Re: Why are coral reefs so colorful?

Hi Pamela,

A good question - it's something everyone 'knows', but isn't so obvious 
when you start to think about it. 

Basically, I think the difference in 'colourfulness' between oceanic and 
freshwaters comes down to 3 main factors - age, depth and size - and I 
shall cover these in turn;

1. Age. Oceanic environments are much more ancient than freshwater ones 
and so have had more time to evolve a great diversity of species - oceans 
are more diverse in form and richness of species as well as just colour. 
Although oceans change slowly along with continental drift and the 
shifting of the sea floor, lakes and rivers change much more quickly. It 
is only the large, deep, ancient lakes such as Lake Turkana (E. Africa) 
and huge rivers like the Amazon which are also very diverse. In these 
cases, they can also be colourful as seen with the array of cichlid fish 
species found in some African lakes. Few freshwater habitats last long 
enough to create huge complex reefs like the Great Barrier Reef. Also, 
many phyla evolved in the oceans and so many phyla remain marine. Thus, 
with most phyla being marine, the diversity of life (and hence the 
opportunity for colourful ariety) is much greater.

2. Depth. Most freshwater habitats are relatively shallow. This means that 
sunlight penetrates to the bottom as so there are green plants and algae 
using this energy for photosynthesis. In the sea, much of this 
photosynthesis is done by plant plankton floating near the surface. Below 
them, green chlorophyll is not so useful, therefore other colours are 
found whereas in a river there would be mostly green submerged vegetation. 
However, below a few tens (a few hundred at most) of metres, marine 
habitats become much less colourful as sunlight fails to penetrate and 
colour is more a matter of camouflage than display, communication or light-
gathering. Also, salinity plays a part as few green plants are salt 
tolerant as well as being able to remain submerged. Thus, they give way to 
the greens, reds, purples, yellows and browns of algae such as seaweeds.

3. Size. The oceans are huge and interconnected, so few marine habitats 
are isolated and there are a huge number of niches for species to adapt 
to. Along with the age of the oceans, this means that a great dversity of 
species have evolved, and again this includes diversity of colour. Rivers 
and lakes are relatively small and in most cases, relatively isolated, 
much like aquatic 'islands' in a terrestrial landscape. This means they 
are likely to be less diverse, with large isolated lakes drawing their 
diversity from a few taxonomic groups that have become isolated and then 
speciated to create endemic forms. Again, this is seen with African 
cichlid in much the same way as Darwin's finches evolved on the Galapagos 
Islands. Larger rivers like the Amazon may be more diverse as they draw 
their biota from a large area as they run through huge areas of other 
habitats. They may also be colourful as seen with the variety of 
iridescent piranhas, to give just one example.

So, several factors interact to cause the colourful nature of some marine 
habitats, but remember that these colourful areas are in a minority - the 
large, famous coral reefs are in shallow tropical areas where the sun's 
energy is concentrated and fuels algal photosynthesis, raising the level 
of primary production, and so fuelling a food chain that leads to high 
levels of biomass and greta diversity. Many other marine habitats are not 
so heavily populated, or in deep water, do not show their diversity in 
terms of colour, because there is little or no light. Also, there are 
exception in the form of clourful river and lake habitats, and these occur 
where the factors leading to marine diversity are also present in 
freshwater.

I hope that answers your question,

Dr David Hubble, UK