The marine realm is one of the most colourful and intriguing of worlds. From translucent jellyfish and rainbow-like parrotfish to the blue-green sea, all the colours in the marine environment (much like all of nature) are a play of light and pigments. We know what makes the oceans blue, but why is bioluminescence blue? Why are deep sea fish mostly red or dull, while coral reef fish come in every colour imaginable?
Sunlight — or white light — is a combination of seven colours (remember VIBGYOR?), and when it splits into its constituent colours and spectra (like visible and UV rays), visual magic happens. Rainbows form when the sun’s rays (which contain UV, visible, and infrared light) are scattered by raindrops (refraction). But when light passes through the ocean, the blue and green rays scatter more than the rest due to their shorter wavelengths, creating the blue of the ocean.
When observing the ocean from the outside, there are variations in the shades of blues and greens. This is primarily due to differences in depths, which affect the scattering and absorption of light, the presence of phytoplankton or algal blooms, vegetation growth, and substrate composition (like sand).
While coral reef denizens are brightly coloured, deep-sea animals have reddish-orange bodies that almost blend in with the darkness or are completely transparent so that all light passes right through them. Even deeper, as red and orange light fade completely, red-coloured animals become invisible in one sense, as there is no red light to reflect off them, making it difficult for predators to detect them.
Human eyes typically have light receptor cells called rods, and red, green, and blue colour receptor cells called cones. Cones enable us to see colours in the visible spectrum (~380-700 nm), and thus, we can see the biofluorescence of the branching coral, Acropora sp., in the lead photo but not the ultraviolet (UV) light shining on it.
While colours, fluorescence, and bioluminescence are all functions of light, pigments, and genetics, there is an evolutionary history behind why marine organisms have these adaptations and what purposes they serve. There are entire spectra of light that human eyes cannot see, but marine life can. For instance, a mantis shrimp can see UV and polarised light in addition to the visible spectrum!
The blue sea
Blue light (followed by green) has the ideal wavelength and energy to travel furthest in water; thus, we see the sea as blue. Other light, at either end of the spectrum (red and violet), is either absorbed or filtered out, giving the ocean its much-loved blues, sea greens, and aquamarines. Photo: Dhritiman Mukherjee.
Cover photo: Umeed Mistry
Invisible deep-sea animals
Beyond the first 10-15 metres depth in the ocean, red light fades first, followed by orange, yellow, and then green (reverse VIBGYOR). Marine life has evolved to use this to its advantage. The most relatable example to illustrate this, especially for divers, is how the red colour of fins and dive gear starts fading as we descend. Many marine organisms are translucent, allowing light to pass through them to avoid predation. Interestingly, these translucent animals are also perfect hosts for algae that utilise the light passing through, or mask their see-through bodies against corals. Examples include (1) jellyfish, which are 95 per cent water and may contain photosynthetic algae; (2) the pink-eyed goby (Bryaninops sp.), which is known to share a commensal relationship with soft corals; and some (3) shrimps that evade predators by being mostly immobile on corals. Video: Neil Shah; photos: Dhritiman Mukherjee.
The dark ones
Some deep-sea animals are nearly black and blend in with the darkness — their colour acting like invisibility cloaks. Due to impenetrable darkness at greater depths, most marine animals there have lost the ability to detect red light or see at all, relying instead on their other senses (that are incredibly sharp) to compensate for their lack of sight. Imagine a sharp-toothed, large-jawed predator sneaking up on you undetected because it has the superpower of invisibility. That is just a regular weekday in the deep blue. Shallow water predators like this moray eel also sport patterns and colours that equip it for a sneak attack by hiding in dark crevices. Photo: Dhritiman Mukherjee
Multicoloured reef life
A combination of genetics, pigments in the skin (or scales), and colours imparted due to the physical structure of scales make coral reef fish some of the most colourful and vibrant creatures in the marine ecosystem. Their patterns and markings serve multiple purposes, including intimidation, defence, camouflage (against equally colourful reefs), attracting a mate, and recognising their own species from others.
Many reef fish are brightly coloured due to the presence of chromatophores — specialised pigment-containing cells. These are the same cells found in animals like chameleons, which allow them to change colour according to their surroundings and stimuli.
Fish sometimes take “you are what you eat” quite literally — their diet affects the vibrance and intensity of their colours. Other factors that can affect the appearance of a fish include age, gender, health, environment, stress, social cues, and injury or trauma — all of which also affect how we humans look. Photos: Dhritiman Mukherjee
Psychedelic corals
Corals are an algae-animal symbiotic complex, and most of their colour comes from the photosynthetic microalgae (zooxanthellae) — like the colours of (1) these branching corals Acropora sp. Sensitive to light and temperature fluctuations, zooxanthellae respond by increasing or decreasing in number, thus making the coral appear darker or lighter in colour or by leaving the coral altogether – (2) coral bleaching like this branching coral Acropora sp. However, the deeper shades and fluorescent colours of the coral structure come from the coral (animal) itself and can be viewed under blue light. Corals contain two kinds of pigment proteins that give them a wider range of colours – chromoproteins and fluorescent pigment proteins. Chromoproteins (colour-producing proteins) absorb visible light to give strong colours like red, blue, and purple. A common example of a chromoprotein is haemoglobin, the pigment that gives blood its intense red colour. Photos: Evan Nazareth (1), Vardhan Patankar (2)
Sea sparkle
Bioluminescence is the inherent ability of an organism to produce its own light through a chemical reaction. Interestingly, most of the bioluminescence on the planet is observed in marine ecosystems, especially in deeper waters. Bioluminescence on land is much rarer (glow worms, fireflies, some mushrooms) and is nearly non-existent around freshwater bodies.
Bioluminescence occurs when a light-emitting molecule (luciferin) reacts with an enzyme (luciferase), and the energy from that reaction is emitted as light. Bioluminescent light is “cold” because the reaction does not generate much heat and is typically blue-green since these colours travel furthest in water.
Organisms exhibit bioluminescence in different ways for different purposes. Microalgae on the surface give off “sparks” if the water is agitated, while vampire squids release a cloud of blinding bioluminescent ink in defence. Anglerfish take it to a whole new level, with females using the bioluminescent lures on their heads to attract prey. These lures glow blue due to symbiotic algae present in the little sac (esca) at the tip of the lure. Interestingly, the same lure acts as a homing beacon for male anglerfish to find their mate! Photo: Sarang Naik.
The mystery of many colours
Animals such as the mantis shrimp have evolved to see multiple light spectra within the ocean and are less limited in their ability to see colours. Their colour vision is four times more complex than ours, and they can see 12 channels of colour compared to the three channels (RGB – red, green, blue) through which we see the world.
Scientists have managed to unearth some of the colour-coded mysteries of the sea in recent decades, but a lot more remains to be explained and understood. However, technology is catching up, and there are sensors that can detect and record colours far beyond what the human eye can see. Until that translates into James Bond-style wearable tech-spectacles, I am happy with the knowledge that mantis shrimps enjoy better views than most living creatures. Photo: Umeed Mistry
About the contributor
Phalguni Ranjan
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