So What Is Sand Really? Small Pieces of Stone, Shells or ... Parrotfish Poo!
Sunbathing on the tropical sands of Hawaii – pure bliss, right? Until you realise you are actually lying on parrotfish excrement. Some of that pearly white sand has passed through the intestines of a parrotfish.
There are as many as 100 different species of parrotfish. These fish mainly feed on algae, and occasionally eat small marine invertebrates, sea grasses and sponges, but parrotfish, especially the larger ones, are also very fond of corals.
Stone in...
Corals may look like colourful rocks, but they are actually colonies of small marine animals that acquire a hard exoskeleton around them as they grow. This exoskeleton is made of calcium carbonate, like the shells of mussels and sea snails. So they really are as hard as a rock.
That is why parrotfish have beak-like mouths. The "beak" actually consists of fused teeth made of fluorapatite, a material stronger than gold or iron. This means they have no difficulty crunching on that hard coral. These fish also constantly grow new teeth to replace the ones that do wear off.
... sand out
A humphead parrotfish consumes an estimated 4.5 tons of reef material or more each year, with coral accounting for more than half of its diet. So does it have large chunks of coral constantly pushing through its intestines? Absolutely not, these fish have a set of pharyngeal jaws ("jaws" at the back of the throat) which help to extract the digestible parts and grind up the rest of the material into small grains. This produces fine sand, which is then excreted by the parrotfish and left on the sea floor. And yes, then it washes up on those tropical beaches.
So is all the sand poo?
The sand on tropical beaches comes from a variety of sources and locations. For example, sand can form when rocks break down through weathering and erosion over thousands or even millions of years. Materials like quartz, the volcanic rock basalt or sea glass are also common constituents of sand. Other grains come from dead organisms, like coral, but mollusc shells and sea urchin spines are common too.
Pearly white sand
If you are in an area with coastal coral reefs, then you are probably unrolling your towel onto a bed of parrotfish droppings. It is estimated that up to 70% of the famous pearly white sands of Hawaii and the Caribbean have passed through the intestines of a parrotfish at some point. But don't let that stop you reaching for your beach towel. On the contrary, why not enjoy the fact that you can relax on a superb story of the circle of life on our wonderful planet.