These settle on the bottom, tuck the tail under, and molt to become crabs. This molts into a cypris larva, which has the job of finding a place to settle. Once it finds a suitable rock, pier piling, or boat, it cements its head down, sticks its feet in the water, and starts growing a shell to act like a barnacle. Probably for dispersal, since the benthic environment is so variable and unpredictable, and this way they hedge their evolutionary bets. Also, this may help with dispersal if the local area gets too crowded.
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.