The ocean is a vast place, most of which does not receive any sunlight, but it is not void of light thanks to bioluminescence! Although typically it appears as blue light, did you know that organisms in these otherwise dark waters also can have red and yellow bioluminescence? First it is important to understand why blue light is so abundant in the deep sea. Water absorbs larger wavelengths, like red, easier than smaller wavelengths, like blue, so when blue light enters the water column it is scattered, refer to figure 1. While sunlight might not reach the deep sea, producing blue light will still have this same effect compared to producing red light and therefore other organisms can see it from further away. 

Figure 1. Light penetration in the open ocean.This figure shows the spectrum of visible light next depth which allows us to see what colors get absorbed first versus which ones penetrate the ocean further. You can see in this figure that red is the first colored absorbed and blue is able to penetrate the deepest.

In fact, red light gets absorbed so quickly that most deep sea organisms can’t even see the color red due to them never needing to be able to. So why would red bioluminescence even be used? Actually, red light not traveling far and being invisible to most species is why some organisms have evolved to use it! Red bioluminescence is a great tool when it comes to hunting prey because it allows the organisms to spot prey that is near it by illuminating it, but by the time the prey realizes (if it realizes) it is too late to react due to the short travel distance of these wavelengths and the fact that chances are, it can’t see red. It is also suspected to be used for intraspecific communication (communicating with members of the same species) since other organisms won’t be able to pick up on their communication due to the red color of their bioluminescence.

I also mentioned yellow bioluminescence, but this is definitely the rarest color that we know of. In fact it is so rare that we only know one genus that uses it,  the polychaete worm Tomopteris! Like red bioluminescence, yellow does not travel far (as seen in figure 1) and most organisms at these depths are unable to process yellow light. This makes it an excellent tool for intraspecific communication and it has been noticed that these organisms are actually attracted to other yellow bioluminescence. It has also been seen used as a defensive mechanism when disturbed. Although predators likely can’t see yellow light, the worms produce such intense light that scientists believe predators can pick up on the contrast difference from the bright yellow to the surrounding darkness. This can startle the predators and give the polychaete worm time to swim away and survive another day! However, yellow bioluminescence in the deep sea is very understudied and we are learning more about it every day.

This image is a head shot of the species M. niger which shows the species red bioluminescence under the eye.

While blue is by far the most common color of bioluminescence present in the deep sea, it is still important to acknowledge there are others and continue to learn about them. The ability to communicate with only your species is very important for these creatures as it helps with finding mates, warning about dangers, and letting friends know about any abundant food supplies. It is also a valuable hunting tool since food can be hard to come by in the deep sea. In an area otherwise filled by darkness, bioluminescence is one of the most helpful tools for survival. There is still so much more to learn about these alternative colors of bioluminescence and I know I will be looking forward to reading these papers as they come out!