Seawater carbonate chemistry and antennular flicking and locomotory behaviours，olfactory nerve responses of Dungeness crab (Metacarcinus magister)@en

Crustacean olfaction is fundamental to most aspects of living and communicating in aquatic environments and more broadly, for individual- and population-level success. Accelerated ocean acidification from elevated CO2 threatens the ability of crabs to detect and respond to important olfactory-related cues. Here, we demonstrate that the ecologically and economically important Dungeness crab (Metacarcinus magister) exhibits reduced olfactory-related antennular flicking responses to a food cue when exposed to near-future CO2 levels, adding to the growing body of evidence of impaired crab behaviour. Underlying this altered behaviour, we find that crabs have lower olfactory nerve sensitivities (twofold reduction in antennular nerve activity) in response to a food cue when exposed to elevated CO2. This suggests that near-future CO2 levels will impact the threshold of detection of food by crabs. We also show that lower olfactory nerve sensitivity in elevated CO2 is accompanied by a decrease in the olfactory sensory neuron (OSN) expression of a principal chemosensory receptor protein, ionotropic receptor 25a (IR25a) which is fundamental for odorant coding and olfactory signalling cascades. The OSNs also exhibit morphological changes in the form of decreased surface areas of their somata. This study provides the first evidence of the effects of high CO2 levels at multiple levels of biological organization in marine crabs, linking physiological and cellular changes with whole animal behavioural responses.甲壳动物嗅觉是水生环境中生存与交流诸多环节的核心基础，同时更是个体存活与种群繁衍成功的关键要素。大气二氧化碳浓度升高引发的海洋酸化加剧，正威胁蟹类感知并响应关键嗅觉相关信号的能力。本研究证实，兼具生态与经济价值的邓杰内斯蟹（Metacarcinus magister）在暴露于近未来二氧化碳浓度水平时，对食物信号的嗅觉相关触角须弹动反应出现减弱，这一发现进一步充实了蟹类行为受损的相关研究证据库。在该行为改变的背后，本研究发现蟹类在高二氧化碳环境下，对食物信号的嗅觉神经敏感度出现下降（触角须神经活动降低至原有水平的一半），这表明近未来的二氧化碳浓度水平将影响蟹类的食物检测阈值。本研究同时发现，高二氧化碳环境下嗅觉神经敏感度降低的同时，嗅觉感觉神经元（OSN）中主要化学感受受体蛋白——离子型受体25a（IR25a）的表达量也出现下降，而该受体对于气味编码与嗅觉信号级联反应至关重要。嗅觉感觉神经元的胞体表面积也出现了形态学层面的改变。本研究首次证实了高二氧化碳浓度对海洋蟹类多层级生物组织的影响，将生理与细胞层面的变化与个体整体的行为反应建立了关联。