Historical thermal regimes define limits to coral acclimatization

Abstract

Abstract. Knowledge of the degree to which corals undergo physiological acclimatization or genetic adaptation in response to changes in their thermal environment is crucial to the success of coral reef conservation strategies. The potential of corals to acclimatize to temperatures exceeding historical thermal regimes was investigated by reciprocal transplantation of Acropora millepora colonies between the warm central and cool southern regions of the Great Barrier Reef (GBR) for a duration of 14 months. Colony fragments retained at native sites remained healthy, whereas transplanted fragments, although healthy over initial months when temperatures remained within native thermal regimes, subsequently bleached and suffered mortality during seasonal temperature extremes. Corals hosting Symbiodinium D transplanted to the southern GBR bleached in winter and the majority suffered whole (40%; n ¼ 20 colonies) or partial (50%) mortality at temperatures 1.18C below their 15-year native minimum. In contrast, corals hosting Symbiodinium C2 transplanted to the central GBR bleached in summer and suffered whole (50%; n ¼ 10 colonies) or partial (42%) mortality at temperatures 2.58C above their 15-year native maximum. During summer bleaching, the dominant Symbiodinium type changed from C2 to D within corals transplanted to the central GBR. Corals transplanted to the cooler, southern GBR grew 74-80% slower than corals at their native site, and only 50% of surviving colonies reproduced, at least partially because of cold water bleaching of transplants. Despite the absence of any visual signs of stress, corals transplanted to the warmer, central GBR grew 52-59% more slowly than corals at their native site before the summer bleaching (i.e., from autumn to spring). Allocation of energy to initial acclimatization or reproduction may explain this pattern, as the majority (65%) of transplants reproduced one month earlier than portions of the same colonies retained at the southern native site. All parameters investigated (bleaching, mortality, Symbiodinium type fidelity, reproductive timing) demonstrated strong interactions between genotype and environment, indicating that the acclimatization potential of A. millepora populations may be limited by adaptation of the holobiont to native thermal regimes.

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…Reef systems that extend across broad latitudes and temperature gradients have provided evidence of juvenile (Woolsey et al, 2015) and adult corals with enhanced thermal tolerance (Ulstrup et al, 2006;van Oppen et al, 2011;Howells et al, 2012Howells et al, , 2013. On the Great Barrier Reef (GBR), mean summer maximum temperatures have been reported to have nearly 2 • C difference at reefs 250 km apart (South Molle Island to Magnetic Island; Howells et al, 2013).…”

“…On the Great Barrier Reef (GBR), mean summer maximum temperatures have been reported to have nearly 2 • C difference at reefs 250 km apart (South Molle Island to Magnetic Island; Howells et al, 2013). Different thermal histories on the GBR have been found to influence bleaching susceptibility (Ulstrup et al, 2006;Ainsworth et al, 2016) and the physiological performance of a generalist coral symbiont (Howells et al, 2012).…”

Mechanisms of Thermal Tolerance in Reef-Building Corals across a Fine-Grained Environmental Mosaic: Lessons from Ofu, American Samoa

“…Certainly, the extreme temperature profiles of the HV pool are not unique to Ofu; corals are found in a variety of extreme environments and are exposed to temperatures that would cause bleaching in their conspecifics from other areas (Coles and Riegl, 2013;Kline et al, 2015;Richards et al, 2015;Camp et al, 2017). Wide variation in thermal tolerance and genetic divergence has been reported across latitudes and at large-spatial scales (Middlebrook et al, 2008;Howells et al, 2013;Dixon et al, 2015;Thomas et al, 2017), and it is becoming increasingly clear that locally adapted thermally tolerant pockets of corals exist at fine-spatial scales within a variety of coral reef systems (Goreau and Macfarlane, 1990;Barshis et al, 2010;Castillo et al, 2012;Kenkel et al, 2013bKenkel et al, , 2015Schoepf et al, 2015). For example, Porites astreiodes colonies from the thermally variable inshore patch reef environment of south Florida have greater thermal tolerance than offshore populations less than 10 km away (Kenkel et al, 2013a).…”

“…This is known as reciprocal home site advantage, where adaptation to one environment comes at a cost of adaptation to other environments (Hereford, 2009). Fitness related trade-offs associated with local adaptation appear to be widespread in corals (Howells et al, 2013;Kenkel et al, 2015;Bay and Palumbi, 2017). A reciprocal transplantation experiment of Acropora millepora between central and southern Great Barrier Reef found signals of local adaptation in fitness related traits including thermal tolerance, reproduction, and growth (Howells et al, 2013).…”

“…Fitness related trade-offs associated with local adaptation appear to be widespread in corals (Howells et al, 2013;Kenkel et al, 2015;Bay and Palumbi, 2017). A reciprocal transplantation experiment of Acropora millepora between central and southern Great Barrier Reef found signals of local adaptation in fitness related traits including thermal tolerance, reproduction, and growth (Howells et al, 2013). In addition, growth trade-offs exist between inshore and offshore populations of P. astreoides (Kenkel et al, 2015) and M. annularis (Hudson, 1981) in Florida.…”