FEEDING ECOLOGY OF BENTHOPELAGIC ZOOPLANKTON ON AN EASTERN TROPICAL PACIFIC SEAMOUNT

The gut contents of dominant deep-sea benthic boundary layer zooplankton (primarily copepods and mysids) and the vacuole contents of phaeodarians collected and preserved in situ at four depths (from 724 to 3112 m) on an eastern tropical Pacific seamount (Volcano 7; 13-degrees 23'N, 102-degrees 27'W) between 23 November and 4 December 1988 were analyzed using transmission electron microscopy (TEM). Suspended, and sinking plus resuspended particles, were quantitatively sampled to characterize potential food sources. The broad oxygen minimum characteristic of this region intersects the summit of the seamount and affects the feeding ecology of these organisms. Several copepods and mysids and an amphipod contained guts packed with what appeared to be grain-positive bacteria, an unusual finding. We hypothesize that the source of these bacteria-like bodies was a mat or aggregate that originated in the oxygen minimum or at its upper or lower interface. The presence of the bacteria-like bodies in 43 to 100% of the particle-feeding zooplankton that were sectioned and that had gut contents, suggests that the bacteria-like bodies are an important food source. The diverse gut and vacuole contents of other detritivores were similar among depths. Particles and microorganisms from the depths were also similar. This finding can be explained by the rapid sinking of particles and aggregates from surface waters and their relatively intact transit through the broad oxygen minimum with its reduced populations of zooplankton. The presence of algal cells in guts and vacuoles of benthic boundary-layer zooplankton suggests that these zooplankton select particles of recent surface origin for consumption. The presence of the guts filled with bacteria-like bodies shows that some deep-sea copepods and mysids that are normally generalist feeders can specialize opportunistically. The similarity of gut contents of crustaceans and vacuole contents or phaeodarians suggests that these two very different groups of particle feeders utilize similar food sources in the deep sea.