Differential feeding and fecal pellet composition of salps and pteropods,and the possible origin of the deep-water flora and olive-green “Cells”

Salps (mainly Salpa fusiformis and, to a lesser extent, Pegea socia) and a web-building pteropod (Corolla spectabilis) were studied in epipelagic waters of the central California Current. Although both kinds of gelatinous zooplankton trap phytoplankton in a mucus net, a fecal pellet analysis indicated that their diet differs significantly when they feed together, probably because of differences both in the pore sizes of their nets and in their feeding methods. Salps have a finemesh filter, on which they can retain even the smallest phytoplankton; thus, when small coccolithophores are abundant, as they were in our study, salp feces contain such cells and the coccoliths derived from them. In contrast, pteropods feeding in the same area produce fecal pellets consisting chiefly of larger phytoplankton, especially diatoms. Since fecal pellets transport most biogenic material to the deep sea, changes in herbivore species composition at a given geographic location can change the chemistry of materials entering deep water; at our study site, the more salps, the greater the calcite flux, and, the more pteropods, the greater the silica flux. In addition, fecal pellets of both salps and pteropods include partially digested residues of phytoplankton that appear as olive-green spheres, having an ultrastructure identical with that of the socalled olive-green cells. Presumably, fecal pellets, after sinking into deep water, ultimately disintegrate. releasing both the viable phytoplankton and the olive-green spheres into aphotic waters. Thus the feces of epipelagic herbivores are likely sources of much of the flora of the deep ocean.