Stress-hardening by environmental priming could increase the odds for
corals to resist ocean warming. Natural environmental fluctuations, such
as those observed on offshore reefs in the Andaman Sea, provide an ideal
natural environment to study these effects. Here, internal waves (IW)
generate short cold-water pulses that peak from January to June and are
absent from August to November. Additionally, only western shores of
islands are exposed to this stress-hardening stimulus of IWs, while
eastern shores remain sheltered. Therefore, this study examined (1)
whether exposed corals were more heat stress resistant than their
sheltered conspecifics and (2) whether this trait would persist during the
season of stimulus absence. We exemplify that thermal regimes featuring
cold-temperature pulses successfully induced thermal stress-hardening in
corals. Corals from the IW-sheltered shore responded strongly to heat
stress irrespective of the season, while stress responses of IW-exposed
corals were either undetectable (during stimulus presence) or very weak
(during stimulus absence). However, this demonstrates the relevance of
stimulus re-occurrence in maintaining heat resistance. Furthermore,
priming stimuli do not need to exceed certain upper thermal thresholds to
be effective and we argue that cooling pulses represent a safer
stress-hardening regimen potentially implemented in conservation
strategies since it avoids warming-stress accumulation.