Non-contact competition between soft and hard corals: a transcriptomic perspective

Abstract

Ecological interactions affect species evolution and, acting in combination with environmental factors, determine the composition of an ecosystem. In the case of coral reefs, the interactions of species with the corals (Anthozoa) is essential in shaping the ecosystem. Competition is particularly intense in coral reef communities because of the limited availability of space where conditions are appropriate (e.g. depth, substrate, currents) for settlement and growth. Space limitation makes the interaction between corals an essential element determining coral assemblages. Competitive interactions are difficult to analyses due to the number and diversity of factors (e.g. environment, life history, genotype) affecting outcomes. In the case of corals, research on competitive interactions has mostly focused on visible signs of aggression, such as measuring the damaged tissue next to a competitor or reporting visual competitive behaviours (e.g. mesenteric filaments). However, competition (particularly non-contact competition) does not always lead to visible symptoms, which has led in some cases to the underestimation of the extent of competitive interactions. For example, many soft corals (Octocorallia) produce secondary metabolites that may be used to compete for space; the production of secondary metabolites is unlikely to be visually obvious, and their impact on competitors may be subtle or cryptic. The outcomes of competitive interactions between individual corals will also be affected by the health and history of those individuals. For example, individuals that are already immune-compromised are unlikely to be able to compete as efficiently as healthier individuals. The immune system is assumed to be a critical component of competitive mechanisms. Research on coral immunity has focused, with few exceptions, on hard corals (Scleractinia), very little information being available on soft corals immune systems. The lack of basic research on soft corals extends to many aspects of their biology, despite the importance and abundance of these organisms in reef ecosystems. More research on soft corals immunity is important in order to better understand how these organisms respond to environmental factors or competition and to better predict the future composition of coral reefs. In this thesis, I have attempted to advance the knowledge of soft coral biology and non-contact competition between soft and hard corals. I analysed, at a transcriptomic level, the response of the soft coral Lobophytum pauciflorum to challenge with the defined immunogen MDP and the effects on both L. pauciflorum and the hard coral Porites cylindrica (hard corals) when these were in noncontact competition. The response of the soft coral to MDP was variable and unexpectedly dominated by genes likely to have functions in the nervous system. Non-contact competition triggered general stress and immune responses in soft corals, as well as differential expression of genes likely to function in secondary metabolite production and others genes that may be involved in tissue remodelling. The transcriptomic response of the hard coral, Porites, on the other hand, suggested cellular stress combined with resistance and aggressive responses. This research also highlights the role of the coral nervous system and behaviour in the stress response, suggesting that neuro-related pathways are closely linked to the immune system. Similarities between the transcriptomic responses to non-contact competition identified here and previously reported responses to environmental stressors (e.g. ubiquitination, antioxidant production), is consistent with the recruitment of common gene repertoires; therefore climate change is likely to effects competitive interactions in complex ways. Finally, the research presented in this thesis demonstrates the extent of variation in the responses of individual corals to stress (immune challenge and competition) and the challenges that this poses particularly for the investigation of the molecular bases of competition. In the future, individual variation needs to be better accommodated for molecular investigations into coral research, which means increasing biological replication and stopping the practice of discarding outliers.