Rock and roll: experiments on substrate movement and coral settlement

Rubble is ubiquitous on coral reefs and can aggregate into fields, forming a significant component of the reef substrate. Rubble fields often remain unconsolidated, with the component rubble pieces subject to movement that is dependent on hydrodynamic forcing, rubble size, shape, and other factors. Settlement of corals to rubble fields has long been assumed, but the dynamic movement of rubble pieces has been presumed to deter settlement and is thought to contribute to high post-settlement mortality. Rubble often forms on coral reefs following severe disturbances, and is predicted to increase under climate change, with the potential to impact settlement and recruitment-dependent recovery processes. Through a series of laboratory and field experiments, we demonstrate that corals from broadcast spawning species on the Great Barrier Reef will settle on unstable substrates, even those in constant motion. We also observed more coral spat on settlement tiles suspended in the water column than those fixed to the reef using a common approach to censusing settlement. Sampling of natural rubble on the reef 50 days after a mass-spawning event confirmed the presence of similar numbers of coral settlers on rubble and on tiles fixed to the reef. These results suggest that rubble fields are places of significant settlement for broadcast spawning corals. Suspended tiles were also surprisingly effective in collecting coral settlers, demonstrating that a change in sampling protocol can produce significant variation in settlement data and strengthening the argument for standardisation of settlement-monitoring protocols, particularly at a time of growing need for reliable metrics. These results also suggest that movement of rubble is not precluding settlement outright, but rather post-settlement processes (i.e. competition, predation, shading or burial by shifting rubble) are limiting recruitment to rubble patches. Consequently, rubble stabilisation may increase the survival of spat that have settled in these environments.