8), 9), and 10) I call lobe jellies. They are cnidarians with lobed bells.
11) is a worm or mollusk with suckers on the trailing edge of each leg.
7) is some sort of planktonic larva with a transparent gut.
6) is a steeple fish, so named for its tall dorsal fin. I don't know what it uses it for.
5) is the arcswimmer. Its fins have two attachment points each, leaving two large holes. I don't know why.
4) is a jellyfish-like creature that can store salt in its body (guess how I got that idea). I tried to come up with some creative way for it to use its salt. I thought that maybe large schools of these creatures could release their salt at once, creating a light-bending barrier where the salt water meets the surrounding freshwater, perhaps to confuse predators. Thinking more about it, I realized this would be so incredibly impractical as to be preposterous. Instead, I decided it just sprinkles poisonous slugs before eating them to neutralize the poison.
3) swims with its four tiny legs, sweeping particles into its net that filters them out of the water. The dots are eyes. It is a rare asymmetrical creature.
2) is a mobile colony. It crawls across the seafloor. The polyps grow in rows that define certain shapes. The lower picture is a "stick-figure" not showing the individual polyps.
1) has a string with special traps for capturing morsels. The string is slowly moved one way through the gut, and then the opposite way, while the morsels are digested. It is a relative of one of these four creatures.