Floaters

First some worldbuilding (not part of the actual prompt) Therinos is an ocean world, it has 2 suns and it's an earth-like world. It has 2 moons, the first one isn't reflective and is hard to see, the second is, and is well to be seen. The atmosphere closely resembles the earth's atmosphere, and it has 1 mountain which is above the surface. There are a few main species on the planet, humans, cave crawlers, floaters, there are a lot more under the surface, and also on the surface, these could be mentioned from time to time.
prompt 4. A species of working animal floaters, a species of animal that floats on the surface of the waterworld, these floaters have lungs and used to live in colonies, but eventually, they became so predominant that the entire surface was filled with them. They use photosynthesis to get energy, but they started to grow on top of each other and evolved legs when the surface was filled. Because of this, they started to grow layers of dead floaters, and on the top, there are alive floaters. Eventually, plants started to grow on top of the floaters, the floaters adapted to this by tapping the energy from the plants, the moving wasn’t necessary anymore so they evolved to lay still again, tapping energy from the trees while giving them minerals from the dead floaters below them.
However, in the north it's too cold for trees to grow, so the floaters there evolved to give the energy they got to the floaters below so they can store it and give it back in the winter when the sun doesn’t give enough energy to stay alive, the floaters have a thicker shell and on the bottom hair which helps to warm the floaters below, which results in them having to use less energy. The humans in the tropical zones go to the north to get these floaters, which give of a lot of energy in the warmer regions. The humans in the north are less developed than the ones in the warmer regions, so they don’t care about the top floaters, they care about the ones below that which stored a lot of energy, which they can use as a heat source for in the winter. Because of the floaters, humans can live high in the north near the north and south pole, and even thrive, because it’s the poles are too cold for the floaters to live is it an excellent fishing spot and many humans live near those lakes (which are frozen, but still more accessible than going through the hard shell of the floaters). The blood of the floaters is green and has special immune cells which the humans use to treat the sick