Positive interactions between corals and damselfish increase coral resistance to temperature stress

By the century’s end, many tropical seas will reach temperatures exceeding
most coral species’ thermal tolerance on an annual basis. The persistence
of corals in these regions will therefore depend on their abilities to
tolerate recurrent thermal stress. Although ecologists have long
recognized that positive interspecific interactions can ameliorate
environmental stress to expand the realized niche of plants and animals,
coral bleaching studies have largely overlooked how interactions with
community members outside of the coral holobiont shape the bleaching
response. Here, we subjected a common coral, Pocillopora grandis, to 10
days of thermal stress in aquaria with- and without the damselfish
Dascyllus flavicaudus (yellowtail dascyllus), which commonly shelter
within these corals, to examine how interactions with damselfish impacted
coral thermal tolerance. Corals often benefit from nutrients excreted by
animals they interact with and prior to thermal stress, corals grown with
damselfish showed improved photophysiology (Fv/Fm) and developed larger
endosymbiont populations. When exposed to thermal stress, corals with fish
performed as well as corals maintained at ambient temperatures without
fish. In contrast, corals exposed to thermal stress without fish
experienced photophysiological impairment, a more than 50% decline in
endosymbiont density, and a 36% decrease in tissue protein content. By the
end of the experiment, thermal stress caused average calcification rates
to decrease by over 80% when damselfish were absent but increase nearly
25% when damselfish were present. Our study indicates that
damselfish-derived nutrients can increase coral thermal tolerance and are
consistent with the Stress Gradient Hypothesis, which predicts that
positive interactions become increasingly important for structuring
communities as environmental stress increases. Because warming of just a
few degrees can exceed corals’ temperature tolerance to trigger bleaching
and mortality, positive interactions could play a critical role in
maintaining some coral species in warming regions until climate change is
aggressively addressed.

到本世纪末，多数热带海域的年均水温将超过绝大多数珊瑚物种的热耐受阈值。因此，这些区域的珊瑚能否存续，将取决于它们耐受反复热应激的能力。

尽管生态学家早已认识到，种间正向互作可缓解环境压力，从而拓展动植物的实际生态位，但珊瑚白化研究却在很大程度上忽视了：珊瑚共生体（coral holobiont）之外的群落成员间的互作，如何影响珊瑚的白化响应。

本研究将常见的巨枝鹿角珊瑚（Pocillopora grandis）分别置于伴生与不伴生黄尾宅泥鱼（Dascyllus flavicaudus, yellowtail dascyllus，该鱼类常栖息于这类珊瑚中）的两组水族环境中，施加为期10天的热应激处理，以探究与宅泥鱼的互作对珊瑚热耐受能力的影响。

珊瑚通常可从与其互作的动物排泄的营养物质中获益；在热应激发生前，与宅泥鱼共养的珊瑚表现出更优的光生理机能（光系统II最大光化学效率Fv/Fm），且体内共生藻种群规模更大。

当暴露于热应激环境时，伴生宅泥鱼的珊瑚的表现，与无鱼且处于常温环境的对照组珊瑚相当。

与之形成鲜明对比的是，未伴生宅泥鱼且受热应激的珊瑚出现了光生理损伤，共生藻密度下降超50%，组织蛋白质含量降低36%。

实验结束时，在无宅泥鱼的情况下，热应激导致珊瑚的平均钙化速率下降逾80%；而当宅泥鱼存在时，钙化速率反而提升近25%。

本研究表明，宅泥鱼来源的营养物质可提升珊瑚的热耐受能力，这与压力梯度假说（Stress Gradient Hypothesis）的预测相符——该假说提出，随着环境压力加剧，正向互作对于群落结构塑造的重要性将愈发凸显。

由于仅升温数度即可突破珊瑚的温度耐受阈值，引发白化与死亡，正向互作或可在应对气候变化的强力措施落地前，为暖化区域的部分珊瑚物种提供存续的关键支撑。