Sparisoma viride (Bonnaterre), the Stoplight Parrotfish, a Major Sediment Producer on Coral Reefs of Bermuda

Gygi, Reinhart

A constantly regrowing dentition enables S. viride to erode and triturate reef rock or calcarenite of any hardness. Smooth-scoured surfaces of subtidal calcarenite and algal-vermetid rock off Bermuda's wave-swept south shore appear to be largely the result of abrasion by grazing herbivorous or omnivorous fish. The erosion rate was estimated in Gravelly Bay on the south shore where S. viride feed almost exclusively on algae. There most of the algae are filamentous and form a short-cut overgrowth on hard calcareous substrates of calcarenite or biolithite. The fish scrape away algae and some of the rock below with strong, beak-like jaws, and crush the mouthful in the pharyngeal mill. Swallowed rock bits are used as a milling agent to rip up algal cells. They outweigh nutrients in the digestive system about 3 to 1 (dry weight), and make up 4% of the fishes' wet body weight. Grain sizes range from less than a micron to 5 mm. If the excreted sediment were deposited on an area equal to the area of erosion, then 2.1 metric tons or 0.12 mm of sediment with a porosity of 40% were laid down per hectar per year, which is equivalent to 1.2 m of uncompacted sediment per 10,000 years or about 100m of limestone per one million years. This rate of a single scarid species indicates that parrotfish are a geologically significant agent in the transformation of reefs into sediment. S. viride prefer agitated water near exposed reefs and shorelines. There the proportion of reworked sediment to detritus from erosion is lowest in the excrements. The erosion rate calculated for Gravelly Bay therefore appears to be above average to high. Boring by the sponge Cliona tampa under optimal conditions may proceed at a hundred times the maximum abrasion rate of S. vir ide. Fish erosion and sedimentation are conspicuous, but in Bermuda possibly of less quantitative importance than erosion and sedimentation by subtidal borers and etching by algal rhizoids. The Rauracian Reef complex (Upper Jurassic) of northwestern Switzerland is in many respects similar to Bermuda. It is conceivable that fish of the genera Mesodon and Microdon (family Pycnodontidae) on these reefs had an ecologic position comparable to recent Scaridae. However, their capacity in the production of reef-derived sediment must have been inferior to that of modern scarids.

Sparisoma viride (Bonnaterre), the Stoplight Parrotfish, a Major Sediment Producer on Coral Reefs of Bermuda

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