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Presented by Charles & Linda Raabe
Mactan Island, The Philippines
© 2008 All Rights Reserved

Tiny Crabs Protect Corals

" 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. "

  It is our hope that by documenting the coral and crab species found as commensals we can help to determine which crab species should be targeted for breeding efforts. If we are to ever transplant any of the branching corals out onto the reefs as a restoration project, we will have to include a variety of coral crab species as well.

                        Specimen #1                                                                                                      Specimen #2
       Photo by Charles Raabe  Trapezia digitalis                                   Photo by Charles Raabe  Tetralia sp.

                         Specimen #3                                                                                                       Specimen #4
       Photo by Charles Raabe  Tetralia glaberrima                               Photo by Charles Raabe  Trapezia rufopunctata

Specimen #5                                                                                                        Specimen #6
         Tetralia nigrolineata                                
Tetralia cymodoce  

Specimen #7                                                                                                       Specimen #8
       Photo by Charles Raabe  Trapezia guttata                                    Photo by Charles Raabe  Trapezia serenei  

Specimen #9                                                                                                      Specimen #10
         Tetralia nigrolineata                               Jonesius triunguiculatus

Specimen #11                                                                                                      Specimen #12
       Tetralia cinctipes                                   Photo by Charles Raabe  Trapezia septata

 Cymo andreossyi : Very common and is considered a coral symbiont, with the caveat that such symbiosis are a trade off between the coral and the crab, the coral gains protection and house cleaning services while giving up a few polyps, mucus and captured/settled food particles in exhange for those services.  A healthy coral should have no problem repairing or replacing lost polyps, but again, in an aquarium environment, an eye should be kept on the coral for excessive damage being done due to the coral being unable to recover as fast or faster than the damage being done by the crab.  In short, its a judgement call that you will have to make.

Also See The Caridean Shrimp

  Related Reading
    Four New Species of Coral Crabs Discovered
    Trapeziid Crabs of New Caledonia, Eastern Australia and the Coral Sea

Identifications made by:
Sandy Trautwein                                            Peter Castro, Ph.D.
Curator of Fish and Invertebrates                      Biological Sciences Department
Aquarium of the Pacific in Long Beach Ca.          California State Polytechnic University

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