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Keywords: that are physically and behaviourally specialised to perform specific tasks. Because such morphs are often sterile, their production represents a fitness cost for the colony and there should be an optimal ratio of the Hamilton’s (1964) theory of kin selection can explain why such individuals forgo their own reproductive self-interest to promote that of highly-related kin, investment into the production of sterile castes still represents a potential fitness cost for the colony as a whole. As such, instead of producing a fixed number of individuals for a certain role, it could be advantageous if the ratio of sterile undergo asexual multiplication within their gastropod first inter-mediate host, resulting in masses of clonal stages known as parthe-nitae that in many species can occupy the entire visceral mass of the snail (Galaktionov and Dobrovolskij, 2003; Lafferty and Kuris, 2009). The main role of the parthenitae in the trematode life cycle is the production of free-living cercariae which typically leave the snail host to infect the second host in the life-cycle. The infection within the snail grows in a modular manner through the continual clonal production of parthenitae by pre-existing individuals. The key aspect of this life history trait is that the parthenitae are clonal, therefore all individuals resulting from a single infection event in