Why parrotfish are important to corals

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A study to be published in a forthcoming issue of the journal Biology Letters has found that parrotfishes not only graze on coral, they can also help disperse the symbiotic algae (zooxanthellae) that live in them.

Colombian scientists Carolina Castro-Sanguino and Juan Sánchez investigated the stoplight parrotfish (Sparisoma viride) as a vector for the zooxanthellae Symbiodinium by quantifying the grazed substrate, the dispersal area, and the genetic diversity and viability of Symbiodinium-cell densities extruded in faeces.

Working in three sites in the Caribbean Colombian coast, the authors observed individual parrotfishes for 5- and 15-minute periods, during which the number of bites, substrate grazed, the number of faeces released, and the substrate type where they were deposited were recorded.

The distances between the areas grazed and where the faeces were deposited were also measured.

The faeces were collected, after which DNA was extracted from them and positive cultures obtained.

The authors found Symbiodinium cells in 63 of 68 samples of parrotfish faeces studied, providing compelling evidence for the role of parrotfishes as vectors for zooxanthellae in coral reefs. The Symbiodinium found were in various reproductive stages, suggesting that they were viable. The faeces were also dispersed up to an area of 190 square metres within the grazing sites.

Although the average densities of Symbiodinium in parrotfish faeces were not significantly different from those identified in sediments, significantly greater concentrations were found in individual faecal samples, suggesting that fishes carry significantly higher levels of Symbiodinium and that the continuous deposition of faeces on the substrate constitutes an important source of free-living cells that could be used to seed new coral growth.

The authors note that spearfishing has resulted in reduced population sizes of the Stoplight parrotfish at one study site (Isla Fuerte), where the lowest Symbiodinium counts were detected, and that the Symbiodinium density is the greatest at depths where parrotfishes are usually more abundant. Together with the results of this study, this points to the importance of parrotfish in maintaining environmental reservoirs of zooxanthellae.

For more information, see the paper: Castro-Sanguino, C and JA Sánchez (2011) Dispersal of Symbiodinium by the stoplight parrotfish Sparisoma viride. Biology Letters doi:10.1098/rsbl.2011.0836

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