27 resultados para juvenile Ciona intestinalis


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Seasonal cycles in the condition index of Mytilus edulis from three sites in southwest England are described. These are analysed in relation to host length, stage of gonad development and parasite burden by linear regression analysis. An effect on the condition index due to the presence of Mytilicola intestinalis can be detected only in the sublittoral mussels in those few winter months when the mean intensity of infestation is over about 25 parasites per host. In all cases studied, the magnitude of the effect due to variation in host length, stage of gonad development, seasonal cycles and environmental factors is greater than that due to parasitism.

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The occurrence of Mytilicola intestinalis in populations of mussels in south-west England is recorded and compared with previous data. Since 1955 there have been two main changes in the distribution of Mytilicola: (a) it has invaded all the major estuarine mussel populations on the Bristol Channel coast, and (b) many previously uninfested open-coast populations all round the peninsula are now lightly infested. It is suggested that differences in infestation levels between estuarine and open-coast populations of mussels are due primarily to differences in the degree of exposure to wave action although factors such as size, population density and location of the hosts also influence infestation. The chance of the establishment of breeding pairs of Mytilicola depends on the parasite population size and its distribution through the host population.

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Data from 40 published studies of the diet composition of larval and juvenile cod (Gadus morhua) from around the northern North Atlantic were summarized to assess generic patterns in ontogenetic and regional variability in the key prey. The results showed that larvae at the northern edge of the latitudinal range of cod depend primarily on development stages of the copepod Calanus finmarchicus, whilst those at the southern edge depend on Para- and Pseudocalanus species. Juvenile cod preyed on a wider range of taxa than larvae, but euphausiids were the main target prey. Analysis of regional variations in the relative abundances of C. finmarchicus and Para/Pseudocalanus spp. in the plankton, as estimated by the continuous plankton recorder (CPR) surveys, showed a similar geographical pattern to the larval cod stomach contents. Comparison of CPR data from the 1960s and 70s with data from the 1990s showed that the boundary between C. finmarchicus and Para/Pseudocalanus spp. dominance has shifted northwards on both sides of the Atlantic, whilst the abundance of euphausiids in the southern cod stock regions has declined. The results are discussed in relation to regional differences in the response of cod stocks to climate variability.