A polymorphism suggestive of a division of labour has recently been observed in colonies of some species of parasitic trematodes. In these colonies there are two distinct castes: one which reproduces and one which is unable to reproduce and provides benefits to the colony in some other way. This recently uncovered trait has been reported from a number of closely related trematode species. It has been hypothesised, based on caste member behaviour as well as parallels with the division of labour in social insects, that the division of labour in trematodes is an evolutionary response to competitive interactions, and that the role of the non-reproductive caste is to defend the colony against co-infecting trematodes.The aim of this thesis was to further understand this division of labour in Philophthalmus sp. colonies within the first intermediate snail host, Zeacumantus subcarinatus. I aimed to determine fitness benefits gained by colonies that invest in non-reproductive caste members as well as conditions under which colony organisation and division of labour changes. This was done using a range of manipulative and observational experiments in vitro and in vivo. Manipulative and observational in vivo experiments were used to quantify the number of individuals in each caste under varying conditions (host stress, inter or intra specific competition, and different geographic locations). Using parallels to the social insect literature, it is predicted that colony demographic traits (such as colony size or the ratio of individuals in each caste) are adaptive and will be optimised to varying environmental conditions. Therefore, information can be gained by observing conditions under which caste ratios are different.Fitness benefits provided by non-reproductive caste members were investigated using manipulative experiments and an in vitro culture system was designed specifically for this study. The method was designed to culture multiple marine trematode species long-term outside of their host. It provides a standardised in vitro culture method for all marine trematode research. It allowed me to quantify colony fitness after manipulating the presence and absence of caste members as well as other co-infecting trematode species. Results of these experiments suggest there is a fitness benefit of non-reproductive caste members to their colony, however, these benefits are not necessarily gained through a competitive advantage over co-infecting trematode colonies. Colonies with a high proportion of non-reproductive individuals were more reproductively successful regardless of the presence of competition. Results also indicate that demographic traits such as colony size and the ratio of individuals in each caste are adaptive to environmental conditions: demographic traits of geographically separate colonies vary as expected based on overall trematode infection prevalence in each locality. There are more non-reproductive individuals in areas of high trematode prevalence, where there is a higher likelihood of intra- and interspecific competition, compared to areas of low trematode prevalence. It would be advantageous for colonies in areas of high trematode prevalence to have relatively more non-reproductive individuals if the role of that caste is defensive, as hypothesised. These traits also appear to be adaptive in colonies kept in laboratory conditions long-term, however, results from field collected snails indicate that colonies may not be able to quickly adapt to environmental conditions in the field. The experiments in this thesis were the first attempts to understand the recently discovered division of labour in a parasitic trematode species, and provide the first explanation of the division of labour in the phylum Platyhelminthes.
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Sociality in parasite colonies: A division of labour in the trematode Philophthalmus sp.
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