait and I want to learn how light, bioluminescence, and vision structures where and what animals are doing in the Earth’s oceans.