DIURNAL VERTICAL MIGRATIONS OF DEEP-WATER PLANKTON

Abstract

A study was made with the aid of closing nets of the diurnal vertical migrations of bathypelagic organisms at a station in continental slope water of the western N. Atlantic. While the hauls were being made a continuous record of the light intensity at the surface was kept. The penetration of light into the upper 84 m. was directly measured photometrically; the average extinction coefficient for green light was k = 0.092. All of the malacostracan Crustacea (to which the detailed results presented in this paper are limited) which occurred in sufficient numbers for analysis exhibited diurnal migrations 200 to possibly 600 m. in vertical extent. The speed of vertical movement in these migrations varied from 24 to 125 m. per hr. among the various crustaceans. A considerable part of the migrations took place while the light intensity even at the surface was no greater than starlight. Several Crustacea living at 800 m. during the day showed extensive diurnal vertical migrations. It is concluded, however, that whether the migrations are regulated by external environmental or by internal physiological factors, at some time of day the organisms concerned are affected by light penetrating from the surface. Calculations made from the light penetration data indicate that the amount of light probably present during the middle of the day at the depths where the animals were migrating was adequate to support this conclusion.