Last night after dinner, with the refreshing thoughts of the hammock still in mind, I thought I would return to my place of rest. The deck of the ship is not well illuminated at night, so a headlamp was necessary to avoid fumbling over the side and becoming lost at sea. My headlamp can be set to incandescent or LED lights. I thought the LED on the low setting would emit less light and not interfere with the bridge’s visibility. The light coming off the headlamp was illuminating the white of the deck in a blue light, as if I was 100 meters below the depths of the ocean, where only blue light penetrates.
Water scatters blue light making the oceans appear blue and blue light is of a higher energy than red and yellow light. Therefore, it travels further into the water, blanketing the depths in blue light. Organisms living at these depths are more likely to utilize bioluminescence. Because light of other colors does not make it to depths greater than about 50 meters, most organisms living in the mesopelagic zone can only see in blue light wavelengths. In most cases, the biological production of light is used as camouflage, or defense, such as squirting a trail of glowing ink behind to confuse a predator. To hide in this environment it is best to be translucent, counter illuminate with light organs, or to have red/orange pigments. Many of the crustaceans from these depths contain orange carotenoid pigments. They get these pigments from their food source, phytoplankton. This pigment does not reflect blue light and most organisms cannot see in the red and yellow wavelengths, making red shrimp nearly invisible. That is except for a few species of fish, including a species in the family Malocosteidae. These fish have outwitted, in a sense, their prey. This group of torch fish emits light not only in the blue color, but red as well. The red light comes from specialized photophores on the head of these fish and illuminates the red colored crustaceans brilliantly. This light is not initially red but is originally generated as blue light. Special filters in the photophores absorb the blue light and re-emit at lower energy, as red light. These fish, in essence, shine a red flashlight into the blue water column. But who is there to see it, since the organisms at these depths cannot see in the red wavelength? This near infrared scan goes undetected to all except the Malacosteid fish himself. These fish not only emit the red light but also have evolved eyes that can see in the color of red. The fish uses a special pigment in its eyes very similar in nature to chorophyll to absorb red light. The Malacosteids are the only group of fish that emit red light and have red receptive pigments in their eyes. Other species of deep-water fish that produce red light have to use special filters in their eyes to convert the red light back to blue. With the small crustaceans now unwittingly targeted visibly red, this small predator can capture its quarry. Nature has such enlightening stories.
It turns out my blue LED was not as dim as I had hoped. Once I had found the hammock, gotten settled, and begun reading, the bridge had a crewmember come down and tell me to shut off my light. Apparently a strange blue light reflecting off the deck was blinding the bridge. Ironically, this may be one way deep-sea animals use light as defense. They may temporarily emit a light bomb to blind their pursuer. Having no intention of blinding the ship’s officers on bridge, I switched my light off and laid in the dark swinging in the hammock for a while. The air had cooled considerably and the clouds were blocking the stars. Chilled and slightly bored, I gingerly worked my way back to the bowels of the ship to call it a night.
Below are two links to information on bioluminescence:
Daily question: To what kingdom do radiolarians belong and what characteristics define this kingdom?
Figure 1. Radiolarians from the Sargasso Sea.
Figure 2. Sunset April 21, 2006.
Figure 3. Planktonic cnidarian.
Figure 4. Sapphire waters.
Figure 5. Out watching the sunset.