Colors in the Open Ocean: Hide and Seek without Structure

If you asked an elementary school student to draw an oceanscape, they would likely draw a vibrant coral reef ecosystem, full of colorful corals and shiny fishes. Although many coral reef ecosystems are losing this color and most reef ecosystems do not actually look like the reef structures in aquaria, coastal seas are typically characterized by this luscious vibe. On the contrary, the open ocean is more monotone and so are the organisms within it.

Reef vs. Pelagic Ocean.

Almost all animals that live in the open ocean have a dark dorsal (top) side and light ventral (bottom) side, a characteristic called countershading. Consider the below image of an Atlantic bluefin tuna (Thunnus thynnus). The dorsal side is a dark blue and the ventral side is white. The main purpose of this is to avoid predation in a 3-dimensional environment (many pelagic organisms in lakes have this as well). Predators hunting from below the tuna will have to look up at a light backdrop where the white ventral side blends in with the environment. The dark dorsal side serves to protect from predators hunting from above, which would be looking into the dark backdrop of the deep ocean.

School of juvenile bluefin tuna.

If you are a SCUBA diver, you may have noticed that certain colors seem to disappear or become distorted as you go deeper. The reason for this is different colors have distinct wavelengths. Humans can only see a portion of the electromagnetic radiation spectrum, and we discern the different wavelengths of this spectrum as gradients of individual colors. As photons (particles of light) hit objects in the ocean (e.x. water molecules, animals, and static objects), the particles are either absorbed or scattered. Different objects absorb different wavelengths of photons, and we see the colors that are not absorbed. For example, water absorbs longer wavelengths (red) and we see the blue wavelengths that are left over. As you dive deeper in the water, all of the red wavelengths, then orange, yellow, and green have been absorbed and objects of these colors disappear into the surrounding environment.

Electromagnetic spectrum showing the wavelengths of light that are visible and some that are not.

An interesting phenomenon in the deep ocean is that many organisms are either dark black or red. In a study published last year in the journal Current Biology, deep-sea fishes were found to have special skin that absorbs greater than 99.5% of light in the visible spectrum! These fishes are essentially invisible in their environment and can operate in a stealth manner while searching for food. Other fishes and shrimp are a bright red color that would stick out in shallow water but are invisible in the deep sea. Below is an image of the aptly named velvet whalefish (Barbourisia rufa) that I took on a research cruise in the Gulf of Mexico. This fish occupies the bathypelagic zone (greater than 1000m (3280 ft) deep) and if you look on the ventral side, you can see that there is likely something in its stomach (black segment in the body). This orangish-red color is nearly invisible in the dark, deep ocean because all red wavelengths will have been absorbed long before they reach the depth range of the velvet whalefish. We see this “reddening” of deep-sea organisms across multiple taxa (groups of organisms that are closely related), including shrimps and crabs.

A velvet whalefish (Barbourusia rufa) captured in the oceanic Gulf of Mexico.

Some species have even evolved ways to see wavelengths of light that humans cannot. A popular marine example is the Mantis shrimp and many terrestrial animals can do this as well. In the open ocean there are species that can also see light on the Ultraviolet (UV) portion of the spectrum. A study published in the Journal of Molecular Phylogenetics and Evolution described that oplophorid shrimps (a family of crustaceans) have eyes that allow them to see UV light. Conveniently, this species also can spew a bioluminescent (animal-produced light) goo when it feels threatened that sticks to the potential predator and illuminates it so others within the vicinity can see that a predator is nearby.

The High Seas are a predominately structureless realm in which organisms must consume any prey that they can find. The ability for prey to remain invisible in the midst of predators is critical to survival. Therefore, many organisms have evolved traits that allow them to blend in with the surrounding environment, effectively acting as an invisibility cloak to which animals use to hide in plain sight. Comparing the animals in the High Seas to those in coastal zones, many organisms in the open ocean have evolved ways to hide from predators, as opposed to producing vibrant displays to gather attention by potential mates.

Suggested Peer-Reviewed Literature (May not be open access)

Wong, J.M., Pérez-Moreno, J.L., Chan, T.Y., Frank, T.M., Bracken-Grissom, H.D., 2015. Phylogenetic and transcriptomic analyses reveal the evolution of bioluminescence and light detection in marine deep-sea shrimps of the family Oplophoridae (Crustacea: Decapoda). Mol Phylogenet Evol 83, 278–292. https://doi.org/10.1016/j.ympev.2014.11.013

Davis, A.L., Thomas, K.N., Goetz, F.E., Robison, B.H., Johnsen, S., Osborn, K.J., 2020. Ultra-black Camouflage in Deep-Sea Fishes. Curr Biol 30, 3470-3476.e3. https://doi.org/10.1016/j.cub.2020.06.044