More Than Just a Pretty Face

Not all sand is created equal. Depending on the location of the beach or dessert, it can be made from a wide variety of materials and in as many different processes. Sand can be made of quartz, basalt, garnet, silica, olivine, glass, coral, and more. It can be made from wind or water currents eroding large rocks down, wave energy pounding the coastline, volcanic eruptions, and so on. One interesting way, my personal favorite, is a bit more unusual. In tropical locations with nearby coral reefs, a special family of fish have been found to be major contributors to the production of sand. This family of fish, known as parrotfish in plain English (Scaridae in scientific classification), are more than just brightly colored and patterned beauties. They have highly specialized teeth (pictured below) that fuse together at the front of their mouths to form a beak like structure (hence their name).

Since it is so strong, it allows them to eat in a unique way. Many parrotfish eat by scraping or outright biting the coral on their reef habitat. They do this to get at algae growing among the coral or to eat the soft parts and symbiotic algae within the coral itself. This also means that they ingest a heck of a lot of hard, calcareous coral skeleton. After they extract the nutrients they want, their digestive tracks excrete the skeletons out as finely ground powder. That’s right folks. You read exactly what you thought you just read. Some of the nicest, softest, tropical sand is in fact parrotfish poop. Hopefully that doesn’t put you off your next vacation! Because the reality is, we are lucky to have that sand.

Sand
Close up of coral sand. PC: Siim Sepp

It turns out that not only do parrotfish help produce A LOT of sand for beaches, but a 2015 study found that they also help build up entire islands. In the Indo-Pacific region, islands like the Maldives are made up entirely from packed sediment from coral reefs. Until recently, how that sediment was made was not well studied however. Where was it coming from? How was it produced? There were guesses and assumptions, but not much in the way of actual data. By looking at what materials made up the sediment on the Maldives island of Vakkaru, the researchers attempted to learn more about those questions. They organized the sand by particle size, where on the surrounding reefs the sand came from, how quickly the sand is produced, and how many animals that are known to generate sand (like parrotfish) are in the area. What they found was that more than 85% (685,000 kg a year!) of the sand making up the structure of the island was the exact size of sand that is known to be produced by parrotfish. Also, ~75% of the sand making up the island came from barely 21% of the surrounding coral reef (a thin outer reef flat facing the open ocean, away from the island). What they didn’t find very much of though was sand smaller than the known parrotfish size. The researchers concluded that this was due to the strong offshore currents surrounding the island. Basically, as the parrotfish make new sand, the old broken-down sand was being washed away.

Soooo, the big question. What does all that mean? Why is any of this important? Well, it means that the populations of parrotfish and their reef habitat are extremely important for the survival of low lying islands and tropical coastal beaches. In these locations, waves and water currents are not making of sand. Instead, they are removing it. So for these places to continue to exist, the parrotfish and the coral reefs they live on and convert into sand need to be healthy enough to do their ecological job. With the warming and rising of ocean water (presenting a whole host of other problems that would take several blog posts to cover alone) the parrotfishes’ role may become even more critical for the survival of these places. If they are over fished and/or their reefs are destroyed, the islands and beaches they surround might disappear too. Granted, each island is unique, and this also does not account for those islands with volcanic activity. However, considering that over 2.3 million people live in these vulnerable regions, any threat could be catastrophic from more than an ecological standpoint and bears taking notice. Taking care of these fish and their reef habitat just makes sense. It doesn’t have to be complicated either. It can be as simple as when on vacation, be respectful of the reefs. Don’t touch or stand on the coral, dispose of trash properly, and don’t eat them if you don’t know how they were caught or if their populations are at healthy levels. Simple steps like these can go a long way!

So the next time you are relaxing under a palm tree, toes in the sand, gazing out at turquoise waves with the hazy shapes of underwater reef structures, thank a parrotfish! They made that Instagram worthy moment possible, free of charge 😉

 

Perry, C., Kench, P., O’Leary, M., Morgan, K., & Januchowski-Hartley, F. (2015, June). Linking reef ecology to island building: parrotfish identified as major producers of island-building sediment in the Maldives. Geology, pp. 43(6) 503 – 506.

 

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