That beautiful white sandy beach? Parrot fish poop. It's true - particularly on sandy coral atolls nearly all the sand you find is actually the product of parrotfish chomping away at algae and coral around the atoll. For this reason, we don't often keep parrotfish in our reef tanks - we like to enjoy the coral ourselves, and a 100 lb parrot fish might also be a bit too large for most of our tanks.

Parrot fish poop sand! They break down coral into small grains, and then deposit the.. remains.. where it eventually makes its way back to shore. This is how over 85% of the sand in some atolls in the Maldives is made. Urchins and halimeda do contribute some as well, but the vast majority is created by the parrotfish!

Kyle M. Morgan, Paul S. Kench,
Parrotfish erosion underpins reef growth, sand talus development and island building in the Maldives,
Sedimentary Geology, Volume 341, 2016, Pages 50-57, ISSN 0037-0738,
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Abstract
Parrotfish play a key functional role on coral reefs as external bioerosion agents and produce large quantities of carbonate sediment as a by-product of grazing on reef surfaces. Parrotfish are therefore an important potential source of sediment for island construction and maintenance within atoll reef environments, particularly under future scenarios of sea level rise and island morphological change. Here, we present the first field-based estimates of excavating parrotfish erosion (Chlorurus sordidus and Chlorurus strongylocephalus) within the Indian Ocean and quantify the contribution of parrotfish to the carbonate and sediment budgets of an atoll interior reef platform in the Maldives. We note that parrotfish erosion rates are high (6.3kgm−2 y−1), generating large amounts of new coral-based sediment (2.6kgm−2 y−1) that has a comparable grain size distribution to island deposits. Mean erosion rates by individual C. strongylocephalus (405kg individual y−1) were higher than C. sordidus (55kg individual y−1), but their contribution to erosion per unit area of reef was less due to a lower relative biomass (C. strongylocephalus: 1.3kgm−2 y−1; C. sordidus: 5.0kgm−2 y−1). Parrotfish also facilitate sediment export from reefs (0.7kgm−2 y−1), which contributes extensively to the development of the sand talus on the fore-reef slope and to the evolution of the wider atoll basin. Our results provide strong evidence that parrotfish erosion (and sediment generation) underpins island morphology on Maldivian reefs and highlight the importance of larger parrotfish as producers of island-grade sediment. Ecological processes must therefore be considered within future coastal management strategies for enhancing island stability.

Urchins, and Halimeda also contribute quite a bit to sands!
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