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
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.
Results: ‘The Guide to Self-Defense by Crab’
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.
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.
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 Pocillopora – Trapezia 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.