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Symbiotic crabs maintain coral health by clearing sediments

Abstract

Stony corals are the foundation of coral reef ecosystems and form associations with other reef species. Many of these associations may be ecologically important and play a role in maintaining the health and diversity of reef systems, rendering it critical to understand the influence of symbiotic organisms in mediating responses to perturbation. This study demonstrates the importance of an association with trapeziid crabs in reducing adverse effects of sediments deposited on corals. In a field experiment, mortality rates of two species of branching corals were significantly lowered by the presence of crabs. All outplanted corals with crabs survived whereas 45–80% of corals without crabs died within a month. For surviving corals that lacked crabs, growth was slower and tissue bleaching and sediment load were higher. Laboratory experiments revealed that corals with crabs shed substantially more of the sediments deposited on coral surfaces, but also that crabs were most effective at removing grain sizes that were most damaging to coral tissues. The mechanism underlying this symbiotic relationship has not been recognized previously, and its role in maintaining coral health is likely to become even more critical as reefs worldwide experience increasing sedimentation.

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Acknowledgments

We thank S. A. Gravem, S. Holloway and K. Seydel for outstanding assistance in the field. This manuscript was improved by the comments of J. Stachowicz and an anonymous reviewer. Thanks to C. McKeon and G. Paulay for species identification. We gratefully acknowledge the support of the National Science Foundation (OCE 99-10677; OCE 04-17412), the W.M. Keck Foundation and the Gordon and Betty Moore Foundation. This is a contribution of the Moorea Coral Reef (MCR) LTER Site and is Contribution No. 91 of the UC Berkeley Gump Research Station.

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Correspondence to Hannah L. Stewart.

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Communicated by Biology Editor H.R. Lasker

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Stewart, H.L., Holbrook, S.J., Schmitt, R.J. et al. Symbiotic crabs maintain coral health by clearing sediments. Coral Reefs 25, 609–615 (2006). https://doi.org/10.1007/s00338-006-0132-7

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Keywords

  • Crabs
  • Corals
  • Sediment
  • Trapeziid
  • Coral bleaching
  • Symbiosis