Algae associated with coral degradation affects risk assessment in coral reef fishes

Habitat degradation alters the chemical landscape through which information about community dynamics is transmitted. Olfactory information is crucial for risk assessment in aquatic organisms as predators release odours when they capture prey that lead to an alarm response in conspecific prey. Recent studies show some coral reef fishes are not unable to use alarm odours when surrounded by dead-degraded coral. Our study examines the spatial and temporal dynamics of this alarm odour-nullifying effect, and which substratum types may be responsible. Field experiments showed that settlement-stage damselfish were not able to detect alarm odours within 2 m downcurrent of degraded coral, and that the antipredator response was re-established 20 - 40 min after transferral to live coral. Laboratory experiments indicate that the chemicals from common components of the degraded habitats, the cyanobacteria, Okeania sp., and diatom, Pseudo-nitzschia sp., prevented an alarm odour response. The same nullifying effect was found for the common red algae, Galaxauria robusta, suggesting that the problem is of a broader nature than previously realised. Those fish species best able to compensate for a lack of olfactory risk information at key times will be those potentially most resilient to the effects of coral degradation that operate through this mechanism.

生境退化（Habitat degradation）会改变群落动态信息传递所依托的化学环境（chemical landscape）。嗅觉信息对水生生物的风险评估至关重要：捕食者在捕获猎物时会释放气味，引发同种猎物的警报反应（alarm response）。近期研究表明，部分珊瑚礁鱼类在暴露于死亡退化珊瑚环境时，无法识别警报气味（alarm odours）。本研究探讨了该警报气味消除效应的时空动态，以及可能介导该效应的底质类型。野外实验结果显示，在退化珊瑚顺流方向2米范围内，定居期雀鲷无法检测到警报气味；而将其转移至活珊瑚环境后，其反捕食响应（antipredator response）会在20至40分钟内恢复。实验室实验表明，退化生境的常见组分——蓝细菌（Cyanobacteria）Okeania属（Okeania sp.）和硅藻（Diatom）伪菱形藻属（Pseudo-nitzschia sp.）所产生的化学物质，会阻断鱼类的警报气味响应。针对常见红藻粗壮花萼藻（Galaxauria robusta）的实验也观测到了相同的消除效应，这表明该问题的影响范围比此前认知的更为广泛。那些能够在关键时期弥补嗅觉风险信息缺失的鱼类物种，将是最有可能通过这一机制抵御珊瑚退化影响的类群。