The Effect of Snail-Associated Chaetogaster (Annelida: Naididae) on Host Behaviour and Fitness.

Members of the genus Chaetogaster (Annelida: Clitellata: Naididae) are small oligochaete worms found in freshwater habitats around the world. Most are free living predators or omnivores, but members of one species group are symbionts of molluscs, particularly snails. Despite being a common symbiotic association, little research has investigated the ecological aspects of this relationship. In particular, where the Chaetogaster-snail association falls on the mutualism-to-parasitism spectrum is still in question, especially since genetically distinct worm- host species pairings might result in different ecological relationships. The broad purpose of this thesis is to investigate the relationship between symbiotic Chaetogaster and host snails from the family Physidae in central Alberta, including how these worms affect snail fitness and behaviour, as well as the potential for Chaetogaster to play a role in host defence against trematode parasites. To address these aims, I conducted a comprehensive survey that explored the host associations and population dynamics of Chaetogaster in the field, and several manipulative laboratory experiments. Chapter 2 details my field-based research, conducting experiments and sampling in water bodies near Morinville and Fort Saskatchewan. My survey determined that abundance of Chaetogaster varies depending on season and snail host size. Sequencing of the ‘barcode’ region of the mitochondrial COI genes of hosts and worms revealed that the ponds surveyed each had two or more species of physid snail present, but only one species of Chaetogaster. My manipulative field experiments involved placing lab-reared Physella acuta snails in small cages in two ponds, half with Chaetogaster and half without. Unfortunately, due to destruction of field cages by wildlife and high mortality of experimental snails, the field experiments did not result in statistically useful data. A correlation analysis on data from field surveys indicated that there is no association between high numbers of external Chaetogaster and trematode metacercariae within snails, suggesting that Chaetogaster may not prevent metacercariae from infecting freshwater snails. Chapter 3 discusses two lab experiments regarding the effect of Chaetogaster colonization on physid fitness. In these experiments, one set of lab-bred Physella acuta were artificially colonized with worms and another set was not. The snails were individually monitored for 5 weeks. Using egg production as a proxy for host fitness, I found in both experiments that snails without Chaetogaster produced significantly more eggs than those with Chaetogaster over the five-week time frame. These results suggest that worm presence decreases host fitness. Chapter 4 includes two experiments examining the behaviour of snails with and without symbiont worms. In the first, I observed snails collected from the field with naturally varying Chaetogaster abundances to quantify the distance that each snail moved over the course of an hour. I found no difference in the movement behaviour of snails regardless of Chaetogaster number. In the second experiment, I exposed lab-bred Physella acuta to conspecifics with and without Chaetogaster to determine if Chaetogaster presence attracts or repels nearby snails, and whether attraction/repulsion is influenced by whether the focal snail itself was carrying Chaetogaster. I did not find a pattern of focal snails preferring or avoiding snails with symbionts, which could either indicate that the snails are indifferent to Chaetogaster or that they are unable to detect the chemical signature of the worms in the water. Although the results of my experiments cannot be used to definitively determine if Chaetogaster symbionts positively or negatively affect the snail species that I studied, for the lab-bred Physella acuta, they do suggest a general negative influence of symbiont worms on host snails. It may be that after generations of living in captivity without Chaetogaster, members of this population of P. acuta have become more sensitive to these worms than snails in wild populations. Future research, particularly into other worm-snail species pairs, will be necessary to further elucidate this complex relationship.

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