Lighting Up The Deep

Happy just after Halloween everyone! And Feliz Dia de los Muertos if you're in Central America! This is one of my favorite times of year, not least of all because I've always had a lot of fun on Halloween. It's one of those few holidays that stay awesome no matter how old you get. When you're young you get to have all the fun of trick-or-treating, and when you're older you get to have parties. I remember one of my favorite things about trick-or-treating was getting to use glow-sticks.

Yeah these things!

Courtesy Timo Newton-Syms via Flickr

Most of the time we humans get to glow for fun, but there are animals in the ocean that glow entirely to survive. So this week we're going to explore some of the beauty of deep sea bioluminescence.  

As I'm sure you know the further down you go in the water the darker it gets. Eventually you lose all light but for a long ways, up to about 1000 meters, small amounts of light still get through. Not all light is created equal though. Different colors have different wavelengths, and therefore have different amounts of energy. Colors like blue and green are very energetic compared to colors like red and orange, so they travel farther through the water. Many a scuba diver can tell you that you don't have to go that far down before everything becomes awash in only blue. This property of light is very important to animals in the deep sea, because it determines what color their bodies are and what  colors their bioluminescence. Watch the video below and see how many different colors of animal made light you can see.

Also, revere the master David Attenborough!

So how many did you count? I'm guessing maybe two if you've got really good eyes or like to be pedantic about blue vs. blue-green. So why do we see basically one color down there? Is the deep sea just racist? Well it comes back to those properties of light; blue literally goes a long way down this far. Communication is one of the important uses for bioluminescence in the ocean and you can signal over a much longer distance with blue light.

Alright so attracting prey and communicating with your own species is a great use for bioluminesence, but there's more you can do with living light. Believe it or not many animals use light to blend in. Even though this seems counter-intuitive animals that do this are using an extension of matching their background like traditional camouflage. Many animals in the region where a little bit of light still lingers, sometimes called the twilight zone, have light emitting organs called photophores on their bodies. The photophores give off the same color of blue that makes it through the water, breaking up their silhouette and blending them in. This is especially true when they're viewed from below because the light comes down from the surface. Many animals do this: from the incredibly numerous lantern fishes (Myctophiformes), to one of my favorite animals, the firefly squid (Watasenia Scintilans) 

Seen here in a festively appropriate form. 

Actual pictures can be seen here.

Now that we know the things most animals ocean animals do with biolumiescence, let's look at an interesting exception. Three genera (one grouping less specific than a species) of dragonfishes (Stomiidae) have photophores that make red light instead of blue. These special light organs, which are just beneath their eyes, actually beam ahead of them like headlights. The reason they use red instead of blue is two fold. One, most animals in the deep have no reason to see red. Dragonfishes' prey evolved in an environment where red light doesn't exist, so they have no need for the eye proteins that see it. And two, many deep sea animals are red. Weird right? But red is great camouflage against everything except for those three groups of dragonfish. Since there isn't any red light, animals with red skin appear completely black in the depths. These three genera of dragonfish are pointing lights that their prey can't see at animals that are lighting up like beacons.

Why are you weirded out? Only two of them have no bottom on their jaws.
By Erich Zugmayer (died 1939) [Public domain], via Wikimedia Commons 

Even more amazing is the fact one of the species in this group of fish, the Northern stoplight loosejaw (Malacosteus niger), regularly eats copepods (open ocean relatives of shrimp) which eat a bacterium that makes a type of chlorophyll that picks up red light. The stoplight loosejaw takes this chlorophyll and produces the pigment its eyes need to see red light. Think about that for a second; this fish uses the food, of its food, to make what it needs, to find food. Cue the theme song from Inception!

BWWWWWWAAAAAAAUUUUUUGGGGGHHHHHH!

References:

Douglas et al., "Enhanced retinal longwave sensetivity using a chlorophyll-derived photosensitizer in Malacosteus niger, a deep-sea dragon fish with far red bioluminescence", Vision Research, Vol. 39 Issue 17, Aug. 1999, DOI 10.1016/S0042-6989(98)00332-0, Accessed via: http://www.sciencedirect.com/science/article/pii/S0042698998003320

Moser, H. Geoffry & Watson, William, "Order Myctophiformes: Blackchins and Lanternfishes" From NOAA, accessed via: http://web.archive.org/web/20011201063212/http://www4.cookman.edu/noaa/Ichthyoplankton/Myctophiformes1.pdf

Malacosteus niger: Northern Stoplight Loosejaw, Encyclopedia of Life
http://eol.org/pages/224918/details

Watasenia scintillans: Sparkling Enope Squid, Encyclopedia of Life
http://eol.org/pages/399186/details