Animals & nature · 1943
Why can laboratory light make Acartia tonsa swim upward while it moves downward in the ocean by day?
The angular distribution of light matters: in highly directional light, this swims toward the light, but in diffuse light it stops swimming and sinks passively. Light measured in the air and sea was generally more diffuse than in the laboratory cylinder, and the laboratory response to diffuse light accords with its daytime downward movement in the ocean.
Details
Laboratory illumination experiments examined the behavior of the copepod Acartia tonsa in a tall glass cylinder, alongside measurements of light distribution inside the cylinder, in the air, and in the sea. When the cylinder was illuminated from above, the animal swam upward. When the cylinder was illuminated obliquely, the animal sank downward.
Measurements inside the cylinder showed that A. tonsa's behavior depended on the angular distribution of light. In highly directional illumination, it responded phototropically, swimming toward the light. In less directional, diffuse illumination, it stopped swimming and sank passively downward.
Measurements in the air and in the sea showed that light was generally more diffuse there than under the conditions in the cylinder. The animal's laboratory response to diffuse light accorded with its downward movement in the ocean during the day. The abstract concludes that its response to the direct light typically used in phototropism experiments has no bearing on its behavior in nature.
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