Abstract

Teleosts exhibit extensive, ongoing neuroproliferation within their brains even as adults, which facilitates indeterminate growth and permits them to recover from injuries to their central nervous system which would be catastrophic in higher vertebrates. As a result, the brains of teleosts show considerable phenotypically plasticity, adopting different morphologies in response to stimuli from the environment. This plasticity manifests as differential regional growth rates within the brain, with the balance between the rates of neuroproliferation and apoptosis determining if a region grows or shrinks. Their adaptive plasticity is constrained by the elevated metabolic cost of neural tissue, which penalizes excess investment in underutilized parts of the brain but also permits patterns of investment to change, leading to substantial intraspecific variation in brain morphology. The plasticity which leads to the morphological variation seen within species can exceed that seen between species and presents both obstacles to and opportunities for those who study teleosts or used them as model species. In this thesis, I set out to explore the potential ramifications of neuroplasticity in the teleost brain.

I begin with a review of the literature (Chapter 1), identifying factors known to influence brain morphology and the predominant methods and model species used to measure it. In Chapter 2, I tested the impact of exposure to predation risk on the brain morphology of juvenile Atlantic salmon (Salmo salar) and adult northern redbelly dace (Chrosomus eos). I found that gross brain morphology can change in under 14 days of elevated predation risk. Chapter 3 expands on this, finding that predation pressure lead to smaller hypothalami and bolder individuals. I also demonstrate that hypothalamic size correlates with shyness (i.e. risk averse phenotypes). In Chapter 4, I used predation-induced brain morphology to test how olfactory and hypothalamic investment influence anti-predator behaviour in proactive and reactive individuals. Finally, in Chapter 5 I conclude with an experimental validation of the methodology used for quantifying brain morphology (cross-sectional area); I tested its performance against the widely used ellipsoid estimation; in most circumstances, they were found to perform comparably. Together, my research provides strong evidence that short-term exposures to local stressors (i.e. predation, captivity) lead to predictable changes in brain morphology; changes that correlate well with studies of behavioural phenotypes.