Mechanisms of coral resilience at the Houtman Abrolhos Islands: resistance, recovery, and adaptation

This thesis aims to increase our understanding of the overall resilience of reef-building coral populations at the Houtman Abrolhos Islands (HAI) to rising temperatures and increasing thermal disturbances associated with anthropogenic climate change. To do this, I apply a number of genetic techniques and explore different components of resilience: resistance — the capacity to persist in the face of a disturbance; recovery — the capacity to recover from disturbance; and adaptation — the capacity to adapt to rapid environmental change.

The first section of this thesis explores the capacity of coral populations to resist bleaching through a mechanism of acclimatization known as symbiont shuffling. In Chapter 2, I apply an amplicon pyrosequencing approach and target the chloroplast 23S and ITS2 rDNA regions of Symbiodinium to characterize the flexibility of the Acropora-algal symbiosis at the HAI. Acropora at the HAI exhibited a high level of specificity to clade C Symbiodinium, suggesting that the shuffling of different clades to increase the thermal tolerance of corals is limited; however, a lack of significant differences in symbiont communities between the high-latitude HAI and the low-latitude extreme environment of the Kimberley Coast implies that a heightened thermal tolerance in Acropora in Western Australia is not facilitated by Symbiodinium alone, but instead is more likely a product of local adaptation of the symbiont, coral host, or their partnership.

The second section of this thesis explores the capacity of coral populations to recover from bleaching events and associated mortality through the dispersal of larvae and maintenance of connectivity. In Chapter 3, I apply a seascape genetics approach and combine population genetic data from a panel of microsatellite markers with an oceanographic dispersal model to better understand patterns of connectivity in the dominant plating coral Acropora spicifera across the HAI. Patterns of connectivity were consistent between the genetic data and the dispersal simulations, and revealed that coral populations throughout the archipelago maintained high levels of connectivity. A small pocket in the southern Pelsaert group, however, was genetically distinct from neighbouring reefs and demographically isolated from the greater larval pool. This chapter shows how complex ocean currents can create significant barriers to dispersal, and that some regions of the HAI are more likely reliant on self-recruitment to maintain healthy population levels. These areas are less likely to recover from disturbance and should be considered a priority area for conservation management.

The third section of this thesis explores the capacity of coral populations to adapt to rising temperatures. In Chapter 4, I apply a genome-wide scan of local adaptation in Pocillopora damicornis across a temperate-tropical transition zone, spanning 10 degrees of latitude and a maximum temperature range of approximately 14°C. By focussing on loci under positive selection, I identified high levels of adaptive differentiation between tropical and high-latitude regions. In contrast to neutral loci, loci under positive selection revealed an increase in genetic diversity with increasing latitude. A number of these loci under positive selection exhibited strong clinal patterns and were linked to known functional genes involved in the coral thermal stress response, such as ubiquitin, which is a cellular protein tag involved in protein turnover and the elimination of damaged cells. The results of Chapter 4 show that high-latitude reefs of Western Australia represent evolutionarily significant units for conservation management distinct from their tropical counterparts, with a reservoir of genetic diversity in ecologically important loci under positive selection. This may prove to be a valuable genetic resource, enhancing their relative capacity to cope with rapid environmental change.

In summary, I apply a variety of genetic techniques to better understand the resilience of reef-building corals through the exploration of population resistance, recovery, and adaptation. This thesis broadens our understanding of coral resilience at this unique high-latitude archipelago and adds to the existing body of literature on the resilience of corals to stressors associated with anthropogenic climate change. It also directs future studies applying genomic techniques to further understand the capacity of corals to cope with a rapidly changing environment.