Abstract
Tylodelphys sp. metacercariae are a unique diplostomid trematode larval-stage that infects the eyes of their second intermediate fish host, the common bully (Gobiomorphus cotidianus). These metacercariae are free-moving (non-encysting) and inhabit the vitreous chamber of the eye, between the lens and the retina. Like other diplostomid trematodes, Tylodelphys sp. is trophically transmitted and dependent on the consumption of the second intermediate host by the definitive host. Considering that visual obstruction resulting from parasite-induced cataracts has been shown to reduce a fish’s ability to avoid artificial avian predation, I hypothesised that selection on Tylodelphys sp. would favour 1) a behaviour that obstructs the vision of its fish host during the foraging time of its avian definitive host, and 2) occupation of the lower region of the eye, obstructing visual information originating from the water’s surface.
Ocular obstruction within the eyes of sedated hosts was assessed via an ophthalmoscope. Tylodelphys sp. exhibited a clear diel behaviour pattern, with the metacercariae primarily occupying the lower region of the vitreous chamber at night and expanding into the central region during the day. This increase in visual obstruction specifically during the foraging time of the parasite’s definitive host strongly suggests that the parasite’s activity pattern is adaptive. Further, an analysis of the visual processing region of the brains of infected bullies revealed an effect of Tylodelphys sp. number on the amount of neural activation. Fish with higher numbers of metacercariae showed a significant increase in neuronal activity when exposed to a flashing light stimulus, a trend not present in control fish (no light exposure).
To determine if the visual obstruction resulting from Tylodelphys sp. infection, and the subsequent alteration to visual processing, impairs the host’s ability to respond to visual cues, bullies were exposed to a bright flashing light during the day and at night. Fish with higher numbers of metacercariae in their eyes were significantly less likely to respond to the stimulus during the day when visual obstruction is greatest; however, this trend did not exist at night when visual obstruction is reduced.
Finally, I assessed the body condition and feeding ability of Tylodelphys sp. infected bullies as a proxy for host foraging ability. Considering the energetic demand parasites exert on their host, a negative selection may occur on a diel behaviour pattern that significantly impairs the host’s ability to forage. However, the results revealed no effect of parasite burden on the body condition of wild bullies or their ability to detect and respond to prey introduced into a tank.
Overall this thesis shows a diurnal activity pattern of Tylodelphys sp. metacercariae that coincides with the foraging time of the definitive host. The resulting obstruction of the visual field appears to be intensity-dependent, resulting in alteration to the visual information processed by the brain, as well as reducing the fish’s response to visual stimuli. This visual obstruction, however, does not appear to impair a bully’s performance and health in the field, or its ability to capture prey under lab conditions.