|Trophodynamics of Pleurobrachia pileus (Ctenophora, Cydippida) and ctenophore summer occurrence off the Norwegian north-west coast|
Bamstedt, U. (1998). Trophodynamics of Pleurobrachia pileus (Ctenophora, Cydippida) and ctenophore summer occurrence off the Norwegian north-west coast. Sarsia 83: 169-181
In: Sarsia. University of Bergen. Universitetsforlaget: Bergen. ISSN 0036-4827, more
Stomach-content analyses and laboratory experiments on Pleurobrachia pileus (Cydippida) showed an average digestion time of 2.0 h at 12 °C and a high potential predation rate with highest daily ration in terms of prey carbon ingested as percent of predator body carbon for the calanoid copepod Calanus finmarchicus, the biggest prey tested. Predation rate increased almost linearly with increased prey abundance over the whole range tested (12-1043 l -1 in start concentration) of mainly small-sized copepods. Tests of the importance of prey size showed an individual clearance rate of 6.1 l day-1 with Calanus prey alone, which was depressed to 29 % of this when smaller prey was also present in high abundance. This is supposed to be an effect of handling time of prey in the feeding process. The laboratory results were used to estimate the impact of this species in Norwegian coastal waters. Abundance data were collected in summer from 56 stations between 63° and 69°N along a cruise track west of Norway. P. pileus was present in the southern part of the investigated area and was restricted to the uppermost 50 m throughout the day. It mainly occurred where its predator, the atentaculate ctenophore Beroe sp., was absent and its abundance was not correlated with the ambient prey biomass. Estimation of the predatory impact on the zooplankton stock in the area where P. pileus occurred, using experimentally determined daily ration, clearance rates, abundance and individual size of ctenophores, and zooplankton biomass in the field, revealed a rather low impact. Integrated over time and depth (down to 100 m) these calculations suggested that up to a few percent of the zooplankton stock could be removed daily by P. pileus predation, but in the surface water, where it was most abundant, it could be of considerable importance in controlling the zooplankton abundance.