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Lernaeocera branchialis (L., 1767) is a parasitic copepod that parasitises a range of gadoids by anchoring in the proximity of the branchial chamber of its host, deriving nutrition from the blood of its host and causing serious pathogenic effects. This study investigates the taxonomy of the juvenile free-swimming stages and host location behaviour in the pre-metamorphosed adult female. The large size and distinctive appearance of the metamorphosed adult female stage, coupled with the wide exploitation and commercial importance of one of its principle final gadoid hosts, the cod (Gadus morhua L.), means that this species has long been recognised in the scientific literature, and here the extensive literature concerning this potentially important and damaging pathogen is re-examined to provide an up to date overview, which includes both aquaculture and wild fisheries perspectives. Due to disagreements between several descriptions of the L. branchialis juvenile stages, and because the majority of the descriptions are over 60 years old, the juvenile free-swimming stages are re-described, using current terminology and a combination of both light and confocal microscopy. The time of hatching and moults in these stages is also examined. Techniques for the automated creation of taxonomic drawings from confocal images using computer software are investigated and the possibilities and implications of this technique are discussed. The method of host location in L. branchialis is unknown but is likely to involve a variety of mechanisms, possibly including chemo-reception, mechano-reception and the use of physical phenomena in the water column, such as haloclines and thermoclines, to search for fish hosts. In this study the role of host-associated chemical cues in host location by adult female L. branchialis is investigated by analysing the parasites behavioural responses to a range of host-derived cues, in both a choice chamber and a 3D tracking arena. To analyse the data from the experiments, specialised computer software (“Paratrack”) was developed to digitise the paths of the parasites’ movements, and calculate a variety of behavioural parameters, allowing behaviour patterns to be identified and compared. The results show that L. branchialis responds to host-associated chemical cues in a similar way to many copepods in the presence of chemical cues. Of the different cues tested, gadoid conditioned water appears to be most attractive to the parasites, although the wide variation in behavioural responses may indicate that other mechanisms are also required for host location. The different behavioural responses of parasites to whiting (Merlangius merlangus L.) and cod (Gadus morhua) conditioned water, which are both definitive hosts, provide some evidence for sub-speciation in L. branchialis. The role of chemical cues in host location of L. branchialis, and the relative importance of chemical and physical cues in host location are discussed. As well as demonstrating several techniques, which show potential for further development, this work has improved our knowledge of the biology and life-cycle of L. branchialis. Further study of this, and other areas of L. branchialis biology and its host-parasite interactions, should assist the development of contingency plans for the effective management and control of this widespread and potentially devastating pathogen.

Certain aspects of the relationship between small mammals and their epifaunistic communities were investigated in a habitat supporting three rodent species. Preliminary studies indicated that, in an area of rough grass and bramble shrub, a community of rodents existed harbouring a wide range of flea ectoparasite species, as well as the apparently rare beetle Leptinus testaceus Muller. Techniques were developed, and are described, for the accurate assessment of the level of epifaunistic infestation of rodents. The infestation of various host species, sex and age categories were compared during the different phases of the host's annual population cycle. The rates of re-infestation of hosts and the levels of infestation with fleas with respect to home range size and migratory/ sedentary host categories were also investigated. The underlying cause of certain observed similarities in the level of infestation of some host categories was studied by the use of mark and recapture of fleas. The possibilities offered by this method of study of ectoparasites and their hosts are discussed. In order to contrast the situation observed in the flea infestation of small mammals a study of the association between hibernating hedgehogs and their flea ectopara-sites was undertaken. A method was developed for the assessment of the alimentary relations of a flea population with its host and this approach, together with the use of other techniques, indicated certain behavioural modifications which allowed the fleas to successfully parasitise their hibernating hosts.