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Weird Science: Parrotfish and Sand

NGSS Crosscutting Concepts

Approximately 90 different species of parrotfish (family Scaridae) are found throughout the world in shallow subtropical and tropical oceans. They are mostly herbivorous grazers. Parrotfish and other reef herbivores help to maintain the structure and function of coral reefs by grazing algae that might smother coral.


Parrotfish have unique teeth that allow them to play an integral role on coral reefs, especially in the Caribbean and Indo-Pacific. Parrotfish teeth are fused together (SF Fig. 5.4 A). This fusion enables them to scrape off fine, filamentous algae that grow within coral skeletons and on dead coral substrates. During this scraping while foraging for algae, parrotfish often ingest chunks of coral. Parrotfish have a special set of toothplates in their throat called a pharyngeal mill (pharyngeal = relating to the pharynx or throat) that acts like additional teeth to assist in grinding up the hard coral. The ground-up coral is passed through their digestive tract and expelled as fine-grain white sand (SF Fig. 5.4 B). The sand that is produced by parrotfish forms beaches and provides valuable habitat for benthic organisms like crabs and shrimp.


SF Fig. 5.4. (A) Close up of parrotfish teeth (Chlorurus microrhinos).

Image courtesy of Nhobgood, from Wikipedia. CC by SA-3.0

SF Fig. 5.4. (B) Parrotfish expelling fine-grain coral sand on a tropical reef at Palmyra Atoll. Palmyra Atoll is in the center of the Pacific ocean basin.

Image by Ingrid Knapp

Scientists estimate that up to 70% of the sand on white sandy beaches in the Caribbean and Hawai‘i has been excreted by parrotfish. A large adult parrotfish can excrete over a ton of sand per year. Areas that have parrotfish produce more sand than similar areas with few or no parrotfish. This sediment production rate is especially important in areas like Hawai‘i, where there is little terrestrial input, and almost all of the sand is of biogenic origin.

Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.