Multiple defender effects: synergistic coral defense by mutualist crustaceans

Abstract

The majority of our understanding of mutualisms comes from studies of pairwise interactions. However, many hosts support mutualist guilds, and interactions among mutualists make the prediction of aggregate effects difficult. Here, we apply a factorial experiment to interactions of 'guard' crustaceans that defend their coral host from seastar predators. Predation was reduced by the presence of mutualists (15% reduction in predation frequency and 45% in volume of coral consumed). The frequency of attacks with both mutualists was lower than with a single species, but it did not differ significantly from the expected frequency of independent effects. In contrast, the combined defensive efficacy of both mutualist species reduced the volume of coral tissue lost by 73%, significantly more than the 38% reduction expected from independent defensive efforts, suggesting the existence of a cooperative synergy in defensive behaviors of 'guard' crustaceans. These emergent 'multiple defender effects' are statistically and ecologically analogous to the emergent concept of 'multiple predator effects' known from the predation literature.

Molecular characterization reveals the complexity of previously overlooked coral-exosymbiont interactions and the implications for coral-guild ecology

“…Finally, ecological specialization to host morphology could also be part of the explanation as it has been observed in other systems, such as cryptic species of barnacles which preferentially associate with different Millepora morphospecies33. Regardless of the mechanisms involved, this represents a rare case where Trapezia species do not display any preferential partnership with a heterosexual pair of A. lottini shrimps111253 or even with other congeners in adult coral hosts.…”

“…The ability of one heterosexual pair of crabs and shrimps to co-occur, while they both exclude congeners, represents a selective advantage to the coral host because the benefits provided to the host facing acute environmental stressors increase with the number and diversity of exosymbionts (e.g., sedimentation load11, predation1249). Therefore, coral hosts associated with a single pair of T. speciosa in the high impacted site at Reunion Island may prove less resilient to future episodes of sedimentation or predator outbreak than hosts harboring multi-specific crab-shrimp communities.…”

“…These exosymbionts are both exclusively associated with pocilloporid corals81112 and provide various cleaning1113, physiological14 and defense services121516 to their host, such as the removal of sediments and the deterrence of large corallivorous predators (e.g., Acanthaster planci ).…”

Wild trees in the Amazon basin harbor a great diversity of beneficial endosymbiotic fungi: is this evidence of protective mutualism?

“…2.5Â more Trichoderma strains than trees in plantations. This supports a "multiple defender effect" hypothesis, which was first described for corals and their mutualistic crustaceans (McKeon et al, 2012). The multiple defender effect hypothesis states that the presence of multiple mutualists creates a more synergistic, additive and protective effect than just the presence of one of them.…”

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“…We chose to use proxies of coral volume and feeding scars calculated as the volume of an ellipsoid using the formula: V = 4/3πabc, where a, b, and c are radii following McKeon et al (2012). After each trial the aquaria were scrubbed, rinsed, drained and refilled.…”