The adaptive significance of die1 vertical migration of zooplankton

Many taxa of both marine and freshwater zooplankton perform diel vertical migrations with amplitudes from a few to 100 metres (Hutchinson, 1967). The 'normal' pattern is an evening ascent and a morning descent, though several cases of 'reversed' migrations have been described (Ohman, Frost & Cohen, 1983; Bayly, 1986). Migrating animals spend the day in deep waters but stay near the surface at night. The amplitude of the movements and the shape of the vertical distribution of the population may be very different between species and between ontogenetic stages of the same species and may be influenced by factors like turbitiy and food abundance (Bohrer, 1980; George, 1983). Zooplankton may either migrate up and down together in a narrow band or may be sharply stratified in deep waters during the day but spread throughout the entire water column at night. However, what we observe are only changes in the population density at different depths of the water column. When taking plankton samples from different water layers, we obtain a vertical profile of animal abundances. Shifts in these vertical distributions are usually interpreted as movements of the population and day-night differences between means or medians of the distributions serve as measures of the vertical range of migration. However, such population responses may be seriously misleading in terms of the behaviour of constituent individuals (Pearre, 1979a). Individuals may vary speed and direction of their movements, so that at any time some animals move upwards while others rest or move downwards. The movement of the population reflects only the net effect. The distance between the mean depths of a population at different times equals the distance travelled by an individual only if all animals migrate synchronously. Otherwise, the movement of an individual can be considerably underestimated. As it is not possible to track small individual zooplankton in situ, this problem is still not solved. In the sixties, the main concern of investigators of vertical migration was the neurophysiological basis of the rhythmic behaviour. Behavioural physiology tried to identify the stimuli for initiation and direction of the migration. The relative change of light intensity has been found to be the proximate cue that controls the upwards and downwards movements. At least in Daphnia, the reaction depends on the level of light adaptation of the animal's eye (Siebeck, 1960; Ringelberg, 1964; McNaught & Hasler, 1964; Ringelberg, Van Kasteel & Servaas, 1967). Today the focus of most research on vertical migration has shifted from the environmental control towards the search for ultimate reasons.