The Ecology of the Ctenophore Pleurobrachia Pileus in Scottish Waters

Abstract

A short review of the literature on the distribution and ecology of Pleurobrachia is given and supplemented by data taken between 1925 and 1968 in the Scottish area. It is a neritic species in the northern North Sea and reaches its greatest abundance in October and November following a possible minor peak of abundance in early summer, but in some years (e.g. 1965) the summer peak can exceed that of the autumn.

Pleurobrachia is a non-selective carnivore, feeding on what is available in the plankton, and swarms can greatly reduce the amount of zooplankton in large areas of water. Spawning is continuous while conditions are suitable, but changing conditions can sometimes give the effect of two separate spawnings in a year.

Although there are reports in the literature of Pleurobrachia eating eggs and larvae to the extent of reducing the recruitment potential of the fish and shellfish, this has not been found in the Scottish area where crustaceans form 80% of the diet, and reaching 97%. Because of the reduction in zooplankton, however, swarms of Pleurobrachia could be responsible for larval fish mortalities, even if the fish themselves are only rarely eaten, and they could affect also adult herring and other plankton feeders.

There is a rapid increase in the numbers of cercarian parasites of Pleurobrachia from August to September.