This behavior has fascinated me ever since I saw it on Youtube. I never thought I could capture it myself on my new microscope.   The water was collected from the backwaters at Lake Havasu, Arizona.  It may be noted that pulling along the detritus doesn’t bother them nor does the innocuous organism, yet to be identified, in their path.

Bacillaria are said to be single celled diatoms.  I don’t know how they aggregate so that they contract, expand and move as a unit.