Coloration of reef organisms
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  hot buttons for colours section of Biology of Caribbean Coral Reefs website
This section deals with how colours are perceived. Topics of HOW COLOURS ARE CREATED and the FUNCTION OF COLOURS can be accessed via the icons.

How colours are perceived

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of blackbar soldierfishes taken from a video

"All divers know about the loss of red wavelengths as they dive deeper. Let's compare the colours of these blackbar soldierfishes in shallow waters with these at a bit deeper depth. Notice how the red colours have attenuated with depth." - Turks & Caicos 2005

NOTE video taken with ambient light

  photo array showing colour perception by a camera at different depths to 20m
Divers naturally assume that what they see is real, but this may not be so. Most underwater photos are taken with flash and thus may show colours that do not exist at the depth at which the photograph was taken. The spectral chart displayed here was photographed at 4 depths down to 20m (66ft) under ambient light (i.e., no flash). Note how the colours change from 0m depth (chart held by the octopus), through 3m, 10m, and 20m depth (chart held by the SCUBA diver). Note that the yellow and blue wavelengths are the ones most readily visible.

illustration showing light-wavelength attenuation with depth in the ocean
Light wavelengths attenuate differentially as they penetrate seawater. Quickest to disappear are reds (about 99% loss for each meter of depth) and deepest penetration is by blue-greens. Divers know that the shallower the dive, the more true will be the colour rendition, that is, relative to the wavelengths of sunlight. Only near the surface will flash-assisted photos match what we see with the unaided eye. Thus, flash-assisted photos at depth are not reality. Nor are the colours seen by the light of a flashlight, for that matter.




The octopus shows us the extent of attenuation
of visible light wavelengths at 20m (66ft) depth

  illustration of red colour being perceived at depth when it doesn't exist
This is all well and good, but how is it that we can see some shades of red at, say, 5m (16.5ft) depth, when red wavelengths at this depth should be reduced to less than one-billionth of their intensity at the surface? The explanation is 2-fold: first our perception may be enhanced in some physiological way and, second, we may have an expectation that something should be red, such as a squirrelfish, a red swimsuit, or even a pink octopus, so the explanation may be partly physiological and partly psychological.
  So, what does a marine organism see at 10m depth? Is it the same as we see, or is the image less sharp, or perhaps in composite form, or maybe something else? Along the bottom row in the table below are 6 possible images that might be seen by the 6 organisms arrayed along the top row.
drawing of a mantid shrimp
photograph of a barracuda photograph of a squid photograph of a dolphin photograph of a clown crab mock-up image of barracuda with spectacles
Mantid shrimp Odontodactylus scyllarus



Platypodiella spectabilis

Barracuda with poor vision Sphyraena barracuda
  Here are a selection of images that might be seen by these animals. You should already be familiar with what some of these organisms will see, and for others you can make an educated guess. For example, does a certain organism see in colour? What is its resolving power...greater or lesser than ours? When you've thought about it and made your pairings, CLICK HERE to see the answers:
fuzzy photo of a queen angelfish photograph in black and white of a queen angelfish drawing of prism-type image of queen angelfsih photograph of a queen angelfish fuzzy photo of queen angelfish black and white
  drawing of visible portion of light spectrum
Reef fishes have abundant rods and cones, and thus appear to have comparable colour sensitivity to our own. However, as noted earlier, studies in Hawai'i show that 47% of reef fishes studied are also sensitive to UV wavelengths and could possibly form images in reflected UV light. Losey et al. 2003 Copeia (3): 433.
While we can't know for sure how different colours or colour combinations would appear to a fish with UV sensitivity, we know that white would appear chromatically flat in UV and we can therefore guess how a banded butterflyfish might appear to another conspecific and perhaps to other fishes as well. Marshall et al. 2003 Copeia (3): 455. photograph of a banded butterflyfish in normal light photograph of banded butterflyfish Chaetodon striatus in simulated UV light
  Banded butterflyfish Chaetodon striatus in
normal light
Same fish in simulated UV light

What possible function could vision in the UV part of spectrum have for a reef fish? Some ideas are explored here. Losey et al. 2003 Copeia (3): 433.

1. It helps zooplanktivorous fishes to see their prey. We know that crustacean zooplankters absorb UV and, because little or no UV is reflected, the crustaceans would appear to a UV-sensitive fish as black objects against a bright back-scatter. simulated view of crustacean zoplankton seen against a clear, blue ocean view of crustacean zooplankton seen under simulated UV radiation
  Reddish-coloured, semi-transparent shrimps in a clear, blue ocean ...the same scene as viewed by a UV-sensitive zooplanktivorous fish

2. Because UV in seawater is polarised it could create discernible patterns to a UV-sensitive fish. Since polarisation occurs even on a cloudy day, orientation to the sun's position would theoretically be possible.


photo simulation of a bluehead wrasse seeing a conspecific in UV
3. Could it aid in species recognition? In theory, yes, although how this would be any better than just seeing a conspecific in normal wavelengths is not clear. Comprehensive studies on Hawai'ian reef fishes have thus far failed to show a correlation between the possession of UV reflectance and an ability to perceive the image. Wrasses, for example, are known to have a complex UV display, but seem to lack the ability to see it. In this scenario, a terminal male bluehead wrasse is looking at another conspecific male, but wonders what he is seeing. Marshall et al. 2003 Copeia (3): 455.

Relevant to this is the fact that a fish's colours, especially reds and oranges, will appear to change as the fish moves up and down in the water column. So what a conspecific or a potential predator sees in visible wavelengths will vary with depth. A certain species will look quite different at 10m depth than it does at the surface. Does a male looking for a female to spawn with, or a predator looking for a familiar prey fish, somehow keep track of this? Or, because colour is so unreliable, do they rely instead on other characteristics, such as behaviour, smell, or vibration? Interestingly, with its blue and green colours that penetrate deeply into the water column, the bluehead wrasse featured here would potentially show less colour change with depth than would a species with red/orange coloration. Something else to think about is that UV wavelengths penetrate relatively deeply into seawater; hence, a UV image would change absolutely less with depth than would an image viewed in conventional wavelengths.

  4. Another possibility is that it helps reef fishes, especially shallow-living ones, to avoid exposure to dangerous UV levels in too-shallow waters.
  5. Finally, as shown in the wrasse example in No. 3 above, the ability of a predator to see its prey in UV might actually help to camouflage the prey. This doesn't help the predator, but it adds another dimension to the problem. A UV-sensitive predator would perceive an image quite different from that perceived by a UV-insensitive one. Therefore, a potential prey fish that looks visually obvious to us might in fact be disguised from a predator. Piscivores that have at least some UV-sensitivity are sharks and lizardfishes; barracudas, however, are thought not be able to see in UV. Losey 2003 Anim Behav 66: 299; Losey et al. 2003 Copeia (3): 433.
  In fact, many colours of reef fishes apparently contain UV-reflective components. In a novel explanation for the funcitonal significance of UV-reflectance and UV-visual sensitivity in reef fishes that possess both, scientists in Hawai'i propose that it could provide a "secret" lilmited-audience means of communications for conspecifics, one denied to neighbouring fishes such as predators. Let's see how this might work with a pair of angelfishes and a predatory barracuda. Marshall et al. 2003 Copeia (3): 455; Losey 2003 Anim Behav 66: 299; Losey et al. 2003 Copeia (3): 433.
illustration to show possible function of UV-sensitivity in fishes illustration to show possible function of UV-sensitivity in reef fishes
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