<I>Trichostrongylus vitrinus </I>is one of the principal causative nematodes of ovine parasitic gastro-enteritis within Scotland and infests the proximal small intestine of the sheep. At present, control is achieved mainly by the administration of anthelmintic drugs, but with the increasing emergence of anthelmintic resistance, much research is now centred on vaccine development. Recent evidence has suggested that the excretory-secretory components (ES) from parasitic nematodes may be an important source of host-protective antigens. The overall aim of the present study was to characterise the nature and properties of <I>T. vitrinus </I>ES components. The initial part of the work involved the partial characterisation of two of the enzymes, acetylcholinesterase (AChE) and proteinases, excreted and secreted during the <I>in vitro</I> culturing of adult <I>T. vitrinus. </I>These enzymes were defined on the basis of their substrate specificity, molecular size, pH optima and inhibitor sensitivity. Attempts were also made to isolate cDNA fragments encoding AChE from an adult <I>T. vitrinus </I>cDNA pool, using the polymerase chain reaction (PCR) and degenerate oligonucleotide primers directed towards highly conserved regions of AChEs that had previously been identified by comparison of known polypeptide sequences from a number of higher eukaryotic organisms. No adult <I>T. vitrinus </I>cDNA fragments encoding AChE were amplified, suggesting that <I>T. vitrinus </I>AChE(s) is/are distinctly different to AChEs from higher eukaryotes, at least at the level of nucleic acid sequence. Subsequent research focused on the molecular characterisation of adult <I>T. vitrinus </I>ES. Immunoscreening of an adult <I>T. vitrinus </I>cDNA lambda gt11 library with antiserum raised against adult <I>T. vitrinus </I>ES, resulted in the isolation of ten immunopositive clones. Their inserts were sequenced and the results were analysed using computer databases. Three of the clones were identified as harbouring inserts that encoded proteins that shared significant homology to myosin heavy chain, vitellogenin and serine proteinase inhibitor (serpin) respectively. The other seven clones contained inserts that showed no significant homology to any of the sequences present in the computer databases.

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The ecology of Polydora, epp was reviewed and the infestation of molluscs by the boring, spionid polychaete, Polydora ciliata, was studied in detail. X-radiography showed that, P. ciailt a infested a wide variety of molluscan substrates, but it . occurred in greatest abundance on large, rough-surfaced, epibenthic shells, which were collected from the-sublittoral zone and the level of extreme low water spring tides, of shores sheltered from wave action. P. ciliata was contagiously dispersed on mussel--shells. Monthly radiographs of mussels indicated that maximum settlement of P. ciliata larvae in the river Fowey, 1 occurred in May. The effect of heavy infestations of P. ciliata on limpets P. vul ata , mussels (Eytilus edulis), and oysters (Crassostrea is) was investigated. At certain times of the year, heavy infestations of P. ciliata reduced the dry weight condition and fecundity of limpets and mussels, but did not seem'to adversely affect C. i ae. Compression tests showed that P. ciliate weakened mussel shells. Predation experiments, with Cancer a s, indicated that this might increase the mussels' vulnerability to predation. Heavy P. ciliata infestation also reduced the mussels' resistance to loads which tended to force the mussele' valves apart. This may decrease the mussels' phyeico-chemical tolerance, and increase their vulnerability to starfish predation. Limpet adhesion, measured on the shore with a simple dynamometer, was impaired by P. ciliata infestation. This may reduce the limpets' resistance to wave action, and increase their vulnerability to predation. Reasons for the adverse effects of P. ciliata infestation on limpets and mussels are suggested, and the ecological and commercial implications of the infestation are discussed.

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