Who doesn’t love a lovely trip to the beach? It can be wonderfully refreshing to breathe in the salty sea air, listen to the waves lapping on the shore and feel the sand between your toes. But have you ever wondered where this sand comes from?

The Earth’s beaches are estimated to contain roughly 7.5 x 1018 grains of sand (that’s five hundred quadrillions, in simpler terms). Each individual grain has a different story to tell, undergoing a unique journey across the planet that eventually ends with it settling on the shore. Most sand grains are formed by the weathering and erosion of rocks over long periods. Often, these rocks start hundreds of miles inland, slowly making their way towards the ocean via streams and rivers. As they do so, they break down into smaller and smaller pieces until they are comparable in size to a simple full stop.

But not all sand grains are formed in this process. Some are produced in a far more unusual way and require the actions of a specific tropical fish – the parrotfish.

A Taste for Algae

Parrotfish make up a family of marine fish found in coral reef habitats and on rocky coastlines, mainly in the Indo-Pacific Ocean. There are about 95 species of parrotfish – many of which are known for their striking colours. They are named after their very odd teeth, which are arranged into parrot-like beaks that extend from the external parts of the jaw bone.

This form of avian imitation is not found in any other group of fish, and it serves a particular purpose in the parrotfish’s way of life. They are strictly herbivorous fish, with algae and bacteria making up the majority of their diet. In tropical reefs, this food can be found growing on the surface of coral, and parrotfish use their highly adapted beaks to scrape the algae or bacteria away. In doing so, they sometimes ingest pieces of coral as a side portion to their main meal, with it being ground up in the throat and transferred to the intestines. The fish cannot digest rigid coral particles, meaning they are eventually excreted from their bodies at the other end. What comes out is perfectly-formed sand.

It may seem unlikely that this process can have any significant effect on beach formation. But one study found that parrotfish in Hawaii can grind up as much as 990 pounds of coral in just one year. Safe to say, this is a substantial amount (more than enough to make a good sandcastle or two). Scaled up to an entire population of parrotfish, this can have an extraordinary effect. In the Maldives, for example, it has been found that 85% of new sand produced in the reef systems surrounding the islands are formed by parrotfish. In fact, on some beaches, parrotfish poop is the main constituent. 

Coral Munchers or Reef Recyclers?

With such rampant coral consumption, it must be questioned whether parrotfish are a danger to tropical reefs – habitats that are increasingly threatened due to the negative impacts of rising sea temperatures. While many parrotfish species do not intentionally feed on live coral, some are known to be active predators of these vital reef-building organisms. However, it is thought that the benefits to the coral outweigh the costs. By removing algae and other materials from the coral’s surface, parrotfish helps promote coral growth, which would otherwise be stunted by the loss of light caused by algal accumulation.

Sadly, just like the corals themselves, parrotfish are in global decline, largely thanks to the impacts of overfishing. As the ocean begins to lose these unique fish, there are likely to be severe consequences for the health of tropical reef habitats as a whole, as well as for the great diversity of species that rely on them. If the reef systems ever disappear, we shall lose some of these sandy beaches that many of us have grown to love.

Thankfully, there is still time for changing human behaviours and increased conservation efforts to stop this from happening. But next time you’re lying on a beach in the tropics, you might want to raise a glass to the hard-working fish who helped produce it.

References

Bonaldo, R.M., Hoey, A.S. and Bellwood, D.R., 2014. The ecosystem roles of parrotfishes on tropical reefs. Oceanography and Marine Biology: An Annual Review, 52, pp.81-132.

Perry, C.T., Kench, P.S., O’Leary, M.J., Morgan, K.M. and Januchowski-Hartley, F., 2015. Linking reef ecology to island building: Parrotfish identified as major producers of island-building sediment in the Maldives. Geology, 43(6), pp.503-506.

Further Reading

https://oceana.org/blog/make-tropical-paradise-start-parrotfish-poop 

https://www.newsweek.com/where-does-sand-come-parrotfish-poop-makes-white-beaches-and-now-scientists-714024 

https://oceanservice.noaa.gov/facts/sand.html