Movement of organisms can be seen by sound.

The echograms below were recorded simultaneously at frequencies of 100 kHz (top) and 25 kHz (bottom). They were seen just after sunset. The V-shaped echoes in both diagrams are recordings of a plankton net with an 80 cm diameter ring frame being towed vertically. In the upper echogram, we can see something moving from the leftmost depth of about 160 m to the surface. In the echoogram below, however, we cannot see it. This is another difference in response due to acoustic characteristics. The identity of this creature was confirmed by the plankton net samples, and as shown in the photograph, it was a krill or a tankard, as the name implies. On the other hand, a "spiral" echo can be seen at a depth of about 60 m. What is this? What is this?