Some Experiments on the Vertical Migration of Zooplankton

Abstract

Using appara-tus consisting of 2 tubes, each 1 m in length, at various depths down to 20 m in daylight in Windermere to test vertical orientation of zooplankton, it was found that Diaptomus gracilis of shallow origin tended to be found in top of tubes, whereas Diaptomus of deep origin tended to be at bottom. For Cyclops strenuus of "shallow" origin, as expts. were carried out at progressively deeper layers, downward aggregation was succeeded by upward aggregation. In Windermere, several spp., when tested at same depth at different times near and during darkness, showed a tendency to reverse their initial reaction as light increased or decreased. For example, Cyclops strenuus in evening was down, and as light decreased it rose. For several spp., behavior at constant depth, with light varying was similar, and in certain spp. identical, to that found at varying depths during daylight. This provided indication that light, rather than depth, was mediating factor in migration. There was a rough relation between depth at which forms were found in daylight and character of migration that they showed in series of expts. at constant depth at dusk, night and dawn. This was true both in Windermere and North Sea. Forms in question were Diaptomus gracilis, Cyclops strenuus stage IV, C. strenuus stage V, Pseudocalanus elongatus and Temora longicornis. Patterns of migration in these were classed as "afternoon", "evening", or "night". In expts. with a tube 13 1/2 ft. long in Windermere, a few diaptomids and Daphnia longispina climbed at a speed of 3.5 m/min. Placing long tube across a thermocline had no effect on distr. of descending animals. Exptl. aggregations are discussed in relation to observed vertical migration in sea. Movements of animals making such migrations could be phototactic or photokinetic. Some evidence that within tubes at any rate, plankton animals were behaving photokinetic ally.