Environmental Effects On Host-Parasite Interactions In The Aquatic Cestode Schistocephalus Solidus and its interactions with aquatic hosts

The broad aim of the work undertaken in this thesis was to increase understanding of how environmental changes in aquatic environments impact host-parasite interactions in the experimentally amenable copepod - three spined stickleback - Schistocephalus solidus model using controlled, laboratory experiments. In natural environments, animals are being exposed to a suite of anthropogenic stressors including temperature and heavy metal pollution. In this thesis the effects of temperature and heavy metals pollution are studied to determine the effect of these stressor on the growth development, survival and life cycle of both hosts and parasites. This thesis first investigated the effect of the temperature on the survival of infective free-swimming stages (coracidia) of Schistocephalus solidus, and their subsequent development as procercoid larvae in copepods (first intermediate hosts). The survival of coracidia was temperature-dependent, with the longest survival times at 10°C, and being reduced at 15°C and 20°C. The growth of procercoids in copepod hosts was faster at 20°C than at 10°C or 15°C. The second part of this thesis investigated the effect of water hardness and the influence of heavy metals on the survival of S. solidus coracidia, and on the development of procercoids in copepods. The survival of coracidia was shown to be sensitive to low water hardness (i.e. soft water), but was unaffected by heavy metals at the concentrations used. Procercoids developing in copepod hosts attained a larger size when reared under heavy metal treatments in hard water. The third part of this thesis investigated the effect of elevated zinc concentration (0.2 ug/L, 2 ug/L, 20 ug/L and 200 ug/L) on the viability of developing S. solidus eggs, on the survival of coracidia and on the development and growth of procercoids in copepod hosts. Under elevated zinc concentrations, S. solidus eggs were able to develop and hatch, and coracidia were shown to survive longer at higher zinc concentrations. After being exposed to zinc, procercoids grew more quickly in copepod hosts. The last part of this thesis investigated how the growth of early larval stages of the parasite in copepod hosts exposed to zinc affected the subsequent performance of the second larval (plerocercoid) stage in the second intermediate host, the three-spined stickleback Gasterosteus aculeatus. Schistocephalus solidus procercoids, which grew larger in copepods exposed to zinc, developed into larger plerocercoids when transmitted to the three-spined stickleback hosts, suggesting carry-over effects in this complex parasite life cycle. The overall findings of the thesis are that environmental stressors in aquatic environments, such as temperature and heavy metal pollutants, can have complex and divergent implications for different parasite life cycle stages, with potentially complex implications for the dynamics of parasite life cycles in ecosystems subject to environmental changes.