Protection mutualists affect colonization and establishment of host-associated species in a coral reef cryptofauna community

Chelsie W. W. Counsell, Biology, Fairfield University, Fairfield, CT; Hawai'i Institute of Marine Biology, University of Hawai'i at Mānoa, Kāne'ohe, HI and Megan J. Donahue, Hawai‘i Institute of Marine Biology, School of Ocean and Earth Science and Technology, University of Hawai‘i, Kāne‘ohe, HI

Background/Question/Methods
Mutualistic relationships are ubiquitous in nature and can have important consequences for the population dynamics of both mutualist partners. Beyond pairwise interactions, mutualists can influence the assembly dynamics of the broader communities in which these mutualisms are embedded. Strong effects on community-level dynamics are expected for protection mutualists that display territorial behaviors, which act as protective services for their host species. Coral guard crabs (Trapezia spp.) and snapping shrimps (Alpheus spp.) form a guild of protection mutualists that aggressively defend their host corals from predators. Working with an experimental array of corals, we manipulated the presence of guard crabs and snapping shrimp to investigate how protection mutualists affect the colonization and establishment of the community associated with the host coral. During our 2-month colonization study, mutualist treatments were maintained and colonizing species were recorded and removed. During our establishment study, mutualist treatments were set and then community trajectories were followed for 6 months without interference.
Results/Conclusions
The presence of protection mutualists led to a more restricted composition for the coral-associated community. This result was driven by species-specific effects of Trapezia intermedia and Alpheus lottini on colonization and establishment. A. lottini inhibited conspecifics but did not affect the arrival of other protection mutualists. T. intermedia did not affect recruitment by Trapezia spp. or Alpheus spp., but did keep other Trapezia spp. that arrived to the host coral from remaining on the coral and becoming established in the community. Both protection mutualists limited colonization and establishment by commensal species, including xanthid and portunid crabs. The presence of protection mutualists affected the 6-month trajectory of communities such that the initial presence of protection mutualists explained 39% of variation in coral-associated community composition. To understand the cumulative effect of protection mutualists on their host, we need to understand how commensal species affect the host and to incorporate the costs of lost interactions.
We found that the coral host’s response to protection mutualists was mixed. Over the two-month colonization study, host corals with both protection mutualists had lower growth rates than corals without either protection mutualist. Over the six-month establishment study, host coral growth did not differ between treatments. These results highlight the context dependent nature of pairwise species interactions. When the protection mutualists were the only members of the established community, the costs of maintaining the protection mutualists outweighed the benefits for the host, but these costs were mitigated as more diverse communities became established.