Journal of Plankton Research Volume 4 Number 2: 189-201 1982

Skidaway Institute of Oceanography, P.O. Box 13687, Savannah, Georgia 31406, USA
(Received August 1980; accepted May 1981)

Abstract. Grazing and ingestion rates of laboratory-born Thalia democratica aggregates and Dolioletta gegenbauri gonozooids, phorozooids and oozooids were determined while fed Isochrysis galbana (4-5 µm diameter) alone or in combination with Peridinium trochoideum (16-18 µm diameter) at concentrations of 0. 15 - 0.70 mm³ x l ¬¹. Grazing rates (ml x zooid ¬¹ x 24 h ¬¹) ranged from 10 to 355, and at zooid weights greater than 5 µg carbon were in order oozooid > gonozooid > aggregate. Grazing rates increased exponentially with increasing zooid weight. Weight-specific grazing rates (ml x µgC ¬¹ x 24 h ¬¹) were independent of the four-fold initial food concentration. Mean weight-specific grazing rates increased linearly with increasing zooid weight for the aggregates and oozooids, but gonozooid mean rates were independent of zooid weight. Aggregate and gonozooid ingestion rates (10^6 µm³ X zooid ¬¹ x 24 h ¬¹) ranged from 4 to 134 while oozooid rates ranged from 3 to 67. An ingestion rates were independent of the initial food concentration but increased linearly with increasing zooid weight at similar rates. All mean weight-specific ingestion rates (ml x µgC ¬¹ x 24 h ¬¹) were independent of zooid weight. The mean aggregate daily ration (µgC ingested x µg body C ¬¹) was 59% and the mean doliolid ration was 132%. Field studies indicate that normal concentrations of D. gegenbauri in the Georgia Bight clear their resident water volume (1 m³) in about 4 months, but that highly concentrated, swarm populations which occur along thermohaline fronts clear their resident water volume in less than 1 day.