Influence of an extreme climatic event on bilateral asymmetry and occurrence of eye flukes in perch (Perca fluviatilis) and roach (Rutilus rutilus) from a lake in southeast England

Eye flukes are common infections of freshwater fish and their occurrence as metacercarial stages may occur non-randomly resulting in an asymmetrical distribution within the host eyes. However, from previous studies the presentation of bias by these trematodes lacks consistency suggesting that congenital asymmetrical effects are unlikely to be the cause and exogenous factors, such as environmental stress, may be more influential. The present study, undertaken over a 4-year period, investigates the impact of an extreme heatwave and drought on the annual bilateral asymmetry and occurrence of two eye fluke species (Diplostomum sp. in the lens and Tylodelphys sp. in the vitreous humour) from perch (Perca fluviatilis) and roach (Rutilus rutilus). The onset of the extreme climatic event resulted in a significant negative effect on the occurrence of the eye flukes. Bilateral asymmetry, which was present within both trematode species and hosts, appeared to be highly variable between eye fluke and fish species and also year of study. However, during the pre-drought period both host species demonstrated significant asymmetry for Tylodelphys sp. but not for Diplostomum sp. while during the drought this bias was reversed. The potential role of fluctuating asymmetry of fish hosts in structuring the bilateral asymmetry of eye flukes is discussed.

How birds cope physiologically and behaviourally with extreme climatic events

As global climate change progresses, the occurrence of potentially disruptive climatic events such as storms are increasing in frequency, duration and intensity resulting in higher mortality and reduced reproductive success. What constitutes an extreme climatic event? First we point out that extreme climatic events in biological contexts can occur in any environment. Focusing on field and laboratory data on wild birds we propose a mechanistic approach to defining and investigating what extreme climatic events are and how animals cope with them at physiological and behavioural levels. The life cycle of birds is made up of life-history stages such as migration, breeding and moult that evolved to match a range of environmental conditions an individual might expect during the year. When environmental conditions deteriorate and deviate from the expected range then the individual must trigger coping mechanisms (emergency life-history stage) that will disrupt the temporal progression of life-history stages, but enhance survival. Using the framework of allostasis, we argue that an extreme climatic event in biological contexts can be defined as when the cumulative resources available to an individual are exceeded by the sum of its energetic costs—a state called allostatic overload. This allostatic overload triggers the emergency life-history stage that temporarily allows the individual to cease regular activities in an attempt to survive extreme conditions. We propose that glucocorticoid hormones play a major role in orchestrating coping mechanisms and are critical for enduring extreme climatic events. This article is part of the themed issue ‘Behavioural, ecological and evolutionary responses to extreme climatic events’.