Experiments on the Vertical Migration of Plankton Animals

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Abstract
Experiments were made with apparatus, largely of glass, specially designed to study the vertical movements of plankton animals under as natural conditions as possible at various depths in the sea. The animals were introduced in darkness into the apparatus which was kept covered with a black clothuntil it was lowered below the surface so that from the time they were caught, in the opaque tow-net bucket, to the time they were returned to the sea in the apparatus, they were never subjected to above-surface illumination. The experiments were begun and ended at the required depth by the dispatch of messenger weights down the suspending wire from above. The copepodCalanus finmarchicus(Gunn.) was used, but a few experiments were also made with another copepod,Euchaeta norvegicaBoeck.The apparatus is described in detail; it takes two forms. No. 1 consists of two vertical parallel glass cylinders AB and αβ closed at the top by a glass plate and at the bottom by a metal one, and having trapdoors at the middle of each which divide them into upper and lower compartments, A and B in one, and α and β in the other. They were filled with sea water and the copepods were introduced injo only one compartment of each cylinder, in standard experiments into A on one side and j3 on the other. The apparatus was now lowered to the required depth and the trapdoors opened so that the copepods were free to move up or down the whole length of the cylinders. At the end of an experiment the trapdoors were closed again so that the percentage proportions that have moved up or down under different conditions could be estimated. In experiments other than standard, one side was left normal as a control and in the other the conditions were altered for the purpose of the experiment, e.g. darkened or the pH of the water increased or decreased. Apparatus No. 2 is a modification, a long tube of seven compartments each separated by a controlled trapdoor.
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