Summary 1. Within‐species phenotypic variation is hugely variable and may play a role in determining the range of habitats a species can exploit. Our study addressed two main questions: 1. does phenotypic variation allow some species (i.e. habitat‐generalists) to use heterogeneous habitats and 2. are habitat‐generalists more variable than species occupying relatively homogeneous environments (i.e. habitat‐specialists)? 2. We examined the morphology of the common bully (Gobiomorphus cotidianus), a habitat‐generalist eleotrid fish found in lakes and rivers throughout New Zealand. We also compared the level of morphological variability in common bullies with that in the closely related redfin bully (Gobiomorphus huttoni), a habitat‐specialist of moderate‐ and fast‐flowing rivers. 3. Common and redfin bullies were collected from the South Island of New Zealand. A series of body and fin measurements were made, and cephalic dorsal head pores of the mechanosensory lateral‐line system were counted. The pores and associated canal neuromasts are important for prey detection and predator avoidance in other species, particularly, in turbulent conditions where the effectiveness of superficial neuromasts may be compromised. 4. The common bully had more dorsal head pores in fish from rivers than in those from lakes. This pattern was apparent only in adults, suggesting that selective pressures associated with adult habitat, be it rivers or lakes, are responsible. 5. As expected, there was greater phenotypic variability in the generalist common bully than in the specialist redfin bully, particularly with regard to the sensory pores, suggesting their importance for survival in turbulence. 6. We identified habitat‐related patterns in phenotypic variability in a generalist species and demonstrated a link between phenotypic variability and habitat breadth. Variation in the common bully may explain its ability to occupy a range of habitats.

Some fish recognize the threat of predatory fish through chemical cues, which may result in variation in diel activity. However, there is little experimental evidence of diel shifts in activity of prey fish in response to the diel activity of a predator. We compared the total prey consumed and the use of cover by common bullies (Gobiomorphus cotidianus), a native benthic feeding eleotrid, when exposed to the odour of an exotic predator, European perch (Perca fluviatilis), over a 12-h period. Our results showed no significant effect of perch odour on feeding activity, but a significant increase in the use of cover at night and a decrease in the use of cover by day. While common bullies may recognize the presence of a predator through chemical cues, dark conditions may inhibit this and other sensory mechanisms, affecting their ability to recognize the proximity of a predator. For example, during the daytime they may rely on visual cues to initiate cover-seeking behavior, but in the dark, vision is impaired giving them less warning of predators, thus potentially making them more vulnerable.

Species termed ‘habitat‐generalists’ are able to survive across a wide range of habitats, potentially enabled by the plastic behaviours and morphologies of individuals. This study aimed to compare habitat‐related adaptive specialisation in habitat‐generalists and specialists, by comparing feeding abilities of fish under varied hydrodynamic conditions. We compared the proportion of prey consumed by a New Zealand fluvial habitat‐specialist, the redfin bully, to a closely related habitat‐generalist found in fluvial and lacustrine habitats, the common bully, over a 6‐h period in either still or turbulent aquaria. We tested for intraspecific habitat‐specific adaptations by comparing feeding of common bullies from habitats with different hydrodynamic conditions. Within common bullies, previous research has documented a pattern of more lateral‐line system oculoscapular canal pores in fluvial habitats compared to lacustrine ones. By testing for a relationship between total pores and feeding success, we also aimed to identify the adaptive value of an increase in pores in turbulence. We identified decreased feeding (i.e., proportion of prey consumed) by both species in turbulence, but less so for redfin bullies. There was no clear indication of adaptation to habitat type by common bullies, with high variability in feeding by fish from all locations. We also found no statistical relationship between feeding and total oculoscapular canal pores. The results of this study suggest specialised adaptations of a habitat‐specialist to turbulent environments. The variability in feeding activity of common bullies also suggests the importance of behavioural plasticity in the survival of a habitat‐generalist across a wide range of conditions.

An investigation of intraspecific habitat-related patterns of variation in oculoscapular lateral-line superficial neuromasts (SN) identified a decrease in the ratio of total SNs to pores, and a trend towards decreased asymmetry in SNs in the habitat-generalist common bully Gobiomorphus cotidianus from fluvial habitats compared to lacustrine habitats, suggesting habitat-related phenotypic variability. A greater ratio of pores to SNs, as well as less variation in the total number and asymmetry of SNs observed in the fluvial habitat-specialist redfin bully Gobiomorphus huttoni may provide further evidence of variations in the oculoscapular lateral-line morphology of fluvial habitat G. cotidianus individuals serving as adaptations to more turbulent environments.