Feeding rhythmicity in a migratory copepod (Pseudodiaptomus hessei (Mrázek))

SUMMARY. This paper describes a day/night feeding rhythm in the copepod Pseudodiaptomus hessei in subtropical Lake Sibaya (27°20' S, 32°40' E) and examines the origin of this rhythmicity on the basis of field and laboratory observations.
Under natural conditions, adult and late copepodite stages (C_IV-C_V) of this copepod were predominantly benthic during daylight and migrated at dusk into pelagic waters in which they remained until dawn. Pronounced changes in feeding activity (based on the proportion of animals which contained food in the gut) were associated with this transition from a benthic to pelagic distribution; feeding activity was low by day and high at night. The younger copepodite stages (C_I-C_III) were less strongly benthic during daylight and showed correspondingly less pronounced diel changes in feeding activity.
Laboratory experiments involving concurrent light/dark and continuous dark regimes showed that batches of adult and C_IV—C_V stages fed throughout 24 h, although faecal pellet production increased with time under both light regimes. The laboratory observations are interpreted as negating the possibility that diel differences observed in the field arose through an endogenous feeding rhythm. The possibility that food resources in near-bottom waters was inadequate to support reasonable feeding activity is rejected on several counts. It is concluded that the feeding rhythm in P. hessei arises from an inferred infaunal distribution during daylight which would preclude filter-feeding activity. The circumstantial evidence supporting this suggestion is given and the suggestion is evaluated.