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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.

Sanguinicola inermis Plehn, 1905, a recently introduced fish pathogen in Britain, has caused considerable damage to the carp industry. Two severely affected fish farms in England were included in the study. The incidence of fish infection for both farms was high, being 74-84% in 0+ and 1+ carp. Studies were made on the infection process and details of route of entry, migration and maturation of the worm are presented. An apharyngeate, furcocercous, lophocercous cercaria which develops in the snail Lymnaea (Radix) peregra (Mü11) was shown experimentally to develop into an adult Sanguinicola inermis. Maximum penetration of cercaria was achieved within 30 minutes and fins appeared to be the preferred site. Large numbers of worms were found to remain in the skin after penetration where they continued to develop to maturity, a previously unreported feature of S. inermis infection. The greatest migration to preferred loci occurred at 60 days post infection immediately prior to egg production. The migratory route used was found to be the loose connective tissue as well as the circulatory system. The distribution of mature worms in major blood vessels changed with season. Light microscopical studies were made on the morphology of specimens collected from the two farms and these were identified as S. inermis. Scanning electron microscopy of the surface topography and transmission electron microscopy revealed details of the tegument and provided evidence for the absence of spines. The major developmental features of the worm are described. A marked increase in size occurred up to 36 hours post penetration. Subsequent growth was slower. Egg production by mature worms began 10 weeks post infection at 15°C. Distribution and development of eggs in tissues is described. Egg production continued throughout the year with peaks during the summer months despite the constant environmental conditions. The growth rate of 0+ fry was studied over a period of 16 weeks and heavily infected fish showed stunted growth and poor Specific Growth Rates, Protein Efficiency Ratios and Food Conversion Ratios. Haematological studies showed that infected fish suffered from hypochromic macrocytic anaemia with leucocytosis and an increased Erkhrocyte Sedimentation Rate. Three different phases of infection were recognized. During phase I, the invasive stage, mortality may occur if infection levels are extremely high, but lightly infected fish present as clinically normal. Phase II was found to be the most critical phase since the majority of fish (over 90%) died at this stage. Histopathology revealed the progress of the infection from cercarial invasion to worm migration, maturation, egg production and miracidial hatching. Pathological changes were observed in the skin in phase I. In phase II, the heart, gills and kidneys were severely affected by both mature worms and developmental stages. The infection reached a chronic stage during phase III evidenced by a granulomatous tissue reaction largely in response to residual eggs in tissues. An attempt was made to integrate the phases of infection, development of the worm and pathogenesis in order to elucidate the host-parasite relationships.

This research study is divided into 4 major sections. Section A deals with the world-wide distribution of the haematoprotozoan parasites of marine fishes. These are tentatively divided into 3 major groups: the Haemogregarines, the Trypanosomes and the Trypanoplasms and one group of ill defined and controversal organisms including Haemohormidium sp. and Dactylosoma sp. The results of extensive surveys of the coastal waters of France, Scotland and Wales with added examples from the Mediterranean Sea substantiate the zoogeographical distribution of these parasites. Two species of haematoprotozoan parasites Haemogregarina simondi and Trypanosoma soleae are re-described, earlier accounts dating from the beginning of the century being considered incomplete. A new species of haemogregarine is described from the farmed turbot (Scophthalmus maximus) as Haemogregarina sachai n. sp. and an unidentified species of Haemohormidium found occasionally in turbot and Dover sole (Solea soles) is also described. In addition Haemogregarina sp. were encountered in certain wild marine fish from the Atlantic coast of France e. g. Zeus faber, Sebastes sp., Trisopterus luscus, Pagellus bogaraveo and Raja sp. and from the coastal waters of Malta e. g. Peristedion cataphractum and Oblade melanura. However, they were not found in sufficient numbers to allow a definite description. The value of surveys of wild fish populations is discussed in the light of zoogeographical distribution, the apparent periodicity of these parasites and a seasonal variation of parasitism. Section B attempts to summarize the knowledge of the mode of transmission of marine haematoprotozoan parasites by piscicolid leeches as intermediate hosts and vectors. The developmental stages of a trypanosome, probably Trypanosoma murmanensis from the Atlantic cod Gadus morhua are described in the marine leech Calliobdella nodulifera. Stages of a haemogregaririe were observed in the same leech. The development of the turbot haemogregarine Haemogregarina sachai n. sp. in artificially infected leeches is also described up to the 20th day post infection. Transmission of this haemogregarine to apparently uninfected turbot via this leech was not successful. Various stages of development of Haemogregarina simondi are described in its apparently natural vector, the marine leech Hemibdella soleae, and transmission with infected leeches to apparently healthy hatchery reared soles was achieved. Thus it was shown for the first time that marine leeches can serve also as vectors for haemogregarines. Stages of this haemogregarine are also described in the blood-sucking ectoparasitic copepod Lernaeocera sp. parasitizing the haemogregarine infected soles. These results are discussed in relation to the feeding behaviour and migration patterns of the fish hosts, the periodicity of the parasites and possible other vectors or other ways of acquiring an infection with these haematoprotozoan parasites. In Section C the pathogenicity of the haematoprotozoan parasites of marine fishes is summarized from previous accounts among wild fish populations and compared with the situation in aquaculture. The pathogenicity of the two haemogregarines, Haemogregarina simondi and Haemogregarina sachai n. sp., accidentally introduced into several fish farming establishments connected with the effluent cooling waters of a nuclear power station is described with special reference to the possible source of the infection. The results of therapy trials and control programs are discussed in the light of the periodic reappearance of the parasites, the possibility of carrier fish existing, the immune status of the host and the possible role of an intermediate host or vector in maintaining the infection. Possible means of controlling the pathogenicity of marine haemogregarines and perhaps other haematoprotozoan parasites when they occur in farmed marine fishes are also discussed. In Section D are described for the first time, the ultrastructural characteristics of various stages of the haematoprotozoan parasites of marine fishes in fish hosts and vectors. The electronmicroscopical studies are limited to Haemogregarina sachai n. sp. and Haemogregarina simondi, for which the ultrastructure of schizonts from the spleen and blood, intracellular merozoites and free gametocytes were contrasted. In addition stages of H. simondi were demonstrated in Hemibdella soleae and Lernaeocera sp. The fine structure of the various organelles encountered was compared with that of related organisms from other cold-and warm-blooded vertebrates. In conclusion attention is drawn to the need for more investigations in this field of host-parasite relationship of marine haematoprotozoan parasites and their vectors and their pathogenic-action as seen in a confined and artificial environment such as the marine aquaculture.