Radiolarians have shells of silica, which produce siliceous oozes in the equatorial regions. They have a ring of cilia with which they swim and capture phytoplankton. Bacteria These are the smallest living things, with cells one to a few micrometers long. They are extraordinarily numerous, with as much as a million cells in a drop of water in productive waters. To put that in perspective, the total biomass of bacteria in San Francisco Bay has been estimated as equal to 57 times the biomass of Humphrey the whale.
Bacteria do a lot of different things in seawater.
Sampling plankton Sampling procedures depend on what is being sampled. For zooplankton usually we use nets with mesh size selected to get the species we are interested in. These nets are usually conical with a jar or other container at the small end and a rigid ring at the open end. The net is towed through the water, then the sample is rinsed down into the net. Often the nets are rigged to open underwater and then close again, so the sample comes from a selected depth.