Structured habitats provide thermal refuges and mitigate effects of heatwaves

Heatwaves are an increasingly common threat for many species, particularly those in intertidal regions. In terres-
trial systems, shading by vegetation can provide beneficial microclimates for other organisms, and artificial shad-
ing is often used in agriculture, but has not been tested in a marine restoration context. We tested how increased
shading affected the survival of a model foundation species, oysters (Crassostrea virginica). We conducted a field
experiment to compare the relative effects of predators and sun exposure on oyster survival across tidal eleva-
tions. Juvenile oysters were planted at two tidal elevations in predator–exposed and predator–protected cages.
Half the oysters in each cage were exposed to sunlight while the other half were shaded to reduce heat stress, and
individual survivorship was assessed periodically over their first month of planting when oysters are most vulner-
able to abiotic stressors and predation. Predators were the greatest source of mortality: uncaged oysters initially
experienced twice the mortality rate of caged oysters and these survivorship differences intensified over the
month. Nevertheless, shading significantly increased survivorship by 32–93 %, with shading benefits increasing
with tidal elevation (i.e. aerial exposure time during low tide). Additionally, temperatures in the shaded treat-
ments matched temperatures within the reef shell-matrix, which were up to 4.66 °C cooler than in direct sun-
light. Thus, shading can be a valuable tool to improve the restoration success of marine species during vulnerable
stages by mimicking natural thermal refuges from healthy, structured oyster reefs. These results highlight how
habitat degradation can increase risk to climate change.