The pathology of the early stages of the crustacean parasite, Lernaeocera branchialis (L.), on Atlantic cod, Gadus morhua L. J L Smith, R Wootten and C Sommerville Institute of Aquaculture, University of Stirling, Stirling, Scotland Abstract Lernaeocera branchialis is a penellid copepod, the adults of which are commonly found on gadoids in the North Atlantic and North Sea. L. branchialis has a severe negative impact on wild gadoids and is a potentially serious pathogen of farmed cod, Gadus morhua. This paper describes an experimental infection by cohabiting the intermediate host, the flounder, Platichthys flesus, heavily infected with juvenile stages of L. branchialis, with hatchery- reared cod in order to study the pathology of the early stages of infection of the final host. Early stages of L. branchialis induce extensive local gill hyperplasia, large intravascular thrombus formation and a moderate cellular immune response in the cardiac and branchial tissues of G. morhua. The parasite appears to migrate within the lumen of the afferent branchial artery, ventral aorta and in turn, that of the bulbus arteriosus. These findings are consistent with the presence of a haematopha- gous, voluminous and rapidly invasive parasite. Keywords: Lernaeocera branchialis, copepod, loca- tion, pathology, experimental infection, Atlantic cod. Introduction The marine sector of the aquaculture industry, and in particular cod farming, is expected to develop substantially in the next 20 years, given the reduc- tion in wild capture fisheries. This expansion in gadoid culture will undoubtedly be challenged by the appearance of new pathogens. Amongst wild fisheries the penellid copepod, Lernaeocera branchi- alis (L.), is not only one of the most prevalent parasites, but is also highly conspicuous and induces dramatic clinical changes (Mann 1952; Khan & Lacey 1986; Khan 1988; Khan, Ryan, Barker & Lee 1993) and thus may contribute to the decline in cod fisheries. Cod mariculture is currently developing in Scotland and cage sites are still few. However, in Norway, Lysne & Skorping (2002) have shown that Lernaeocera branchialis can infect cage-cultured cod. Lernaeocera branchialis possesses a two-host life cycle. The mature adult is commonly found on gadoids in the North Atlantic and North Sea: principally Atlantic cod, Gadus morhua L., and haddock, Melanogrammus aeglefinus (L.) (Wilson 1917; Capart 1948; Kabata 1958). Flounder, Platichthys flesus L., and lumpsucker, Cyclopterus lumpus L., are the most documented intermediate hosts (Kabata 1961; Templeman, Hodder & Fleming 1976), but juvenile stages of L. branchialis have been found on species such as Dover sole, Solea solea (L.) (Slinn 1970), plaice, Pleuronectes platessa L. (Capart 1948), and dab, Limanda limanda (L.) (Begg & Bruno 1999). Upon attachment to the gills of the gadoid, the adult female undergoes a complex metamorphosis, which results in the extension of the feeding apparatus, thorax and corresponding appendages towards the cardiac chamber of the host by elongation of the abdominal segment (Sproston 1942; Capart 1948). The parasite then develops a holdfast or anchor, consisting of three ramified antlers, in the cardiac region, often involving the bulbus arteriosus (Schuurmans Stekhoven 1936; Sundnes 1970). The external position of these parasites on wild caught hosts is at the base of the gill arch (Sundnes 1970). The number of Journal of Fish Diseases 2007, 30, 1–11 Correspondence Prof. C Sommerville, Institute of Aquaculture, University of Stirling, Stirling, Scotland, FK9 4LA (cs3@stir.ac.uk) 1 Ó 2007 Blackwell Publishing Ltd