DISTRIBUTION AND DIEL VERTICAL MIGRATION OF PSEUDODIAPTOMUS-HESSEI (MRAZEK) (CALANOIDA-COPEPODA) IN A SUBTROPICAL LAKE IN SOUTHERN AFRICA

The vertical distribution and diel migratory behavior of P. hessei in a freshwater lake is described. All stages showed a pronounced diel migration. During daylight naupliar and copepodite instars were almost exclusively benthic in shallow areas (< 10 m). Copepodite V and adult stages were predominantly benthic even at 40 m. The diurnal vertical distribution pattern implies age-related differences in photosensitivity and a possible depth-regulatory mechanism, based on the existence of differential photosensitivity, is offered to account for day-depth control. Nocturnal vertical distribution, studied approximately fortnightly during 1970-71, showed no clear seasonal variation. Variations in pattern, most obvious in adult instars, correlated with prevalent lunar conditions. Nocturnal or midnight sinking was evident, particularly in post-naupliar stages. Dusk rise and dawn descent were performed at very low light intensities and paralleled changes in light penetration. Dawn descent was less rapid than the dusk rise. An ephemeral dawn rise was sometimes observed but was not a consistent feature of all stages. An endogenous rhythm of locomotor activity was recorded under laboratory conditions. Activity was low during daylight, increased sharply at dusk and then decreased through the night to reach daytime levels at dawn. The stimulus provided by changes in light intensity and/or endogenous activity changes can account for the dusk and dawn movements of P. hessei but a definitive identification of the direct migratory stimulus is not possible. Attention is called to a closer examination of the role of endogenous activity rhythms in vertical migratory behavior. The possible adaptive value of vertical migration to P. hessei is examined briefly.