Nephrops norvegicus (L.) from fishing grounds on the west coast of Scotland has been found to harbour infection by a species of parasitic dinoflagellate. Chromosome morphology and ultrastructural features suggest that the parasite is a member of the botanical order Syndiniales, possibly related to Hematodinium perezi Chatton & Poisson 1931. Cells invading the haemal spaces, however, show no signs of flagella. Mode of transmission IS not yet known, and a flagellate spore stage has not been identified. Infection appears to be fatal to its host, the main cause of death possibly being disruption of gas transport and tissue anoxia caused by the proliferation of large numbers of dinoflagellate cells in the haemolymph. Severe infection has an adverse effect on meat quality that has provoked comment from fisherman and processors. Affected lobsters have been found at all west coast sites surveyed, with peak infection rates reaching 70 % of trawled samples. Infection occurrence shows marked seasonality coincident with the annual moult perlod of N. norvegicus. An increased prevalence of infection has been observed recently in some areas.

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The two major field methods (body colour and pleopod index) for the diagnosis of infection in the Norway lobster, Nephrops norvegicus by the dinoflagellate parasite Hematodinium were compared. Using the more reliable method, the pleopod index, which scores infection severity on a four-point scale, infection prevalence data were collected from a single fishing ground in the Clyde Sea area, western Scotland over a continuous period of 31 months. Peak infection prevalence occurs during the spring and is highest in small N. norvegicus [mean carapace length 28.1 ( 0.67) mm in females and 30.9 ( 0.50) mm in males] and in females. Mean infection severity increased from 1.05 0.03 during the low season (July-December) to 2.59 0.19 by the end of the main infection season (May), highlighting the progressive nature of patent infection over time. Infection prevalence is synchronous between the sexes in some seasons but not in others. Additionally, the proportion of recently moulted, infected males in the late season extends the duration of peak infection prevalence in certain years. Data from two adjacent fishing grounds in the Clyde Sea area also show that infection prevalence depends upon the population structure at a given site -the site with smaller animals showing the higher prevalence. An evaluation of methods for monitoring Hematodinium infection suggests that the pleopod method gives the most accurate estimation of prevalence in field-caught lobsters.2001 International Council for the Exploration of the Sea

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