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The effect of temperature on cercaria emergence for three larval trematode species infecting the california horn snail, cerithidiopsis californica (gastropoda: potamididae)
Current debate centers on whether parasite transmission rates will increase as the world warms. Several factors can influence transmission rates of parasites, including how temperature influences the physiology and development of parasites. Trematodes are parasitic flatworms that are ecologically important in estuarine ecosystems. Their importance stems from their interactions with free-living organisms and is substantiated by the large amount of biomass they can represent in estuarine food webs. Trematodes are characterized by multi-host life cycles that commonly include gastropods as first intermediate hosts, a wide range of invertebrates or ectothermic vertebrates as second intermediate hosts, and ecto- and endothermic vertebrate groups as final hosts for adult parasites. Cercariae are free-living stages that leave the first intermediate host to seek out and infect second intermediate hosts. Cercaria emergence from snails has been shown to be positively associated with increased temperature. However, at some point, higher temperatures will negatively impact biological processes and emergence is likely to decline. Hence, the relationship between cercaria emergence and temperature should be hump-shaped. Previous research examining how temperature influences cercaria emergence has not uncovered a hump, at least in part because realistically high temperatures have not been examined. Previous research has also potentially suffered by confounding low dissolved oxygen (DO) levels with increased temperature. Hence, to predict how global warming will influence trematode infection rates, population dynamics, and their role in ecosystems, we require a better understanding of how temperature influences cercaria emergence. My study examined how temperature influences the emergence of cercariae belonging to three digenean trematode species (Himasthla rhigedana, Euhaplorchis californiensis, and Stictodora hancocki) that commonly infect Cerithidiopsis californica, the California horn snail, as first intermediate host. I collected and identified naturally infected snails from Carpinteria Salt Marsh (CSM). In the laboratory, I quantified cercaria emergence at temperatures ranging from 11.6 to 49.5 °C. I examined two aspects of emergence: the probability of emergence, and the number of cercaria that emerged. After documenting that DO can indeed confound the effects of temperature, I controlled for DO level in my experiments and investigated the effect of low DO on cercaria emergence. I hypothesized that daily temperature levels directly affects the probability of cercaria emergence and the number of cercaria that emerge and that a decline in emergence would be evident at high, but realistic temperatures. Although the probability of cercaria emergence did monotonically increase with temperature, the number of cercariae that emerged with increasing temperature was best described by a hump-shaped curve. Cercaria emergence quantitatively increased with temperature up to an optimum, around 33 °C, and then decreased with further temperature increases. Temperature responses may have been even more peaked than observed if differences in emergence during and after acclimation had been separated. Also interestingly, the peak in cercaria emergence matched the thermal optima experienced by their snail host, C. californica, at CSM. Overall, cercaria emergence did exhibit a clear, overarching hump-shaped relationship with temperature. The focus of my study was on how temperature influences one aspect of the performance of trematodes-cercaria emergence from the snail intermediate host. However, other aspects of cercaria performance, survival and infectivity to the next host in the life cycle, must be known to better predict how increased temperatures will influence trematode population dynamics and the role of these parasites in estuarine ecosystems.