These are rotifers , copepod nauplii , tintinnids , and peritrichs . Rotifers are small multicellular organisms that have a characteristic crown of fine hairs or cilia surrounding the mouth area. Rotifers primarily eat phytoplankton, bacteria, and detritus. Some will occasionally eat another rotifer if the prey is small enough. This rotifer is a Brachionus sp.
Copepods occur in all aquatic environments.
These creatures eat phytoplankton and small microzooplankton. You can see these in this photo. There are other animals with stages called nauplii in addition to copepods, like the barnacle, so not all nauplii are the same. Ciliates are a group of organisms that are single celled and occur in all aquatic ecosystems. Unlike the rotifers, they usually have cilia all over their entire bodies.