Disturbance and distribution gradients influence resource availability and feeding behaviours in corallivore fishes following a warm-water anomaly

Abstract

Understanding interactions between spatial gradients in disturbances, species distributions and species’ resilience mechanisms is critical to identifying processes that mediate environmental change. On coral reefs, a global expansion of coral bleaching is likely to drive spatiotemporal pulses in resource quality for obligate coral associates. Using technical diving and statistical modelling we evaluated how depth gradients in coral distribution, coral bleaching, and competitor density interact with the quality, preference and use of coral resources by corallivore fishes immediately following a warm-water anomaly. Bleaching responses varied among coral genera and depths but attenuated substantially between 3 and 47 m for key prey genera (Acropora and Pocillopora). While total coral cover declined with depth, the cover of pigmented corals increased slightly. The abundances of three focal obligate-corallivore butterflyfish species also decreased with depth and were not related to spatial patterns in coral bleaching. Overall, all species selectively foraged on pigmented corals. However, the most abundant species avoided feeding on bleached corals more successfully in deeper waters, where bleaching prevalence and conspecific densities were lower. These results suggest that, as coral bleaching increases, energy trade-offs related to distributions and resource acquisition will vary with depth for some coral-associated species.

Shifts in predator behaviour following climate induced disturbance on coral reefs

“…From our study, it is clear that corallivorous parrotfishes avoid consumption of live, bleached coral, suggesting either that the trophic benefits of corallivory are severely diminished or erased during bleaching, or that parrotfish do not recognize bleached corals as a viable food source. Our findings are consistent with the only other study (to our knowledge) to examine real-time behavioural responses of corallivores to environmentally induced bleaching [ 22 ], who found that three species of obligately corallivorous butterflyfish also actively avoided grazing on bleached corals. While neither study can distinguish between the mechanisms determining fish foraging behaviour, it is plausible that fish might be visually responsive to changes in Symbiodiniaceae species, or symbiont density, as previous work has demonstrated that grazed colonies have lower symbiont densities [ 37 ].…”

“…MacDonald et al . [22] observed depth-dependent grazing selectivity, which may also be consistent with this idea, as host corals are known to shift their association with symbionts across depth gradients [50,51]. Alternatively, MacDonald et al .…”

“…Alternatively, MacDonald et al . [ 22 ] postulate a density-mediated scenario, where higher or lower abundances of corals in shallow water may influence the grazing shift to deeper waters. As such, while bleaching may trigger a change in trophic grazing response, this grazing response might also influence depth-dependent effects of bleaching.…”

“…By contrast, the eastern triangle butterflyfish, Chaetodon baronessa [ 20 ] and the blueblotch butterflyfish, Chaetodon plebius [ 21 ] consistently preferred healthy colonies over bleached colonies, even during early onset bleaching. Chaetodon ornatissimus , Chaetodon pelewensis and Chaetodon reticulatus also prefer healthy over bleached colonies, but the strength of this selectivity appears to be depth-dependent [ 22 ]. The oval butterflyfish ( Chaetodon lunulatus ) does not appear to show grazing discrimination based on bleached status [ 21 ].…”

Consumer feces impact coral health in guild-specific ways

“…Given that corallivore feces contain many of bacterial ASVs associated with locally abundant corals, and previous work has shown that such feces contain high densities of live coral-associated Symbiodiniaceae cells ( 13, 14, 39 ), corallivore feces may in some cases be expected to have a similar “stabilizing” effect on the partnership between corals and their microbiomes. This is especially likely given reports that some corallivorous fish preferentially feed on heat tolerant corals during bleaching events ( 92, 93 ), thereby potentially transferring microbiota associated with stress-resistant colonies to surrounding stress-susceptible colonies. Empirical studies are needed to explicitly test this hypothesis.…”