Ocean warming and acidification degrade shoaling performance and lateralisation of novel tropical–temperate fish shoals

Gregarious behaviours are common in
animals and provide various benefits such as food acquisition and protection
against predators. Many gregarious tropical species are shifting poleward under
current ocean warming, creating novel species and social interactions with
local temperate taxa. However, how the dynamics of these novel shoals might be
altered by future ocean warming and acidification remains untested. Here we
evaluate how novel species interactions, ocean acidification, and warming affect
shoaling dynamics, motor lateralisation, and boldness of range-extending
tropical and co-shoaling temperate fishes under controlled laboratory
conditions. Fishes were exposed to one of twelve treatments (combinations of three
temperature levels, two pCO2 levels, and two shoal type levels:
mixed-species or temperate-only) for 38 days. Lateralisation (a measure of asymmetric
expression of cognitive function in group coordination and predator escape) of
tropical and temperate species was right-side biased under present-day
conditions, but side bias significantly diminished in tropical and temperate
fish under ocean acidification. Ocean acidification also decreased shoal
cohesion irrespective of shoaling type, with mixed-species shoals showing significantly
lower cohesion than temperate-only shoals irrespective of climate stressors.
Tropical fish became bolder under ocean acidification (after four weeks), and
temperate fish became bolder with increasing temperature, while ocean
acidification dampened temperate fish. Our findings highlight the direct effect
of climate stressors on fish behaviour and the interplay with the indirect
effects of novel species interactions. Because strong shoal cohesion and lateralisation
are key determinants of species fitness, their degradation under ocean warming
and acidification could adversely affect species performance in novel
assemblages in a future ocean, and might slow down tropical species range
extensions.

群居行为在动物中普遍存在，并带来诸如食物获取和抵御捕食者等种种益处。众多群居的热带物种正随着当前海洋变暖而向北极迁移，从而与当地温带物种产生新的物种和社会互动。然而，这些新群落的动态如何受到未来海洋变暖和酸化的影响尚未得到验证。本研究评估了新物种互动、海洋酸化和变暖如何影响在控制实验室条件下扩展分布范围的热带和共群居温带鱼类的群居动态、运动侧化和领域性行为。鱼类在38天内暴露于12种处理方法之一（包括三种温度水平、两种二氧化碳分压水平和两种群居类型水平：混合物种或仅温带物种）中。在当前条件下，热带和温带物种的侧化（衡量群体协调和捕食者逃避中认知功能非对称表达的一种指标）偏向右侧，但在海洋酸化条件下，热带和温带鱼的侧化偏向显著减弱。无论群居类型如何，海洋酸化都降低了群居的凝聚力，混合物种群居的凝聚力显著低于仅温带物种群居，无论气候压力因素如何。在海洋酸化下（经过四周），热带鱼变得更加大胆，而温带鱼随着温度的升高而变得更加大胆，而海洋酸化则抑制了温带鱼。我们的研究突出了气候压力对鱼类行为的直接影响以及与新型物种互动的间接效应之间的相互作用。由于强群居凝聚力和侧化是物种适应性的关键决定因素，它们在海洋变暖和酸化条件下的退化可能会对新型海洋群落中的物种表现产生不利影响，并可能减缓热带物种的分布范围扩展。