The effects of microplastic ingestion and environmental warming on camouflage and growth in common shore crabs

Anthropogenic stressors are becoming increasingly prevalent in the marine environment, both as direct pollutants (e.g. microplastics and noise), and indirectly through climate change (e.g. environmental warming and ocean acidification). Microplastics in particular are considered to be hazardous, due to their bioavailability through primary (diet), and secondary (respiration) means. This is owed to their small size (<5mm), ubiquity in the marine environment, and close resemblance of small prey items. Plastic pollution also frequently co-occurs with other ecological stressors, such as environmental warming through climate change. There is a growing body of evidence to suggest that exposure to multiple interacting stressors can magnify their adverse effects. However, little research exists on how stressors such as environmental warming and microplastics affect juvenile marine invertebrates, what this means for subsequent life-stages, and what coping mechanisms they may possess. Furthermore, stressor effects on antipredator behaviours such as colour change for camouflage have also received very little attention, despite these behaviours being pervasive among aquatic species. This limits our ability to make predictions on the impacts of stressor exposure on marine species, and what the subsequent implications are for their survival. Here, I address these knowledge gaps through a series of laboratory-based feeding studies, using environmentally relevant quantities of microplastics (0.5% by feed weight), and two temperature treatments 14°C (ambient environmental temperature) and 24°C (unseasonably high environmental temperature) on juvenile shore crabs (Carcinus maenas). In Chapter 2, I examine the effects of microplastic ingestion as a singular stressor on camouflage efficacy and growth in juvenile shore crabs. Individuals were exposed to microplastic particles through feed over a period of 8 weeks. Weight, incidence of moulting, and carapace diameter were recorded on a weekly basis as proxies for juvenile growth. The level of luminance change (brightness) and subsequent camouflage were quantified in an ecologically relevant context using digital photo analysis, and a model of avian predator vision. Microplastic ingestion alone did not affect luminance change, and subsequent background matching in juvenile shore crabs. Additionally, it was also not found to significantly affect the incidence of moulting, or growth (weight change and carapace diameter). It however found that juvenile shore crabs possess the capacity to remove sequestered microplastics from their gill surface through the process of moulting, thereby mitigating the possible negative effects of microplastic ingestion. In Chapter 3 I build upon the findings of Chapter 2, and investigate the effects of combined stressors (microplastic ingestion and environmental warming) on camouflage and growth using the same methodology. The level of growth per moult (carapace diameter and weight) was found to be significantly reduced in individuals exposed to combined stressors. There was also an initial delay in moulting, as well as an overall reduction in moult frequency. However, the level of luminance change and camouflage remained unaffected by exposure to combined stressors, with any changes being attributed to exposure to a warmer thermal environment. Overall, this thesis indicates that common, co-occurring marine stressors have the capacity to interfere with fundamental physiological processes in marine species when stressors interact. Furthermore it suggests there are potential implications for future fitness and survival in juveniles.