Phenotypic responses of coral reef fishes to extreme and novel environments

In an increasingly variable and rapidly warming global environment, there is still little consensus as to the biological mechanisms that may facilitate population persistence of coral reef fishes. While a range of studies have shown substantial losses of coral reef fishes with environmental impacts, there are an increasing number of studies which have shown the importance of plasticity in behavioural, demographic and life-history traits in sustaining coral reef fish populations. As such, the main goal of this thesis was to investigate whether phenotypic plasticity plays an important role in determining coral reef species survival in an increasingly challenging environment. To do so, I focused on three phylogenetically and ecologically diverse coral reef fish species that thrive in the extreme environment of the Arabian/Persian Gulf (hereafter ‘Arabian Gulf’). Within the Arabian Gulf summer temperatures are in line with those projected for the tropical oceans by 2100, while winter condition can be so severe as to induce cold water coral bleaching. Moreover, the Arabian Gulf’s waters are characterized by hypersaline conditions that are the highest reported for coral reefs. These conditions provide an ideal setting to investigate the traits which allow resident coral reef fish populations to cope with predicted extremes and high variability in environmental conditions. In parallel, as many Red Sea coral reef fishes have recently invaded and established populations in the Mediterranean Sea (i.e. Lessepsian migration), I focused on the potential role of phenotypic plasticity in facilitating marine fish invasion within a temperate rocky reef ecosystem, as well as investigating the ecological interaction between invasive species and the wider native and non-native fish community. As behavioural change is the first response of an organism to environmental stressors, in Chapter 2 I investigated the role of behavioural plasticity in the reef-associated fish Pomacentrus trichrourus in mediating seasonal extremes in temperature (winter low: SST = 21 °C; summer high: SST > 34 °C) in the Arabian Gulf. Through in situ and ex situ observations, I found that this species downregulates metabolically costly activity during winter and summer while upregulating activity and increasing energy stores in spring. Seasonal changes in the level of activity may represent a mechanism to reduce metabolic stress when temperatures are extreme, while accumulation of energy stores during the benign season for activity (i.e. spring, SST = 27°C) will provide such stores to be utilised during environmentally extreme seasons. One of the most important factors which may determine species success within an extreme environment is how population demography is impacted by environmental change. Therefore, in Chapter 3 I investigated whether fish populations in the Arabian Gulf, compared to populations from the less variable and less extreme Oman Sea, would follow the generalised pattern for ectotherms of size reduction at a higher temperature (i.e. temperature size rule – TSR). Furthermore, I investigated whether the extreme and highly variable water temperature and salinity (i.e. mean annual salinity 42 psu) within the Arabian Gulf, would be the driver of expected body size reduction. Contrary to the prediction of the TSR, I found that salinity was the best environmental predictor of interannual growth across species and regions, with low growth corresponding to more saline conditions. These results highlight the importance of osmoregulation cost in impacting growth, as well as suggesting that future studies will need to consider the effect of multiple stressors when investigating the consequences of future climate change on fish demography. Invasive species commonly show a high degree of phenotypic plasticity which may facilitate their acclimation to novel introduced environments. Despite this, there is still debate as to the particular traits which may enhance invasion success. Therefore, in Chapter 4 I investigated the key behavioural and ecological traits of Lessepsian lionfish and the resident fish community in the Mediterranean Sea that may contribute to lionfish invasion success, as well as investigating the potential impact on the recipient native fish community. My findings suggest that Mediterranean lionfish are crepuscular generalist predators, though that feed more substantially on solitary, small-bodied benthic or bentho-pelagic associated species. Such trophic behaviour is expected to increase the likelihood of better dealing with the environmental stochasticity of a novel environment. Additionally, native prey fishes show greater naiveté towards lionfish than Lessepsian prey species. Naiveté of resident fishes towards a novel predator may enhance the population success of the invasive species through increased feeding rate and reduced competition with native predators, potentially mitigating any metabolic costs associated with invasion into a new environment