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The goal of this study was to determine the morphology and ultrastructure of doliolid pellets using light, epifluorescence and transmission electron microscopy and compare the results to observations of calanoid copepod pellets. For (ultra)structural analyses, pellets of gonozooids of  Dolioletta gegenbauri  and females of the copepod  Eucalanus pileatus  were produced in feeding experiments at close to environmental food concentrations. Thin sections of a representative doliolid pellet revealed that these pellets were mainly composed of intact diatom valves, a few fragmented valves and intact flagellates. While the larger diatoms,  Rhizosolenia alata , were completely digested (empty valves), the smaller diatoms,  Thalassiosira weissflogii , were partly, or not digested at all. The phytoflagellate,  Isochrysis galbana , appeared to be hardly digested. Aggregations of bacteria occurred mostly inside pellets associated closely with intact  I. galbana  flagellates and partly digested  T. weissflogii  cells; some scattered bacteria were found among fragmented valves. No, or little, bacterial colonization was associated with empty  R. alata  valves, and hardly digested  T. weissflogii  cells. Whereas doliolid fecal pellets were loosely packed and composed of fully, incompletely and/or hardly digested food particles, pellets of the copepod  E. pileatus  were densely packed and consisted mainly of fragmented diatom valves. Pellets of doliolids and calanoid copepods can represent a high percentage of the particulate organic carbon in the water-column on subtropical continental shelves.

The ultrastructure of a doliolid and a copepod fecal pellet