How efficiently can doliolids (Tunicata, Thaliacea) utilize phytoplankton and their own fecal pellets?

Abstract

The quantitative utilization of detritus as food for zooplankton has been often addressed. Yet studies on the topic are lacking. Thus, the goal of our study was to determine the food absorption efficiency (AE) of the doliolid Dolioletta gegenbauri being first offered phytoplankton and then, subsequently, fecal pellets as a representative of detritus. The AE feeding on phytoplankton was 38.7%, and subsequently, on their own fecal pellets 22.7%. The comparatively low value of 38.7% is due to the doliolids' inability to mechanically destroy the ingested microalgal cells. Part of the remaining nutritious contents of the cells constituting the fecal pellets are then removed when those pellets (first-generation pellets) are ingested by doliolids. After removing 22.7% of the remaining cell contents, considerable amounts of carbon and nitrogen should even remain in the second-generation pellets for subsequent zooplankton consumers. In conclusion the frequent occurrence of doliolid pellets could contribute significantly to the probability of zooplankton survival when phytoplankton abundance is low.