Today's post comes from retired Monterey Bay Aquarium Senior Marine Biologist and lifelong educator Steve Webster.
Divers soon recognize that below a depth of 15 or 20 feet everything appears various shades of blue. To return warm colors to a photographic image or video clip, divers have to carry underwater photo strobes or video lights. Using lights returns the oranges, yellows and reds that would otherwise be missing from the image.
A coral reef crest at the North Save-a-Tack dive site in the Namena Marine Reserve at a depth of 40 feet.
A Fiji reef photographed with light strobes
This raises an interesting philosophical question. What is the true color of a red fish at a depth of 30 feet? Without the lights it will look dull brown or even black to the naked eye. While with the lights it maybe fire engine red or cherry red. In this post we present some images of animals on Fijian coral reefs with the natural light photograph side by side with the photo strobe photograph. Take your pick; are these animals really red or not?
Pink anemonefish Amphiprion periderion in Heteractis magnifica anemone tentacles, pictured at 30 feet deep without strobe
With light strobes
So why do these animals bother to produce red pigment if they don’t look red at depth? One possibility is that since red absorbs blue and green light (remember your high school physics?) a red animal will be a dull brown or even black in the dim light of the evening or morning just before sunset or sunrise. This could be important camouflage for prey species, or disguise for the predators (like a red grouper). Many of the predators on the reef make their living during these times of dim light. Looking brown or black instead of red allows them to ambush a smaller fish or octopus without being seen.
The same principle holds for deep sea fishes, squids, shrimps, etc. where the only light is bioluminescence – light in the blue-green spectrum produced by the animals themselves.
Phyllidia varicosa nudibranch (which mimics the highly toxic juvenile Graeffe's sea cucumber, another interesting story of deception like this one) seen at a depth of 60 feet
The same nudibranch Phyllidia varicosa seen now with camera's strobe lighting
So, all these colorful underwater photographs – are they really an accurate representation of the ocean world? Or are they just an artifact of the photographer’s lights?
Steve Webster, Ph.D, is one of the founding team of the Monterey Bay Aquarium, veteran educator, recently retired Monterey Bay Aquarium Senior Marine Biologist, and veteran of at least 15 Fiji diving expeditions.