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By Charles & Linda Raabe
Mactan Island, The Philippines

© 2009  ALL RIGHTS RESERVED

  I have never found a crab yet that has hitchhiked into my tank that I would consider reef safe. The only crabs that I consider to be truly reef safe are the commensal types such as the porcelain crab and coral crabs. The only reason such crabs can be considered reef safe is that they never leave their hosting spot. The hermit crabs that are intentionally introduced to our tanks are not to be trusted or considered reef safe either, all crabs are omnivores and will eat what ever they can. While most corals are safe from hermit types, other inverts and fleshy polyp corals are not.

CRAB ARTICLES


Photo by Charles Raabe  Photo by Charles Raabe5
A Female crab (left) and a Male crab (right)


    The Xanthidae Crabs  (link - extensive photo gallery)  None of which can be considered reef safe and can be the most destructive of the crab types. While very small, they seem to pick at algae on the rocks, but just as with other crabs species, once they gain a bit of size, their ability to pick off and eat any number of invertebrates and corals also increases. They also have a bad habit of enlarging their hiding places within live rock and will, in due time, weaken the rock considerably.
      Below, a commonly found Xanthid species living within live rock.
   Destructive xanthid rock crab  Photo by Charles Raabe  Photo by Charles Raabe
      Below, yet another rock living Xanthid species.
   Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe
    Below is a Xanthid species found on Acropora corals and assume they prey upon the corals polyps or tissue.
   Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe
      Various other Xanthid species.
   Another Xanthid type, NOT reef safe!  Photo by Charles Raabe  Photo by Charles Raabe
   Another Xanthid type...outta the pool!!!  Destructive Xanthid type crab, outta the pool!!!


      The  Hermit Crabs     Hermit ID site          Paguritta Hermit Crab    
   Cute for a crab, but still a crab!  Filter feeding Hermit Crab, will not leave its burrow and is reef safe


      The  Mithrax Crabs  ( Cling Crabs)   None of which I would consider to be reef safe, members of this family include the popular "emerald" crabs that are used for algae control, which may be fine for awhile, but with all crabs being omnivores, it is usually only a matter of time before these crabs discover that there are meatier items to be had within your aquarium.
   Gets large and is destructive  Not Reef Safe! 
   Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe
   Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe
   Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe


    The Porcellanidae Crabs   Photo Link #2   Photo Link #3   Can be considered Reef Aquarium Safe
   Commensal with anemone, not found as a hitch hiker  Photo by Charles Raabe  Photo by Charles Raabe
   Photo by Charles Raabe  Photo by Charles Raabe  Photo Credit: by TSR770
   Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe
     The Anomuran crabs, part of the Porcellanidae family, relatives of Petrolisthes and Porcellanella.
   Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe
       Petrolisthes species
   Photo by Charles Raabe  Photo by Charles Raabe


      The Galatheid Crabs - Also known as squat lobsters for their appearing very lobster like. All of the species that I have found on live rock have been very small and seem to be somewhat harmless, although they may feed upon small sessile invertebrates.
   Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe
   Photo Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe
   Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe

      The Allogalatheid Crabs -  also known as squat lobsters but only lives on crinoids. Color patterns usually consists of alternating dark & light colored bands with white leg tips & claws; the colors always match the host crinoid. They are klepto-parasites that don't directly harm the host but steal its food.
               Allogalathea elegans
   Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe




     The Dromiidae Crabs - Specializes in carrying sponges and even zoanthids as a means of camouflage, it does so by means of its last set of legs which are modified to grasp onto its captive audience. I have had this specimen drop its sponge and snip out a square inch section of zoanthids off of their rock and form the zoanthids into the same shape as the sponge pictured below. I have seen others on the reef slopes carrying about both sponges and zoanthids in this manner.
   Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe
   Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe


      The Portunidae ( Swimming Crabs) Note the paddle shaped last pair of legs.
   Photo by Charles Raabe   Photo by Charles Raabe   Photo by Charles Raabe

   Photo by Charles Raabe   Photo by Charles Raabe   Photo by Charles Raabe
    Below:  A Lissocarcinus laevis, a symbiont of sea anemone.
   Photo by Charles Raabe   Photo by Charles Raabe   Photo by Charles Raabe
     Below:  Various other species
   Photo by Charles Raabe   Photo by Charles Raabe


      The Majoidea SuperFamily  ( spider crabs / decorator crabs)
   Photo by Charles Raabe   Photo by Charles Raabe   Photo by Charles Raabe
      Below:  Xenocarcinus tuberculatus   A member of the Pisidae family
   Photo by Charles Raabe   Photo by Charles Raabe   Photo by Charles Raabe

           Below:   Achaeus japonicus
         

      Below: Various other unknown species
   Photo by Charles Raabe   Photo by Charles Raabe   Photo by Charles Raabe
   Photo by Charles Raabe   Photo by Charles Raabe   


      The Parthenopidae Family, commonly called the elbow crabs. Below is a possible Daldorfia horrida.
   Photo by Charles Raabe   Photo by Charles Raabe   Photo by Charles Raabe
       Below, possibly a juvenile Daldorfia glasselli.
   
Photo by Charles Raabe   Photo by Charles Raabe   Photo by Charles Raabe


      The Calappa Crabs  ( box crabs )    
                    Gravid Female                                                  Ready to hatch eggs                                            Two day old Larvae
   Photo by Charles Raabe   Photo by Charles Raabe   Photo by Charles Raabe


      The Leucosiidae Crabs  ( Pebble Crabs)
            Below, a member of the Leucosia genus
   Photo by Charles Raabe   Photo by Charles Raabe   Photo by Charles Raabe


       
The Cryptochirid Crabs  ( Gall Crabs )
   Photo by Charles Raabe   Photo by Charles Raabe   Photo by Charles Raabe

       
The Pinnotheridae Crabs  ( Pea Crabs)  -  Pea crabs are commensals that associate with various echinoderms, molluscs, polychaetes, brachiopods, etc.  Some are external while many of them are internal living inside their hosts' gill chambers or body cavities.  (photos by Beth LeBlanc)
   Photo by Beth LeBlanc   Photo by Beth LeBlanc
   
         
      The Stenorhynchus Crabs  ( arrow crabs )  Normally not considered reef aquarium safe
   Not normaly considered reef safe   Photo by Charles Raabe
 


Presented by Charles & Linda Raabe
Mactan Island, The Philippines
© 2009 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.

                                                             Click Links to view photographic details

                   Specimen #1                                        Specimen #2                                          Specimen #3      
  Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe
               Trapezia digitalis                                     Tetralia sp.                                    Tetralia glaberrima  

                   Specimen #4                                         Specimen #5                                         Specimen #6
  Photo by Charles Raabe    
         
 Trapezia rufopunctata                          Tetralia nigrolineata                           Tetralia cymodoce  
                                                                                     
                    Specimen #7                                        Specimen #8                                       Specimen #9      
  Photo by Charles Raabe  Photo by Charles Raabe  
             
 Trapezia guttata                                   Trapezia serenei                              Tetralia nigrolineata

   
               Specimen #10                                      Specimen #11                                      Specimen #12
     Photo by Charles Raabe
           Jonesius triunguiculatus                         Tetralia cinctipes                                  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.




    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



Download a .pdf version of the publication shown below

Every Crab in the World
(click above link)


 


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