Life history variation along environmental gradients in a freshwater fish, the southern pygmy perch Nannoperca australis

2017-05-15T07:04:03Z (GMT) by Morrongiello, John Reginald
Life history theory facilitates an understanding of how traits, evolved or expressed under varying environmental conditions, affect the fitness of individuals, populations and species. An individual’s phenotype is comprised of a series of traits that govern growth, survival and reproduction, and environmental gradients provide a valuable context within which to elucidate the causes and consequences of variation in these. Freshwaters are naturally heterogeneous environments and aquatic animals are exposed to a diversity of abiotic and biotic gradients that can act as potent selective agents. Much work has focussed on both assemblage level responses to, and amongspecies comparisons along, freshwater environmental gradients. Less, however, is known of how species themselves respond to changing environmental conditions. The freshwaters of south-east Australia encompass a range of environmental gradients associated with (among other factors) a diverse range of hydrological conditions. My thesis investigates the fitness consequences of variable life history trait expression, within and among populations, along environmental gradients in a freshwater fish, the southern pygmy perch (Nannoperca australis). I focus on traits associated with mate choice, reproductive investment and larval quality, spanning major fitness components within the life-cycle of this species. Larger and better-conditioned male N. australis had redder and blacker nuptial coloration, indicating that these are honest signals of male quality or dominance (Chapter 2). The average redness of males within a population was also positively correlated to increased water clarity such that signal efficacy is apparently optimised to the contextual environment. These results agree with predictions made by the sensory drive hypothesis. Female N. australis traded-off egg size and egg number along a flow ephemerality gradient, but contrary to theoretical predictions, mothers in ephemeral streams produced more numerous and smaller eggs whilst those from perennial streams produced fewer, larger eggs (Chapter 3). I propose that variable flow and frequent disturbance from drought render larval mortality size-independent. Maternal fitness is thus maximised by producing many offspring, increasing the likelihood of at least some individuals colonising permanent habitat. Environmental predictability also plays a role in determining within-clutch egg size variability. I provide empirical evidence that mothers adopt diversified bet-hedging as an adaptive strategy, whereby eggs of more variable size are produced when future environmental quality is uncertain (Chapter 3). These results have relevance to a wide range of fishes inhabiting freshwaters with variable or unpredictable hydrology. I experimentally explored the effect Eucalyptus leaf leachate (a major source of dissolved organic carbon [DOC] and natural stressor in these streams) had on reproductive effort (Chapter 4). Overall, leachate-exposed mothers were generally less likely to spawn. However, females from a naturally high leachate environment spawned more readily, raising the potential for some local adaptation to stressful conditions. Leachate exposure did not affect egg size or fecundity, and evidence confirms that egg size is a plastic trait as captive females produced smaller eggs than their wild counterparts. Nannoperca australis larvae chronically exposed to high polyphenol (a toxic compound in DOC) levels grew slower and had lower survival, but their swimming ability remained unaffected (Chapter 5). Larger individuals had greater resistance to polyphenols, indicating that exposure timing may be important in determining the lethal and sub-lethal impacts of DOC. The expression of phenotypes across life history stages is shaped by a range of environmental interactions. The results of my thesis clearly support this proposition and reinforce the importance of considering how multiple environmental gradients affect fitness within a species. Nannoperca australis has evolved in a variable and sometimes unpredictable environment, resulting in individuals and populations diverging in the expression of traits to ensure fitness is optimised.