Davis, A.L., Sutton, T.T., Kier, W.M., Johnsen, S. 2020. Evidence that the eye-facing photophores serve as a reference for counterillumination in an order of deep sea fishes. Poc. R. Soc. B 287: 20192918.
The deep ocean is one of the most challenging places for any animal to live. Creatures that live in the deep ocean cope with high water pressures, cold temperatures, food scarcity, and a lack of sunlight. There are three “zones” of the ocean, which are defined by how much sunlight they get. Sea water scatters and absorbs light, so the deeper you go, the darker it becomes. The surface zone is referred to as the “sunlight zone”. This is where most marine species live, and where plankton, kelp, and algae are able to photosynthesize. At about 650 feet, a fish will enter what is known as the “twilight zone”, where sunlight rapidly decreases with depth. At 3,300 feet down, in the “midnight zone”, there is no light other than the light that animals themselves create.
A big problem that fish in the twilight zone face is how to camouflage themselves. With low levels of sunlight coming from above and no light source below them, the fish appear as obvious silhouettes to predators swimming below and make for easy meals. However there are certain fish families, such as dragonfish, lanternfish, and hatchetfish, which use counter-illumination to camouflage themselves from predators below.
Counter-illumination is when an animal emits light from its underside to avoid being seen from below. Fish that are able to counter-illuminate have special organs called photophores on their bottom half, or ventral side, to create light. They create this light either through a chemical reaction, or through a symbiotic relationship with bacteria that live within the photophore.
But how do fish know how bright their photophores should be? Just like on land, light in the ocean fluctuates based on factors like the weather and what time of day it is. These fish can adjust the strength of the light produced by their photophores to match the intensity of the light coming from above them, but most fish have forward facing eyes, so there is no way for them to see the light they are producing on the ventral side of their body. Davis and co-authors aimed to solve this mystery by looking at photophores found on the faces of these deep sea fish.
What did they find?
Davis and co-authors used 21 fish that were collected and preserved on research cruises between 2009 and 2017 from the Gulf of Mexico, as well as 15 preserved specimens that they borrowed from the Smithsonian Museum of Natural History. They targeted specific species that had photophores around their eyes, but did not use the photophores on their face as a “search light”. Species that used photophores as a search light use the light they produce as a flashlight to attract and hunt prey, like the angler fish from Finding Nemo.
Once they had collected and described these fish and their relationship to one another, they looked at the direction in which the photophores were facing using a microscope and a miniature version of a CT scan. They found photophores that faced the fish’s eyes in all of the species that had photophores used for camouflage. All of these eye facing photophores were also surrounded by a layer of melanin, the same pigment that is responsible for determining the color of human skin and hair color. This layer of melanin focused the light from the photophores towards the fish’s eyes. In some of the fish species the eye had adapted to have a special opening in its lens, called the aphakic gap, which was lined up with the light produced by their photophores.
The researchers concluded that fish that had eye facing photophores used them as a calibration tool to match the intensity of the light around them. They used this specialized photophore as a proxy for the brightness of their ventral photophores, and then were able to adjust the light they were producing to eliminate their silhouettes.
Why does this matter?
This research is a great example of how scientists use morphology, or the form of an animal, to see how it is related to other animals and how its body has evolved over time. Davis and co- authors were able to construe that these two traits, having photophores on their bellies and photophores facing their eyes, evolved dependently. Dependent trait evolution occurs when two traits that are tied to each other evolve at the same time. Because the photophores on the face of the fish determined how they were able to control their counter-illumination, and better counter-illumination led to better camouflage, fish that evolved to have both of these traits had a greater chance of survival and a greater opportunity to reproduce to pass down those traits to their offspring.
I am currently a Master’s candidate in Environmental and Ocean Sciences at the University of San Diego, and I study the stickiness of phytoplankton using 3D images. By tracking collisions of phytoplankton, I can see how sticky they are by observing how often they stay together when they collide instead of bouncing off of each other. When I am not working on my thesis you can find me on the beach, reading a book, or working on a painting!