Light and Colour under the Sea

In the International Year of the Reef 2018, Dr Anjani Ganase speculates how the loss of colour (through bleaching) on coral reefs might affect resident fish populations. Follow IYOR-T&T (@IYORTT) and AnjGanase (@AnjGanase) on twitter 

Have a look around, and see if you can distinguish the shades and intensities of colours that surround you in different levels of light. Remind yourself how vulnerable you feel in the dark. We have evolved sensitivity to colour and intensities of light, a quality crucial for survival. In order to forage for food efficiently, primates must distinguish between red and green colouration, so that in the forests we can pick out the ripe fruits, and also identify any danger lurking among the foliage (Gerl and Morris 2008). For many animals, colour along with other cues, assisted in mating; consider the plumage display of male birds or changes that indicate the females are fertile. For modern man, not being able to colour coordinate may not mean life and death but it may lower your appeal. Colour distinctions also vary considerably among species, among habitats and even between sexes. Many women are genetically able to distinguish more colours than males, and this is why more women have a better eye for colour details; colour blindness is more common in men (Gerl and Morris 2008). 

The diver hovers in the blue over a coral reef dominant in colours of yellow and brown in Curaçao. Photo by Anjani Ganase

In the marine environment, being able to distinguish colour is just as important, especially as water does funny things to sunlight. Natural (white) light is made up of colours of different frequencies (and energy levels). When light passes through water in the atmosphere, the different colours are dispersed in different directions and a rainbow is the result. When light travels through the water column of the ocean, here’s what happens: red light, the lowest energy, gets absorbed in the upper 20 m of the water column; higher energy green penetrates deeper; but blue light with the highest energy penetrates deepest. This is why the deeper you descend, the bluer the marine world appears. Eventually, all the light gets absorbed (at or beyond 150 m) and the marine world goes dark. Apart from the visible light, high energy UV light also penetrates the water column. This is the light that causes your skin to tan and burn and why we use sunblock. The perception of changes in the spectrum of light and its intensity with depth may be crucial for the survival of marine organisms, allowing them to feed without getting eaten at the same time. Variations in light intensity and colour with depth may limit their range. In addition, the changes in the coloration of a marine organism with depth will determine whether it stands out or blends in: a Red Snapper will appear black at greater depths where it can hunt in sleuth mode.

School of French Grunts blending together as a form of camouflage. Photo by Anjani Ganase

On shallow, brightly lit coral reefs the vibrant colours and patterns are alluring to divers and snorkelers. However, our biased visualisation of the reefs differs from how reef fish, or squid or a mantis shrimp might view the reef world and has limited our understanding of the many uses for light by marine organisms on coral reefs. For the residents of coral reefs, the use of specific colours, patterns, intensities and even UV and polarised light is strategic. Many inhabitants of coral reefs are capable of visualising UV (ultra violet) light. Markings on fish that are only visible under UV light allow for secret underwater communication and recognition among fish of the same species without notifying predators and competitors (Siebeck et al 2010). Other fish can differentiate polarised light, to hunt for transparent more elusive prey in the water column.

The colorations of the fish and other invertebrates are used to disguise, distract predators and camouflage themselves (Marshall 1998). Classic cryptic camouflaging of reef fish and other organisms is common: coloration and texture allow fish to be almost indistinguishable from the substrate, coral or algae. There is the contrasting strategy: to stand out from the crowd by displaying colours that spell danger, such as the blue ringed octopus, and some nudibranchs with colours that repel predators (Marshall 1998). Shiny silvery bodies reflect the ocean blue and allow fish to blend into their surroundings. Blue and yellow - highly contrasting colours - are often found in varying patterns on reef fish and the contrasting effects are often preserved with depth (Marshal 2000). Through the eyes of a fish, the yellow colour blends better with the reef background, while blue blends better with the water column; up close the fish may be striking; but at a distance and especially when close to the reef-water interface these patterns may actually work as a disruptive camouflage (Marshall 2000). The distinct blue and yellow patterns blur or break up how the fish are seen against the branching and complex background of the coral reef framework. This is similar to how the stripes of zebras grouped tightly together may confuse predators which may be unable to pick an individual out. Other colouration strategies include the placement of an eye pattern on the tail of a fish confusing the predator about the direction the fish is facing (Marshall 1998). The behaviour of the fish based on their colouration also governs the degree to which they are seen by predators and mates. 

The contrasting colours of a blue ocean and a yellow reef evolutionary influenced the colouration of reef fish.  Photo by Anjani Ganase

Now imagine a world where the colour of your habitat has been stripped away. Coral reef fish and invertebrates, if not impacted directly by warming water of climate change, can suffer the after effect of having their coral homes bleach. Coral-associated fish that camouflage on coral-dominant reefs are more likely to be attacked and eaten by predators as they stand out against the white background of bleached or deceased corals (Coker et al 2009). If bleaching continues to be sustained on reefs and they transition to algae or soft coral dominant reefs, we may expect a corresponding loss in coral dependent fish communities and transition to fish communities with different colourations, more reliant on macroalgae and soft corals. 

References

https://www.whoi.edu/oceanus/feature/shedding-light-on-light-in-the-ocean

Coker, Darren J., Morgan S. Pratchett, and Philip L. Munday. "Coral bleaching and habitat degradation increase susceptibility to predation for coral-dwelling fishes." Behavioral Ecology 20.6 (2009): 1204-1210.

Gerl, E.J. & Morris, M.R. Evo Edu Outreach (2008) 1: 476. https://doi.org/10.1007/s12052-008-0088-x

Marshall N.J (1998) Why are reef fish so colorful? Scientific American. 54-57

Marshall, N J. “Communication and Camouflage with the Same ‘Bright’ Colours in Reef Fishes.” Philosophical Transactions of the Royal Society B: Biological Sciences 355.1401 (2000): 1243–1248. Print.

Siebeck, Ulrike E., et al. "A species of reef fish that uses ultraviolet patterns for covert face recognition." Current Biology 20.5 (2010): 407-410