Authored by Koaw 12/6/2016
WHY IS THE OCEAN BLUE or green, or bluish-green, or brown, or red? And why is the sky blue? How do fishes see in the water? The basics to all of those questions will be answered below.
The short answer to why water (most of the time) is blue is because the water absorbs certain wavelengths within the red spectrum, leaving us to witness the intrinsic blueness that is reflected. But really, it's a bit more complicated than that.
The short answer to why the sky is blue is because the molecules in the atmosphere are smaller than wavelengths of light; the shorter wavelengths within the blue spectrum are reflected more efficiently than longer wavelengths, thus redirected into your eyes – and again – it's a bit more complicated than that.
The short answer to how do fishes see is that fishes see much the same way as we see, except within the multitude of species, there are some unique and interesting adaptations. And yes, most fishes see in color.
Essentially three things are happening for light to be perceived as color by us and by any fish.
- A source creates the light; the sun is a great example. Or in the water, a point source of bioluminescence in the deep ocean, such as a fish producing light by way of photophores (an organ that produces light).
- Light will travel in a straight line until something disturbs it, such as the molecules of nitrogen and oxygen in our atmosphere, which constitute 78.09% and 20.95% of our atmosphere, respectively. And in water, the water molecules and suspended particulates will also redirect and absorb light.
- And finally, a detector is needed. Eyes are the receptors that will intercept the light and send the visual information via electrical signals to the brain, informing what is seen.
WHITE LIGHT & VISIBLE LIGHT:
The light emitted from the sun is white light, which is electromagnetic radiation containing all colors that we can perceive and we call this visible light.
Remember ROYGBIV? Red, orange, yellow, green, blue, indigo and violet; (FYI that indigo is more appropriately a cyan). Each color range of visible light corresponds to a wavelength range between about 390 – 700 nanometers (nm); reds and oranges have the longest wavelengths (at the 700 nm end) and blues and violets have the shortest wavelengths (at the 390 nm end). A white fish is reflecting all colors and a black fish is absorbing all colors.
THE SKY IS BLUE BECAUSE…
Those oxygen and nitrogen molecules that are so abundant in the atmosphere are smaller than the wavelengths of light; and the shortest wavelengths of light, which are the blues and violets, get deflected more. (This effect is known as Rayleigh Scattering, named after Lord John Rayleigh who discovered it in the 1870’s). And really, the colors perceived depend entirely upon the angles between the source of light and the perceiver. (Check out this site for the angular physics behind the scattering).
You may be wondering: If violet is of a shorter spectrum of wavelengths than of the blue spectrum wavelengths, then why isn’t the sky violet instead of blue? Well, for us humans, our eyes are more sensitive to detecting blue light; also the sun emits more energy as blue light than violet light (NOAA NWS DOC, 2016). Humans have more difficulty seeing the violet spectrum within a rainbow; (doesn’t this make you want to go find a rainbow to stare at?)
At sunset and sunrise, the light from the sun has more atmosphere to travel through and the shorter blue wavelengths have been reflected more and more, allowing the longer red and orange wavelengths to reach the eye.
In space, where there is no atmosphere, it appears black because the light is not reflecting, or interacting with anything as it travels.
TO THE WATER!
Pour some water in a glass. Looks clear right? Well, it actually has a deep blue hue that is imperceptible at the low volume in the glass (put a larger quantity of that same water in a pool with a white bottom and the deep blue hue is readily seen). This is because of selective absorption of certain wavelengths of red in the visible spectrum, which is due to a phenomenon of highly excited vibrational transitions causing nuclear motions in the water molecules. (Check out this journal article for a thorough explanation.)
Basically, we could say that blue light penetrates water best because the red-orange spectrums that are > 550 nm are readily absorbed by water.
But what about an ocean? Sometimes waters look dark blue, or almost tinted with tropical teal, or even a hue of brown. Again, there are many variables coming into play. It depends on the localities of the source of light and the perceiver. Only from the surface, the water reflects a certain amount of light. It will mirror (to a certain extent) the light incidence upon it (think of your reflection in a calm, waveless pond; it’s recognizable but not exact, as a certain amount of your image in the water is being absorbed); so a blue sky above will have the water reflecting an amount of those blue hues. Also, the suspended particulates in the water will scatter light differently, and in fact, for the blue hues absorbed by water to be reflected back to the perceiver, suspended particulates in the water are required (Braun & Smirnov, 1993). If the water is very turbid, say from mining in an upstream location, the particulate matter will reflect back that brownish hue. Reddish water is likely due to a swarm of red algae, or certain phytoplankton or dinoflagellates (din-uh-flaj-uh-leyts), which are planktonic microorganisms; this phenomenon is often called red tide and can be harmful to life.
WHAT ABOUT THE FISHES?
Fishes see and perceive color much the same as humans and many other terrestrial vertebrates. Most fishes do see in color, and their capabilities, depending on the species, can extend into seeing past our visible spectrum such as into ultraviolet light.
Within the water, light is readily absorbed. At about 200 m, only 1% of the light has penetrated. And for humans, visible light to the human eye vanishes in the 200-800 m zone. Deepsea fishes are 15-30 times more sensitive to light and can detect light around 700 -1300 m (Helfman et al., 2009).
The light in the mesopelagic, or twilight zone, is near nil and is reflecting short blue-green wavelengths around 490 nm. Fishes living in the low-light twilight zone are rarely a bluish-green color; if they were, they would be reflecting all of that blue light and be readily seen by predators. The bathypelagic, or midnight zone, is continually dark, aside from all the creatures creating their point source bioluminescence illuminations. Thus, fishes in the mesopelagic zone will often have large eyes (averaging 50% of their head length) and sometimes tubular eyes and/or eyes that point upwards, trying to gather as much of that minuscule amount of light as possible. And bathyal fishes (those living in the bathypelagic zone) have small eyes. Why? Eyes are very expensive to operate in terms of oxygen and ATP usage (Wong-Riley, 2010). At such extreme depths, with no light, except those from point sources (such as the bioluminescent illuminations of photophores of fishes), there is no need to have large, energy-expensive eyes.
THE BASIC PROCESSES OF THE FISH’S EYE:
Light will travel through the transparent cornea and enter the eye through the pupil. The light will then travel through the lens (more spherical in fishes than in other terrestrial vertebrates) and continue through the liquid center, then through three layers of nerve cells before hitting the photoreceptor cells of the retina.
Fishes have two types of sensory cells in the retina: rods & cones.
Rods are sensitive to lower levels of light and provide low temporal and spatial resolution. These are common in deepsea, nocturnal, and crepuscular fishes (active at dawn and dusk) and these types of fishes usually have a larger rod:cone ratio. And some species only have rods.
Cones are sensitive to higher levels of light and provide high temporal and spatial resolution. Fishes utilizing cones need more intense light and so they are usually found in the epipelagic zone and in waters receiving sufficient sunlight. Fishes can see color because of different types of cones within the retina, each sensitive to different wavelengths of light and all comparing their different absorbances, relaying that information accordingly to the brain. The photoreceptive glycoproteins (opsins) in cones vary according to the type of cone, and fishes may have 2 or 3 of the following types of cones:
Porphyopsins – in cones sensitive to yellow-red light. Remember that red is a longer wavelength and it attenuates (is absorbed) more rapidly in water. These are in species inhabiting shallow, inshore, and often turbid waters, exposed to the red-yellow light before it is absorbed.
Rhodopsins – in cones sensitive to blue-green light. Species inhabiting open water and deeper depths have these types of cones. Most sharks, skates, rays, and sawfishes have rhodopsins but usually lack porphyopsins.
Chrysopsins – in cones sensitive to deep blue light. This is the type of light that penetrates furthest into water and so these cones are found in deepsea fishes.
Fishes that are capable of perceiving ultraviolet light (around 360 nm) inhabit shallow waters as UV light does not penetrate that far into water. UV vision is mediated by certain pigments in the short wavelength-sensitive group (SWS1) that absorb wavelengths at around 360 nm (Shi & Yokoyama, 2003). These fishes can have UV reflectors on their body, likely for conspecific communication (communicating with the same species), be it for predator warnings or finding a mate. This offers a sort of “secret communication channel” that other animals will be unable to see (Siebeck et al., 2010).
There is an IMMENSE amount of diversification in how fishes see; this article provided you with a basic introduction to tickle your interests.
Main reference for this article: (Helfman et al., 2009)
Braun, C. L., & Smirnov, S. N. (1993). Why is Water Blue? Journal of Chemical Education, 70(8), 612. Retrieved from http://www.dartmouth.edu/~etrnsfer/water.htm
Carnegie Library of Pittsburgh. Science and Technology Department. (1997). The Handy Science Answer Book. Visible Ink Press.
Helfman, G. S., Collette, B. B., Facey, D. E., & Bowen, B. W. (2009). The Diversity of Fishes: Biology, Evolution, and Ecology (2nd Edition ed.). Wiley-Blackwell.
Nave, C. R. (2016, December). HyperPhysics. Atlanta, Georgia. Retrieved from http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c3
NOAA NWS DOC. (2016). Why is the sky blue? Retrieved from National Weather Service Forcast Office: http://www.wrh.noaa.gov/fgz/science/skyblue.php?wfo=fgz
Shi, Y., & Yokoyama, S. (2003). Molecular analysis of the evolutionary significance of ultraviolet vision in vertebrates. PNAS, 100(14), 8308-8313. doi:10.1073/pnas.1532535100
Siebeck, E. U., Parker, N. A., Sprenger, D., Mäthger, L. M., & Wallis, G. (2010). A Species of Reef Fish that Uses Ultraviolet Patterns for Covert Face Recognition. Current Biology, 20(5), 407'410.
Wong-Riley, M. T. (2010). Energy metabolism of the visual system. HHS Author Manuscripts, 2010(2), 99-116. doi:10.2147/EB.S9078