We have been thinking about deep sea corals a lot in the LOS offices lately. The pictures and videos of the coral observed during the Finding Coral Expedition are so beautiful and inspiring. It makes one wonder, however, why do deep sea corals have such brilliant colors even though they live where there is no light?
We asked Dr. Tom Shirley (pictured in the sub) this question, and here is his very illuminating response:
The absence of light at depth means that any color works equally well. Hence, that color which requires the least energy to produce (or not produce) works well. The predominant colors at very deep depths appear to be red or white, but silver and black and other colors also occur. Red is a part of the spectrum lost early in the water column, hence red appears black in low light situations.
Keep in mind, however, that many species have extensive depth ranges: hence, many of the colors observed in sea stars (as an example) may have positive survival benefits for members of their species in shallower water, or for their offspring if the larvae settle in shallower water. Also, many of the fish and invertebrates have daily, seasonal or ontogenetic (changes with life history) migrations – hence a particular color or pattern may offer an advantage at some time in their life. Many of the abyssal fishes spend their early life history in shallow waters where there is more food. And, all species carry genetic baggage: structures (such as an appendix) or traits that may have been useful at times in their past. This may explain why many deep sea organisms (especially fish, squid and octopus, and crustaceans) have color vision, even though they live in total darkness.
Also, keep in mind that the visual sensitivity of many species exceeds ours by orders of magnitude. Crustaceans can see in light levels that we perceive as total darkness. We cannot see any light below 200 m, but some (tiny) portions of sunlight remain at depths to 1000 m. A common myth (even among scientists) is that most animals can’t see in red light. That works well for most terrestrial mammals (who are color-blind), but has no bearing for most invertebrates such as crustaceans, which see equally well in red.
Bioluminescence complicates things somewhat, as many animals can produce light – some for signaling, attraction, or distraction, but apparently some to find prey – as we might use a flashlight.
So, the answer is complex. It varies with what group of animals you are discussing, and what depth you are referring to.
Sorry, but life is messy; very few true or false answers.
Dr. Tom Shirley is the Endowed Chair of Biodiversity and Conservation Science, Harte Research Institute, Texas A&M University, Corpus Christi
Tom Shirley and Cup Coral Photos Credit: Living Oceans Society