The survival of Euastacus, spiny crayfish, in Australia is under increasing threat due to land clearing, water pollution and climate change. The spiny crayfish is host to Temnocephala ectosymbiotic flatworms, and very little is known about the role they play in maintaining the health of their host and how it relates to water quality, apart from anecdotal evidence which suggests that crayfish living in poor water quality areas are host to fewer temnocephalans. In order to disentangle this question, we seek to understand Temnocephala on the Murray Crayfish E. armatus and the New Hairy Crayfish E. neohirsutus in a cross-disciplinary manner. We are using veterinary techniques such as THC (Total Haemocyte Counts), differential counts of haemolymph smears, and haemolymph protein, lactate and glucose levels, as well as ecological techniques such as removal and dietary experiments as tools for monitoring crayfish health and the effects of temnocephalans on these key indicators. Here we discuss the usefulness of these techniques and the evidence supporting the existence of a parasitic to mutualistic continuum in Temnocephala-crayfish interactions. We propose the use of the temnocephalan-crayfish as a model across fisheries biology to advance our understanding of how environmental change will influence the ecological outcomes of symbiotic relationships.