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Get crabby! A coral’s guide to self-defense

McKeon, CS., Moore, JM., (2014).  Species and size diversity in protective services offered by coral guard-crabs.  Peer J:e574. DOI: 10.7717/peerj.574

A crab guarding its host coral.

A crab guarding its host Pocilloporid coral. (Photo credit: Seabird McKeon)

Life can be tough if you are a coral of the Pocillopora variety living near Mo’orea, an island in French Polynesia. Chances are you settled in a nice community with plenty of other Pocilloporid corals, and now you all need to worry about being eaten by a whole suite of coral specialists. A smart coral like yourself knows that mutualistic relationships can significantly increase your chance of survival–a mutualistic relationship with algae already helps supplement your diet.  Lucky for you, there are several options for mutualism that can bulk up your physical protection. And for the small price of allowing a few polyp tips to be grazed, you would be crazy not to host some crabby bodyguards, usually in mating pairs, to fight off hungry predators. Crabs in the genus Trapezia come in many sizes and species.  They are not only good housekeepers, clearing sediment that might otherwise smother you, they are also great defenders.  While just the presence of a symbiont (organisms that live and interact in close physical association with each other) can increase your chance of survival, are different species more effective than others? Find out in ‘The Guide to Self-Defense by Crab’.

McKeon & Moore tested the defensive services of four Trapezia species to host corals against three species of corallivores (coral eaters) both in the lab and in the field. Here’s how:

In the lab
Different size classes of Pocillopora corals armed with mating pairs of three different size classes of four Trapezia crab species were thrown into the ring, in this case, aquaria or pools with seawater flow.  Then, two species of coral-eating jerks; cushion sea stars (Culcita novaeguineae) and horn drupe snails (Drupella cornus), that had been starved for 48-72 hours were unleashed. These predators are among the most common on Indo-Pacific reefs, terrorizing corals in the middle of the night and leaving scars on their victims the next day.  The same experiments were then repeated without any defending crabs.

 In the field
Between 2008-2009, there was an outbreak of crown-of-thorns seastar, Acanthaster planci in Mo’orea. In 2008, from October to November, McKeon & Moore consistently removed yellow-spotted guard crabs T. flavopunctata (other symbionts such as fish and other Trapezia crabs were not removed) from 45 corals.  A control set of 45 corals were left to their own devices with their resident T. flavopunctata.  All corals were checked for feeding scars every 48 hours.

Following each experiment in the lab and in the field, total coral volume proxy (TCVP) or the amount of coral tissue devoured was estimated using proxies of coral volume and size of feeding scars.


Three guard crabs: T. biodentata, T. flavopunctata and T. serenei are shown.  Photo credits: David Liittschwager left and right, Seabird McKeon, middle)

Three guard crabs: T. biodentata, T. flavopunctata and T. serenei are shown. Photo credits: David Liittschwager left and right, Seabird McKeon, middle)



Results: ‘The Guide to Self-Defense by Crab’

Percentage of coral tissue volume proxy eaten by D. cornus in corals with no guards, corals with T. serenei and corals with T. punctimanus both small of small class sizes present.

Figure 1: Percentage of coral tissue volume proxy eaten by D. cornus in corals with no guard crabs, with T. serenei and with T. punctimanus present, both of small class sizes.




For best chances against the cushion seastar, host pairs of T. punctimanus for most effective results. In lab trials, mean tissue loss was significantly less with T. punctimanus present (0.61% TCVP) than without (5.7% TCVP). Small T. serenei (a nice, peachy rose crab) pairs are not a bad choice either with 1.3% TCVP and 8.7% TCVP without.  See Figure 1.  Interestingly, medium T. serenei do not seem to have any significant defense effect.




Percentage of coral tissue volume proxy eaten by C. novaeguineae in corals with guard crabs removed, with  T. bidentata and T. serenei, both of large size classes.

Figure 2: Percentage of coral tissue volume proxy eaten by C. novaeguineae in corals with absence of crabs, with T. bidentata present and with T. serenei present, both of large size classes.



For best chances against the horn drupe snail, larger T. serenei pairs can throw down better than T. bidentata (cute little orange crabs). The presence of these bad boys prevented the most mean tissue loss with 19% TCVP, vs 37% TCVP with T. biodentata present–see Figure 2. Although, if you are a small coral, it does not matter if you have small T. serenei.  In the lab, small corals with or without small T. serenei were completely gobbled up.





A crown-of-thorns sea star digs into a coral with no guard-crabs, leaving a trail of coral skeleton in its wake.  (Photo credit: Jenna Moore)

A crown-of-thorns sea star digs into a coral with no guard-crabs, leaving a trail of coral skeleton in its wake. (Photo credit: Jenna Moore)


For best chances against the crown-of-thorns sea star, A. planci, yellow-spotted guard crabs T. flavopunctata will lessen the rate of attack. In the field, 64% of corals were attacked without T. flavopunctata (but with other symbionts), significantly less than 18% with T. flavopunctata present.  Mean coral tissue loss was only 2% TCVP in guarded corals while unguarded corals suffered 22% TCVP.




Turns out, species matter for PocilloporaTrapezia mutualistic relationships. Different species of crabs are more defensively effective in different species of coral, although respective effectiveness is most likely a result of size classes. Having functionally diverse symbionts may outfit corals with a better defense against a range of predators. The positive impacts of mutualistic symbionts can also extend beyond individual coral colonies, as smaller surrounding corals can find refuge in or nearby guarded corals.  This research reveals yet another layer in the story of coral mutualisms, and further brings to light the importance of maintaining functional diversity for healthy coral reefs and oceans.


Seabird McKeon dives a coral reef in search of coral guard-crabs. (Photo credit: Gary Cranitch)

Megan Chen
I graduated with a Masters of Coastal & Marine Management from the University of Akureyri in Iceland, and am currently working at the Smithsonian Institution’s National Museum of Natural History in Ocean Education. I am interested in smart and feasible ocean solutions, especially in fisheries management, and the incredible adaptations marine life has come up with. In my spare time, I like to stargaze, watch talks on random topics and explore different corners of the world.


7 Responses to “Get crabby! A coral’s guide to self-defense”

  1. The article was very informative in the way that crabs defend corals in the symbiotic relationship that they have together. This taught me more about how not all animals are competing to survive but some of them help each other in order to survive. This article talked and described the relationship that crabs and corals have. It is similar to algae and coral where both organisms help each other in order to stay alive. Crabs defend the coral from predators that try to harm it while the coral provides the crab a home in which it can live in. This is similar to how coral gives algae the resources necessary to perform photosynthesis while the algae gives coral oxygen that it needs and it cleans out the coral. Both relationships are very similar and if you think about it, each organism wouldn’t survive without the other one. Without coral, many some of the crab species we know today may have gone extinct already because they wouldn’t have a home to protect them. The same goes for the coral. Without the crabs, all coral may have gone extinct by now because there is nothing protecting them from the danger of predators. If either of these happened, the whole world would experience a drastic change because in reality, all organisms depend on each other for survival. This is what people overlook when they say how useless an organism can be. Relating to the article, I have a question. If symbiotic relationships can benefits the organisms in it so much, why haven’t almost all organisms evolved to be able to have a symbiotic relationship since it is so helpful to have one?

    Posted by Jesus Lauro | May 25, 2016, 9:53 pm
    • Megan Chen

      Hi Jesus,

      Thanks for reading! Your comment shows that you have a deep understanding of coral/crab symbiosis. I especially like how you thought beyond coral/crab interdependence to the fact that that humans are part of the natural world and are also dependent on a healthy functioning ecosystem. You pose a great question! My best answer is that organisms have evolved in response to a variety of pressures. What works for one species may not work for another, so while some organisms have evolved to form critical, beneficial relationships, others found it advantageous to work alone. There may also be symbiotic relationships that occur on different levels and are not outwardly visible. For example, the human body plays host to over tens of thousands different kinds of microbes. While microbe interactions with the human body have not been well studied, some research shows that some microbes help our bodies function and gain something in return. Hope this was helpful (anyone else, please feel free to jump in) and thanks again for your thought-provoking question!

      Posted by Megan Chen | May 26, 2016, 10:49 am
  2. I really enjoyed the article of coral and crabs. This was interesting to me because this is the reason why coral becomes coral reefs with help from crabs. This helps the coral because of their mutual relationship with algae which they also have with crabs who live in the French Polynesians. The coral has been protected by crabs since they were just a little polyp. The crabs protect the coral from sea stars and drupe snails by using camouflage from the sediment to smooth themselves. Without crabs coral will be eaten and not form a coral reef. My question about that is when do the coral reefs normally get attacked, as a full grown reef or when they are just starting out as a polyp? Another question is that which coral do the crabs protect more and why?

    Posted by Luc Slater | May 24, 2016, 7:21 pm
    • Megan Chen

      Thanks for reading, Luc and great questions! Corals are more vulnerable at earlier life stages and probably get picked off more often. Larger coral reefs should be more resilient to predators (safety in numbers) and be able to bounce back (depending on the predation rate). I’m not a coral expert so bear with me..I would think that the type of coral that crabs protect depends on the species or even individual crab. Evolution has led to species that have different preferences and requirements, so some crabs may prefer living in some corals with certain shapes that offer more effective protection than others. Hope this helps, and as you can see there’s so much more to discover. I hope you continue to investigate!

      Posted by Megan Chen | May 26, 2016, 11:22 am
  3. Good job presenting science research results in a way that would be engaging to non-scientists! This would be a great resource for a student needing to write a paper.

    Posted by E.S. Ivy | October 20, 2014, 11:11 am

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