Benni Winding Hansen

Aspects of feeding, growth and stage development by trochophora larvae of the boreal polychaete Mediomastus fragile (Rasmussen) (Capitellidae)

Abstract Planktotrophic larvae of the boreal capitellid polychaete Mediomastus fragile (Rasmussen) were hatched and reared to metamorphosis. Larvae were fed Isochrysis galbana and grazing rate, growth and developmental stage were recorded as a function of food concentration, 0–1350 μgC·1 −1 . The larvae exhibited delayed stage shifts and reduced growth rates at low food concentrations, −1 . Maximum specific growth rate and the maintenance energy requirement were notably low, indicating adaptation to low in situ food concentrations. Larvae were able to capture and ingest particles in the size spectrum between 2 and 10 μm. The optimal particle size was 7 μm. The larvae enter the plankton in the early spring, when the phytoplankton size spectrum is typically dominated by large algal cells, exceeding the size for efficient uptake. The physical limitations for particle capture are therefore a potential limit for grazing. The ability to delay larval development is an advantage for a planktotrophic larva functioning as a growing dispersive organism.

Effects of large nongrazable particles on clearance and swimming behaviour of zooplankton

Abstract Latex beads (diameter 45 μm) were used to mimic pelagic particle composition dominated by large nongrazable particles and to examine their effect on clearance and swimming behaviour of a tintinnid, Favella ehrenbergii , a rotifer, Brachionus plicatilis , a gastropod veliger, Philine aperta , and copepodites of the neritic copepod Acartia tonsa . Clearance was measured as a function of bead concentration and compared with swimming patterns and grazing behaviour. The ciliates and the rotifers were not sensitive to mechanical obstruction by the polystyrene beads. The gastropod veligers and the copepodites, however, change swimming behaviour by increasing and jumping frequencies accompanied by a significant reduction in clearance. These observations imply nthat laboratory grazing rates may overestimate in situ grazing activity if larger, noningestible particles are present in the water coloumn.