such 12 kHz scattering show steep rises during solar eclipses, too, although just as the
moon fully covers the sun the isolumes rise too fast for the swimmers to keep pace.
This is a clear illustration of the simplest response pattern.
(^) Clean experimental demonstrations of the response to light can be produced with
freshwater cladocerans. Harris and Wolfe (1955) held Daphnia in a glass cylinder
filled with an India-ink suspension to raise the light extinction coefficient, thus
collapsing into a short length the gradient from surface irradiance to darkness. The
cylinder was periodically photographed from the side with red light as a
programmable rheostat varied the surface light intensity through a cycle simulating a
day in a few hours. Daphnia were counted in short increments down the photos of the
column (Fig. 8.18).
Fig. 8.18 Artificially induced, accelerated, and vertically compressed migration cycle
in Daphnia held in a 32 cm cylinder of India-ink suspension. Positions of 50 Daphnia
are represented for 10-minute intervals by widths of the “kite diagrams”, as shown by
the scale at the top. Surface illumination in lux was progressively changed as
indicated by the numbers above the diagrams. DR indicates behavior equivalent to a
dawn rise; MS indicates behavior equivalent to midnight sinking.
(After Harris & Wolfe 1955.)
In complete darkness the animals sank to the bottom. As the illumination increased,
they swam up to near the surface, a phenomenon known as a “dawn rise”. Then as
illumination increased more they swam back down, finding a “mid-day” depth. The
mid-day depth showed signs of “accommodation” to the light intensity; that is, the
animals moved up somewhat during the period of strongest illumination. Then, as the
illumination faded, they accelerated toward the surface. Once their preferred level of
illumination was gone, they sank back to the bottom. All the features of vertical
migration in the field are reproduced and can be manipulated. A full migratory cycle
can be compressed into an hour or several, since intensity variation is such a dominant
control on upward and downward movement. Close coupling of the movement of the