Visual Contrast Threshold of Japanese Common Squid <i>Todarodes pacificus Steenstrup</i>

Abstract

The visual contrast threshold of Japanese common squid Todarodes pacificus Steenstrup was sought through a series of experiments by observing their responses to contrast test boards. The contrast test boards consisted of three vertical stripes of which the outer two were painted in black and the middle band was in 11 different hues from white to black, exhibiting the apparent contrast values of 31.3 to zero (O). The experiments were carried on under the light intensity of 1.01x at the water surface, and the total number of squid used was 68. When an experimental squid was confronted with the test board of which apparent contrast was less than 0.63, he made a jet-hit response to it. This jet-hit response was particularly prominent when the apparent contrast was less than 0.008 and decreased in frequency with the increase of contrast value. At contrast values above 1.45, he avoided the middle band. By means of the 50-percent point of jet-hit response probability, the visual contrast threshold of the squid was estimated to be 0.006.