The Role of Conspecific Damage-Released Alarm Cue and Plastic Pollution on Caddisfly Larvae Case Construction Behaviour
Organisms interact with their environment using sensory systems to forage, communicate with other organisms, and detect predators. Prey can detect and assess predation risk using predator-related chemical cues, and this can influence their decision-making process. Antipredator responses are essential for the survival of an organism, and these responses can be displayed physiologically, morphologically, and behaviourally. While some prey rely on single antipredator responses, others combine several antipredator responses to increase their chances of survival. For instance, caddisfly larvae are known to display both morphological changes and behavioural responses when exposed to predation risk (i.e., their case construction behaviour). The portable case of an actively foraging caddisfly larva is generally understood to have evolved as a predator defence strategy. Larvae of many caddisfly species construct transportable cases of different sizes and shapes with various surrounding materials present using self-secreted silk to bind them together. This case construction behaviour can be influenced by predation risk and the surrounding sediments. Thus, this thesis examined the effect of these two factors on caddisfly larvae case construction behaviour. In the first experiment, denuded Limnephilus spp. larvae were exposed to conspecific damaged-released alarm cue, a predator-related chemical cue, twice within 48 hours during case reconstruction. Larvae exposed to alarm cue added more sticks to their cases than those exposed to dechlorinated tap water (control) after 24 hours of the first exposure. However, alarm cue had no effect on larval case length and the rate at which they constructed new cases when compared to larvae in the control group after 24 and 48 hours of exposure. In the second experiment, Phryganea spp. larval preference for different case construction materials was examined based on their surrounding materials. The experiment also investigated which case construction material offered more protection when the Phryganea spp. larvae were exposed to predatory crayfish. As an addition to the second experiment, larval recognition by predatory crayfish using empty larval cases was examined. Larvae were removed from their cases and provided with either their native construction materials (leaves), non- native case construction materials (plastics), or a mixture of both materials for case construction. Larvae were exposed to crayfish after 72 hours of case construction to determine which case type offers more protection against predation. Larvae constructed their cases using the materials provided, and these materials did not affect the proportion of larvae that constructed new cases in each group. However, larvae provided with both leaves and plastics preferred their native case construction materials (leaves). Also, larval survival when exposed to crayfish was not affected by their case type, rather, larval survival depends on the predator’s experience with case-building caddisfly larvae. Similarly, only those crayfish that had previously eaten larvae attacked the empty cases. These two studies also show that predation risk and surrounding materials could influence case construction behaviour. Larvae adjusted their behaviour based on the information perceived from their environment. In this study, Limnephilus spp. larvae were able to modify their case building behaviour to match the intensity of the perceived predation risk by adding more sticks to their cases. Although the protective function of Phryganea spp. larval cases based on material type could not be assessed in the second experiment, the result shows that prey recognition needed to be learned to initiate predatory attack. The assessment of larvae case construction behaviour can help understand larval interactions with their environments, such as predation risk and human activities (plastic pollution). Caddisfly larvae offer various ecosystem services in the freshwater habitat, and their case construction behaviour has been linked to their survival. Hence, natural (e.g., predation risk) and anthropogenic activities (e.g., plastic pollution) that can influence this construction behaviour needs to be investigated and monitored to ensure that their role within the ecosystem is not altered.
caddisfly larva, case construction behaviour, alarm cue, surrounding materials
Master of Science (M.Sc.)