Long-term food modification by Acartia clausi: A preliminary view

Abstract

After acclimation, the copepod Acartia clausi was allowed to graze for 5 days in a mixed suspension of two discrete size classes (species) of the chaining diatoms Thalassiosira spp. derived from continuous culture. Total particle numbers and particle size distributions of Thalassiosira spp. were stable throughout the 5 days, indicating that the effects of algal removal and modification due to grazing were balanced by algal growth. Grazer ingestion is the predominant process affecting all size classes of the smaller diatom population (T. nordenskioldii); however, both ingestion and chain modification are observed with the larger diatom (T. gravida). Although the greatest percentage removal of algal volume occurs in the largest algal size classes for each algal species, the greatest volume removal occurs at the modal peak (T. nordenskioldii) or just to the right of the modal peak (T. gravida). Flask-to-flask replicability of experiments was tested with the single-celled T. fluviatilis, and these tests were compared to the long-term experiments with T. nordenskioldii and T. gravida. Net particle removal occurs on both the large and small algal species in mixed suspension, not just on the larger-cell-side of the T. gravida distribution. Although 80% of the particles ingested are from the T. nordenskioldii distribution, 80% of the volume ingested is from the T. gravida distribution. The apparent multi-peak selection observed in our data is discussed in reference to two separate hypotheses and in light of other recent work pertaining to selective grazing by copepods.