Laboratory-measured grazing and ingestion rates of the salp, Thalia democratica Forskal, and the doliolid, Dolioletta gegenbauri

Abstract. Grazing and ingestion rates of laboratory-born Thalia democratica aggregates and Doliolet-ta gegenbauri gonozooids, phorozooids and oozooids were determined while fed Isochrysis galbana (4-5 fim diameter) alone or in combination with Peridinium trochoideum (16-18 /tm diameter) at con-centrations of 0.15-0.70 mm3 x I"1. Grazing rates (ml x zooid"1 x 24 h "•) ranged from 10 to 355, and at zooid weights greater than 5(ig carbon were in order oozooid> gonozooid> aggregate. Graz-ing rates increased exponentially with increasing zooid weight. Weight-specific grazing rates (ml x pgC"1 x 24 h~x) 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 (106/»m3 x zooid" ' x 24 h"*) ranged from 4 to 134 while oozooid rates ranged from 3 to 67. All inges-tion 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 /igC ' x 24 h~') were in-dependent of zooid weight. The mean aggregate daily ration (/igC ingested x /tg body C"1) was 59% and the mean doliolid ration was 132%. Field studies indicate that normal concentrations of D. gegen-bauri 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