Habitat-related variation of the lateral-line mechanosensory system of the habitat-specialist redfin bully and habitat-generalist common bully

Abstract

Through natural selection, it is anticipated that a species will evolve traits that optimise survivability in their habitat. Habitat-specialist species, which use habitats within a relatively narrow range of conditions often display morphological adaptations specialized to their particular habitat. In contrast, ‘habitat-generalists’ have managed to successfully inhabit a variety of habitats, often through phenotypic and behavioural variability. Relationships between interspecific and intraspecific variability in habitat use and morphology are generally not well understood.

Initially I conducted a study of common bully diel activity in response to chemical cues of predatory perch, identifying an increase in the use of cover by night in the presence of perch odour. This may reflect recognition of a threat, but their inability to rely on visual cues to determine proximity of the predator. The results of this research were not overly encouraging, but the ability of common bullies to feed in complete darkness suggested the importance of mechanosensory systems, which are know to be variable within the species. I therefore changed the scope of my thesis to an investigation of the relationships between habitat use, behaviour and morphology within the fluvial habitat-specialist redfin bully (Gobiomorphus huttoni), and the habitat-generalist common bully (Gobiomorphus cotidanus), with a particular focus on the mechanosensory lateral-line system. The lateral-line system is a network of mechanoreceptor organs located externally along the surface of the fish (superficial neuromasts), and within sub-epidermal canals (canal neuromasts) connected to the epidermal surface by pores. It is important for such abilities as rheotaxis, predator avoidance and locating prey, and therefore, variation in the arrangement and number of these structures may have important implications on survival strategies.

A series of morphological measurements were made for common and redfin bullies from various habitat-types to identify patterns of habitat-related variation within common bullies and make interspecific comparisons of levels of variation. I conducted feeding experiments to test for morphological and behavioural adaptations to hydrodynamic conditions. Within the common bully, I observed habitat-related patterns of variation in lateral-line morphology, with more oculoscapular canal pores in fluvial than lacustrine fish. I also identified greater lateral-line system variation in the common bully compared to the redfin bully, suggesting a more refined, specialized mechanosensory system in the redfin bully as an adaptation to fluvial habitats. Feeding abilities of redfin bullies appeared to be less negatively affected by turbulence, further indicating specialized morphological, as well as behavioural adaptations to turbulence in fluvial habitats.

Overall, this thesis suggests habitat specialization of the lateral-line system of a narrow habitat-niche species, and habitat-related patterns of variation in a habitat-generalist, which might allow the common bully to occur across a range of hydrodynamic conditions. However, I also observed more within-habitat variation in common bully morphology and feeding abilities than anticipated. This might be an indication of individual adaptation to particular microhabitat, or microhabitat-selection by phenotype-appropriate individuals. My results provide detailed morphological and behavioural information about a generalist and specialist species, previously unavailable in the scientific literature, and are thus valuable for future ecological research and conservation.