This thesis examines the importance of sensory perception in our understanding of trophic biology. The morphological properties of the visual, chemical and lateral line sensory organs of three congeneric species of sillaginid, Sillago analis, S. ciliata and S. maculata, were investigated to compare attributes of their sensory systems and feeding behaviour.
Whiting chemosensory systems were found to be well developed compared with many teleost fishes. Sequential sectioning of the olfactory pits revealed that for fish of comparable length S. maculata had the largest pits, whereas S. ciliata possessed the highest mean number of olfactory lamellae (14 ± 3). Surface staining with Pontamine Sky Blue 6B demonstrated interspecific differences in the distribution of external taste buds. In all species, taste bud densities were highest around the tip of the snout and decreased caudally. Sillago analis and S. maculata both possessed dorso-ventral taste bud asymmetry, with highest taste bud densities on the ventral region, whereas S. ciliata had a more uniform dorso-ventral distribution of taste buds.
The cephalic portion of the lateral line canal system in whiting was extensive and interconnected. All three species possessed the same distribution of canals, however differences were observed in the number of pores and tubules in most canals. This was assumed to represent differences in the numbers of neuromasts in the canals (Coombs et al. 1988). Sillago ciliata possessed the lowest mean number of tubules in the preopercularmandibular canal (25 ± 2.8), but had the most suborbital canal pores (8 ± 1.1). Sillago maculata had the lowest number of pre-orbital (10 ± 0.6) and infra-orbital (1 ± 1.0) canal pores, but had the equal highest preopercular-mandibular pore count (33 ± 2.2). Sillago analis possessed the fewest trunk canal pores (57 ± 1.3) and the most infra-orbital canal pores (17 ± 1.6).
Sectioning revealed that the eyes of the three whiting species were similar in gross morphology, although there were significant interspecific differences in relative retinal cell densities. Sillago ciliata had the most well developed eyes. This species had the highest convergence rate of cells from the external nuclear layer to ganglion cells (40:1) and highest histological visual acuity (6.23'). Eyes of S. maculata showed relatively low convergence (25:1) and the lowest estimated visual acuity (8.53'). The convergence ratio (38:1) and estimated visual acuity (7.03') of S. analis were intermediate.
Investigation of the gross morphology of major brain subdivisions associated with sensory input, revealed only subtle differences in the relative proportions of the divisions occur between these three species.
Differences in the structure of the sensory systems between whiting species appear to reflect their feeding habits and the environments they inhabit. Turbulence and turbidity are suggested to be the two most influential environmental factors that determine the relative development of whiting sensory systems.
The relative importance of visual, chemical and mechanical stimuli were investigated in controlled conditions. Prey representative of the natural diet of whiting, bloodworms (Marphysa sanguinea), yabbies (Trypaea australiensis) and barred estuarine shrimp (Palaemon serrifer), were presented in tubes in which the levels of stimuli were manipulated. Whiting were able to locate prey using only visual or chemical cues. In some cases prey were also detected using only mechanical stimuli. Increases in stimulus intensity led to significant increases in the number of approaches to prey. Prey activity also significantly increased the number of responses not only to visual stimuli, but also to chemical and mechanical stimuli. Interspecific differences in behavioural responses to the various stimulus types generally correlated with morphological differences in the sensory systems found in the morphological survey.
Behavioural and morphological differences were observed in two groups of juvenile Sillago maculata fed two different diets for a period of four months. Measurements of gross morphology, taste bud distribution and feeding responses were taken at 0, 30, 60 and 120 days. Significant differences in external taste bud densities were observed between fish on the two diets, with divergence of their taste bud densities increasing with the length of time spent on the diets. It is proposed that this was due to the different stimuli required to locate food in each diet. Pellets were primarily approached using chemical cues, whilst live prey were principally detected via visual stimuli. The response time of whiting to locate the alternate prey to that on which they had been maintained, decreased with increased time on the diet new diet, whilst the response time to locate food of the maintenance diet increased. Variations in sensory morphology and prior experience with prey are likely to have caused these effects. The behavioural differences correlated with variations in gross and sensory morphology. The results of this study have implications for improving our understanding of the influence of rearing environment on fish and allow us to address the issue of low survival in fish reared and released in restocking programs.
I highlight the importance of viewing trophic biology from the perspective of the predator and the role that prey detection plays in the process of prey selection. Predators cannot intentionally consume prey if they cannot detect them, therefore the sensory abilities of a predator will influence prey choice. A clearer understanding of fish sensory perception is therefore required to clarify the relationship between the range of potential prey in the environment and those that are eaten by fish.