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Crassostrea gigas (Thunberg, 1793) is native to Japan and Korea, but has achieved global distribution through human mediated dispersal pathways and natural larval dispersal. Considerable variation in recruitment to wild aggregations has been seen regionally across the globe. Wild recruitment of C. gigas in England has increased in frequency since the millennia however a detailed understanding of their occurrence is limited to an area within the Thames estuary. There have been no English studies to date that reveal how C. gigas interacts with recipient ecosystems, or what impacts winter conditions have. Furthermore conclusive evidence has yet to be presented that feral C. gigas in England are self-sustaining. Intertidal surveys found substrate type and shore height to have the greatest impact on the locality and abundance of C. gigas recruitment. Gametogenesis initiated in C. gigas when water temperatures increased above 9.5 °C. Maturity was generally reached in the summer, however spawning differed between locations. Wild, intertidal C. gigas were found to spawn twice in a single reproductive season. Initially, spawning was triggered through tidally induced temperature shocking as water temperatures increased above 18 °C. It is thought that the second spawning was triggered by a combination of warm water (+18 °C) and an increase in phytoplankton abundance. Farmed, subtidal C. gigas spawned once, coinciding with the 2nd spawning of intertidal oysters. Rapid growth rates allow a size refuge from the greatest predation pressure to be achieved before growth rates decline over winter. In particular Carcinus maenas is capable of predating C. gigas with a shell length of up to at least 50 mm. Winter conditions experienced in England are typically colder than those experienced with in the native range of C. gigas, and as such, were detrimental to C. gigas. Juvenile oysters with 4 ± 0.5 mg (dry flesh weight) had significantly higher respiration rates than 6 and 9 ± 0.5 mg juvenile oysters at water temperatures of 7 °C. Temperatures within the pallial cavity of adult C. gigas remained similar to the ambient environment, changing rapidly with tidal immersion/emersion. Furthermore pallial temperatures below 0 °C were recorded and found to reduce gaping activity. Juvenile oysters are the most vulnerable to cold temperatures and are likely affected most years, however only particularly cold years with extensive frosts impacted on adults. This study expands the base-line knowledge of C. gigas distribution in England to include Southampton Water and Poole Harbour, and investigates the impacts that annual water temperatures and predation have on reproduction and recruitment. The accumulated knowledge and information will enable a better understanding of how establishing C. gigas is likely to develop in the future, and has the potential for use in predictive models and other conservation tools.

Laboratory infection studies involving Bulinus species from the area of the South Chad Irrigation Project, Borno State, Nigeria, were performed using Schistosoma haematobium obtained during a survey of schistosome and intestinal helminth infections in pupils at a local secondary school. These revealed the presence of both typical truncatus and africanus borne strains of the parasite with the former developing in Bulinus rohlfsi and the latter in one of two populations of B. globosus the Ngala population). The forskali group snail, B. senegalensis, was susceptible to infection with both parasite strains, while B. globosus Lake Chad failed to become infected with either. Investigations into the development and productivity of S. haematobium, and the effects of infections on the growth and reproductive biology of intermediate hosts, revealed that although initial establishment of an infection in a given bulinid depended on the strain to which it had been exposed, there was no correlation between parasite strain and subsequent interactions. Observed differences between each bulinid/ schistosome combination depended on the species of host involved, and could be linked with aspects of the mollusc's biology. B. globosus Ngala exposed to compatible S. haematobium produced long-lived infections. Detailed studies of the changes in cercarial output and host oviposition over the course of the infection in this case showed patterns that were not evident in the other shorter lived species. Laboratory experiments revealed that immature schistosome infections were capable of surviving in aestivating B. senegalensis for periods of 56 days, developing to release cercariae after snails were revived. Morphological comparisons of the susceptible B. globosus Ngala and refractory Lake Chad populations showed significant differences in the dimensions of their male copulatory organs. Breeding experiments, however, -proved that members of the two populations are capable of successful cross-fertilization.

The common cockle Cerastoderma edule (L.) is an ecologically and commercially important species in Wales, and other North West European tidal flats and estuaries. Cockle populations often exhibit high inter-annual variability in recruitment strength but also spatially within population distributions. The post-larval stage is a critical period for determining recruitment strength in cockle populations, and likely to be a key period for creating spatial structure. This thesis aimed to record and quantify the spatio-temporal changes in post-larval cockle (newly settled O-group individuals) density over large and small spatial scales, and investigate what processes may create or maintain these patterns. I found that over large scales (100s of metres) the initial settlement can be restricted to certain areas of the shore with high densities, but as time progresses, post-settlement redistributions move post-larvae across the shore. These redistributions recorded over two years extended post-larvae from initial low shore settlements into the mid and high shore, and resulted in a distribution similar to that of the adult population. Over small spatial scales (10s of metres) highly patchy and intense initial settlement of C. edule was also observed. Over time the spatial distribution of post-larvae became more dispersed resulting in significant positive correlation with the adult population post-settlement. To understand what is driving changes in density, experiments were conducted in the field and laboratory. Lugworms and adult cockles can be very abundant and disturb surface sediment during feeding and movements, which may kill or negatively affect C. edule post-larvae. Lugworms and adult cockles were excluded from areas of sediment in the field and the effect on post-larval densities over time investigated. Removing lugworms increased the densities of post-larvae but removing adult cockles decreased the post-larval density. Laboratory experiments looking at the possible role of lugworms and adults cockles causing direct mortality or increasing benthic and pelagic migrations showed no evidence that these were producing the results seen from the field experiment. I also tested if patchiness of post-larval cockles, compared to a uniform distribution, was beneficial for survival under predation from juvenile shore crabs Carcinus maenas, which experience a negative effect of interference competition on their foraging efficiency. Due to the behaviour of 'take away foraging' in the juvenile shore crabs, C. edule post-larvae still had higher mortality when aggregated. It is concluded in this thesis that spatial distribution and structure of post-larval C. edule change significantly during the postsettlement period, over large and small scales, and that macrofaunal interactions and predation may be a significant factor in producing these observations.

Two predatory muricid gastropods, Thalessa savignyi and Ergalatax junionae commonly occur on the rocky intertidal shores of Kuwait. Three shores, Ra’s Ajūzah, Ra’s Eqaila and Ra’s al Qulay’ah between the north and south of Kuwait covering a distance of ~90Km, were studied to determine the geographical and seasonal abundance of these two muricids. Thalessa savignyi was absent from Ra’s Ajūzah in the north, but increased in abundance towards the south of Kuwait. By contrast the smaller gastropod E. junionae was present on the three shores but decreased in abundance towards the southernmost site, Ra’s al Qulay’ah. Thalessa savignyi prefers rocky platforms and associated fissures which are present on the southern shore whilst small boulders and crevices on the shores in the north of Kuwait were favoured by E. junionae. Egg laying by both species occurred in the laboratory in late June and these egg masses hatched ~30days later in late July. Intertidal recruitment of juveniles (<10mm) appeared into the populations at Ra´s Ajūzah and Ra´s Eqaila between October and January whilst no recruitment was observed at Ra´s al Qulay´ah. Polymodal size frequency distributions in some of the populations were separated into their component size (age) classes using the method of Bhattacharya. Using these estimates of the modal size and ages, Von Bertalanffy growth (VBG) curves were constructed to compare the growth of both species. Thalessa savignyi grew faster than E. junionae and both grew faster at Ra´s al Qulay´ah compared with the other two sites. Growth rings on the operculum and in the statoliths were used to estimate the age of T. savingnyi and E. junionae. Age estimates using whole and polished sections of the operculae were problematic because rings could not be seen through the thick and opaque operculum of T. savignyi. The operculum of E. junionae was thin and transparent but the number of rings counted was almost double those found in the statoliths. Operculae sections did not reveal clear growth rings. A latitudinal variation in the definition and the number of weak growth rings in the statoliths was observed. Generally rings in statoliths of both muricids from the northern sites in Kuwait were clearly defined and they contained less weak rings than statoliths from muricids from the southern site. These statoliths contained a wide variety of strongly and weakly defined rings. It was established from seasonal collections that statolith growth rings are deposited annually between November and January. A larval ring, deposited during the period of life in the egg and a settlement ring, formed at the time of metamorphosis from the plankton, were validated in developing T. savignyi larvae in egg masses and from newly hatched larvae. The age of both species was determined successfully using statoliths and VBG population growth curves constructed. Thalessa savignyi was found to be a faster growing species with a shorter longevity (4 years) than the smaller, longer-lived (5 years) and slower growing E. junionae. In the laboratory both muricids were found to be predators of the tube worm Septibranchia krausii and the mussel Brachidontes variabilis. A comprehensive seasonal survey of the three intertidal shores documented the tidal distributions of the macrofauna and macroalgae. No correlation between the presence of potential prey species and the distribution of T. savignyi and E. junionae was found.

The whelk, Buccinum undatum, is a commercially important shellfish species found throughout the North Atlantic. Many problems face fisheries scientists attempting to assess and monitor populations of this species, due to several life history traits such as highly variable growth rates and sizes at maturity both between and within populations. One major problem has been the lack of a reliable ageing tool, limiting the ability of scientists to develop accurate population age structure estimates for stock assessments. This thesis focuses on three major areas of research: the reproductive assessment, growth and age of Buccinum undatum collected from three geographically distinct populations from the Shetlands, North Wales and the Jersey Islands. Reprodictive assessments of several B. undatum populations across the UK were undertaken by utilising several previously published maturity estimation techniques to ascertain the most accurate and reliable methods. It was found that size (total shell length) at maturity was highly variable, even over relatively small geographical distances. The timing of the reproductive cycle was assessed in a whelk population from the Menai Strait (North Wales, UK) to investigate changes in maturity assessments over an annual seasonal cycle. It was discovered that size at maturity varied throughout an annual cycle, suggesting an optimal season for maturity assessments. Spatial variations in shell morphology between populations highlighted clear differences driven by the shape of the aperture and the angle of the shell apex. The crystalline structure of B. undatum shells was also investigated for the first time; using a combination of analytical techniques a multi-layered aragonite shell structure was uncovered. The shells of B. undatum were also assessed for their potential as environmental recorders. The oxygen isotope ratio contained within B. undatum shells was found to be a reliable proxy for seawater temperature, allowing the development of a species-specific palaeotemperature equation that will greatly aid future studies of this species where seawater temperature reconstructions are required. Population growth rates were also assessed, using the isotope data, uncovering clear differences in patterns of shell growth between populations. A novel age determination technique for B. undatum was developed that utilised growth rings present in in a pair of calcium carbonate statoliths contained within the foot matrix of each whelk. An annual periodicity of the growth rings was determined by measuring growth in monthly samples of statoliths removed from laboratory reared juvenile whelks of known provenance and age. Cutting edge analytical techniques were also used to assess the structure and seasonal variations in the chemical composition of the statoliths. Following this, the use of statolith growth rings for fisheries monitoring purposes was assessed and compared to the currently used age determination method, operculum growth rings. Using data on size at age, population growth curves were constructed for a range of whelk populations across the U.K. It was confirmed that counting the annual rings in statoliths providea a more reliable assessment of the age of B. undatum, delivering a new more reliable tool for fisheries scientists to assess population age structure.

Shellfish have been recognised as an important human food source since Roman times and are now routinely consumed by inhabitants across five continents. However, shellfish are also well known vectors for human illness as they are capable of bio-accumulating pathogenic micro-organisms from the wider environment within somatic tissues and hence, are capable of transferring these pathogens into the human food chain. Current European efforts to safeguard consumers include the routine bacteriological monitoring of shellfish tissues using E. coli as a proxy for potential pathogenic micro-organisms. The aims of this thesis were firstly, to identify and quantify the bacterial reservoirs present in commercial shellfish harvesting areas. Secondly, to determine the relative contribution of these reservoirs under different mitigation techniques, and thirdly, to examine the relationship between the bacterial and viral reservoirs present within shellfish tissues. A single commercial mussel (Mytilus edulis) bed was intensively surveyed to identify both spatial and temporal changes in the bacterial reservoir present within mussel tissues and to examine the relationship between the bacterial reservoir present within the mussel tissues and concentrations of bacteria present in the underlying sediment. This study concluded that the underlying sediments represented a greater bacterial reservoir than within the mussel tissues, however no spatial relationship between the two reservoirs was evident. In addition, we investigated the potential of epizoic organisms to act as a bacterial reservoir. The findings from this study demonstrated that epizoic barnacles contained more than 80% of the total coliform bacteria present and, as such, represent a previously unidentified, but significant bacterial reservoir in shellfish harvesting areas. Based on previous findings, the effect of standard mitigation techniques i.e. depuration and offshore relaying on the previously identified bacterial reservoirs were investigated. A standard 48 hour depuration treatment was shown to be effective in the reduction of indicator bacteria from shellfish tissues, but ineffective in reducing the bacterial content of epizoic organisms to beneath acceptable levels. Offshore relaying was shown to be an effective measure to reduce both bacterial and viral concentrations in shellfish tissues, however, these concentrations demonstrated no relationship with the bacterial content of the surrounding waters. Finally the relationship between the bacterial and viral content of shellfish tissues was examined and compared to modelled E. coli concentrations in the water surrounding a sewage outfall. No relationship between the bacterial and viral content of the shellfish was observed. This finding supports previous studies suggesting that bacterial indicators are poor surrogates for viral contamination of shellfish. However, interestingly the determined concentrations of norovirus within shellfish tissues were more closely correlated with the modelled predictions than the determined E. coli concentrations from the shellfish tissues. In conclusion, the bacterial reservoir within shellfish flesh may be over-shadowed by larger bacterial reservoirs present within the wider shellfish harvesting area. The interaction between these environmental bacterial reservoirs and the bacterial reservoirs within shellfish flesh remains largely unknown, and represents an area for further study, especially with regard to the impact of alternative environmental bacterial reservoirs on the bacterial content of shellfish destined for human consumption.

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