The causes and consequences of personality variation in the red cherry shrimp, Neocaridina heteropoda.

Abstract

Behaviour is arguably the most flexible aspect of the phenotype. It is also an important determinant for vital components of fitness including survival, reproduction, and growth. When the local environment is changeable, complete behavioural flexibility (or plasticity) might seem as though it is the optimal strategy, allowing for rapid phenotypic responses to maintain fitness. However, this level of behavioural flexibility is not typically found, perhaps indicating such a strategy would have costs arising from other adaptive evolutionary processes or is constrained by the genetics underlying behavioural traits and plasticity. Such costs and constraints could also help to explain the phenomenon of animal personality, which is defined as among-individual differences in behaviour that are repeatable across time and/or context. Although animal personality is now widely studied, the empirical literature remains dominated by work on vertebrates and much less is known about this phenomenon in invertebrates, which represents >90% of the Earth’s animal diversity. This creates an exciting research opportunity that I address in this thesis, using the red cherry shrimp as a model for personality research in decapod crustaceans. I present two empirical studies focussing on shy-bold variation and explore possible mechanisms underpinning both behavioural consistency and plasticity. Firstly (Chapter 2), I use a repeated measures design coupled to multivariate behavioural phenotyping to investigate personality structure. I find significant individual-level variation in, and covariation among, behaviours putatively indicative of boldness, although my results do not align fully with a priori predictions for a simple shy-bold personality axis. I also show sexual dimorphism and size dependence do not make important contributions to among-individual variation. Secondly (Chapter 3), I take advantage of the short generation time and ease of breeding to conduct a quantitative genetic study, using a full-sib/half-sib breeding design to estimate the genetic contribution to personality and test the interplay of (heritable) personality and plasticity in the presence of predator cues. I find behavioural responses are weakly plastic to perceived predation risk and estimate a moderate heritability of time in refuge (used as a measure of boldness), although the significance in the additive genetic contribution, is marginally non-significant. I further find no support for genotype-by-environmental interactions (GxE), the presence of which would imply plasticity was also heritable. My research demonstrates behavioural differences among individual shrimp which are repeatable across two simple testing paradigms. Although some statistical caveats apply, it shows that boldness-related traits (e.g. time in the refuge) have moderate heritability and should evolve if under directional selection. Conversely, any further adaptive evolution of behavioural plasticity may be constrained. My description of personality and plasticity in the red cherry shrimp further adds to the growing picture of these phenomena in decapod crustaceans. While this research certainly sets the foundations for more fundamental studies, improved knowledge of personality may also have applications for welfare, management, and conservation efforts in decapods.