It is widely documented that non-human organisms express individual differences in behavioural patterns. For example individuals can be categorised as bold or shy and when these individual behavioural differences are consistent through time, they are termed behavioural types (BTs). In recent years research has identified that BTs often correlate across contexts/situations and these correlations are referred to as behavioural syndromes. Behavioural types and syndromes (i.e. personality) have also been implicated as major factors shaping population dynamics and the ability to buffer environmental disturbance. Recent theoretical predictions have proposed that BT variation may be underpinned by life-history strategies; however, these predictions have been little studied to date. Moreover, little research has focused upon environmental influences and the ontogeny of personality. In this thesis I use the Mangrove killifish (Kryptolebias marmoratus), a naturally occurring clonal vertebrate, as a model organism. This species presents a powerful tool providing the ability to replicate within and between isogenic genotypes in a controlled manner. Moreover the natural clonality expressed by this species permits environmental effects upon BT plasticity and BT-life-history interactions to be investigated within a developmental framework. In chapter 2, I present microsatellite genotyping results which show that the founding individuals used to propagate a laboratory population at The University of Exeter represent 20 genetically distinct homozygous genotypes. I additionally address five research questions exploring genotypic, environmental, and developmental effects upon three commonly studied BTs (exploration, boldness and aggression): Firstly; I ask do adult hermaphrodite and secondary males exhibit personality i.e. repeatable BT expression? In chapter 3, I present results showing that both of the sexes express short term personality. Moreover, I show that that genotype is an important factor influencing BTs expressed, regardless of sex, indicating underlying genetic control. Secondly I ask; does genotype level life-history variation underpin personality trait variation during ontogeny? In chapter 4, I show considerable developmental plasticity in behavioural expression between genotypes but not life-history and I find limited behaviour-life-history relationships during development. Thirdly I ask; does the rearing environment influence life-history and behavioural plasticity? In chapter 5, I show that in comparison to a control treatment, the presence of conspecifics during ontogeny results in an average reduction in behavioural scores; however, life-history was unaffected. In addition, I show that development in a low food environment lowered average exploration and growth rate but had no effect on boldness or aggression. Furthermore, fish exposed to a predation risk simulation during ontogeny exhibited similar behavioural scores as the control, yet this treatment generated BTs i.e. personality. My fourth question asks; does the parental rearing environment (utilised in chapter 5) influence behavioural expression in the next generation? In chapter 6, I show that transgeneratonal effects of each parental rearing environment influenced life-history but had a minimal effect upon behaviour in the next generation. Finally I ask; does kin or familiarity influence plasticity in associations and aggression? In chapter 7, I show that genotypes have the ability to discriminate kin and familiars and modulate aggression and association accordingly. These results support the concept that developmental and environmental induced plasticity may be more important than life-history in shaping behaviour. Furthermore, although adults exhibit personality and genotypic effects appear important, genotype interacts with environmental/experiential influences to differentially shape behavioural plasticity during ontogeny. I suggest that theoretical predictions regarding life-history may be insufficient to explain the complexity of animal personality in this species. I discuss these results within developmental and epigenetic frameworks with reference to the ecological significance of these patterns within this species and the animal kingdom as a whole.

The mangrove killifish, Kryptolebias marmoratus, has the potential of becoming a strong model organism for a range of biological disciplines thanks to its ability to self-fertilise, a process only known to occur in invertebrate animals until its discovery. Selfing, a natural occurrence in this species, has lead to the formation of clonal lineages composed of highly homozygous individuals. The aim of this thesis was to further establish K. marmoratus in the field of developmental biology by providing an information infrastructure to help advance research on this peculiar animal and further promote its place in the pantheon of model organisms. To do so, I first set out to standardise K. marmoratus embryology by providing defined developmental stages with clear visual representations of key embryonic structures. This staging series is an essential tool that will ensure repeatability and consistency within and across different laboratories. Secondly, I examined several techniques for embryonic manipulation and for imaging that can be used in an array of experimental designs. Using these techniques I demonstrated microinjection of embryos by monitoring the yolk syncytial layer and its nuclei, and time-lapse analyses of the yolk surface during embryonic development. Finally, I applied the knowledge gained from my first two studies and examined Bmp signalling in K. marmoratus embryos and its influence on body patterning. By inhibiting this pathway, I found a new phenotype characterised by an extremely short and split body axis. These data highlighted the importance of studying known signalling pathways in unknown organisms as species-specific differences may improve our understanding of fundamental developmental processes. This thesis demonstrates that with its easily obtainable and manipulated embryos, K. marmoratus can be used for embryological research in the same light as other model organisms such as zebrafish or medaka. The rising amount of information on mangrove killifish will help further take advantage of this unique and intriguing species, and supports the use of this hermaphroditic vertebrate as a strong comparative model in developmental biology.

The self-fertilizing mangrove rivulus, Kryptolebias marmoratus, is an amphibious fish capable of reversible gill remodelling when moving between aquatic and terrestrial environments. In this thesis I determined how plastic morphological and physiological respiratory traits were integrated during transitions between environments. In two isogenic lineages, I found that behaviour (increased emersion) of individual fish caused gill morphological changes (enlargement of the interlamellar cell mass (ILCM)) that reduced gill surface area. I also found that large ILCMs that formed after 7 d of air exposure increased both gill ventilation and critical oxygen tension (Pcrit) when fish returned to water. These results indicate that large ILCMs reduce aquatic respiratory function, and increased gill ventilation was unable to maintain oxygen uptake at extreme levels of hypoxia. Ultimately, this study highlights the trade-offs in gill structure and function during the transition between air and water, and demonstrates that differences in behaviour can generate morphological variation.