Subweb
In their quests for solid surfaces to grow between, interwebs often stuck to the undersides of matleaves and tried to form webs in the gaps between. This worked fine in terms of building their webs, but it was rather disastrous when it came to having much to eat. Lying nearly flat by the surface of the water, very little of their prey had any reason to swim up through it. Any interwebs that extended downward had a greater chance of finding food, and so a new species, the subwebs, arose that specialized in dangling beneath the matleaves.
Like the interweb, subwebs consist of clumps of four cells in which each cell has a tentacle that it can use to stick to things, including other cell clumps, allowing them to exchange nutrients. These tentacles hold the cell-clumps far apart from each other, making the web as a whole light and insubstantial, mostly empty. As before, they first grow by forming lines to cross gaps, now always the spaces between matleaves on between lobes on a single matleaf. But the subweb forms additional long dangling lines that hang below, with further lines holding them together into the web form. They will then, even more thinly, fill the space between this web with more connected cell-clumps, connected to each other but very diffuse. (They may tangle among the matleaves' filter baits too.)
Subwebs mostly grow by binary fission, as each four-cell clump will split in two as a unit. New clumps may extend or fill in the web or may drift away. Webs also grow as cell-clumps congregate, unrelated clumps joining together into the same web or adding on to an existing one. Individual webs won't get much larger than a single matleaf, but as neighboring webs grow, they'll come into contact with each other, able to further share nutrients and swap newly-fissioned cell-clumps; such larger web-colonies can reach five centimeters across.
Much of the time, subwebs subsist on filter-feeding, digesting any microbes that get stuck to them, but larger prey organisms are liable to get stuck too. Being so diaphanous, even the lines of the web are difficult to see. When a web ensnares large enough prey, cell-clumps will contract around it for a feeding frenzy, holding it stuck and secreting digestive enzymes. If the prey item is too big or manages to tear its way out, causing the web to break apart, the web will fall apart into fragments, each of which may go off to found a new web or join another one.
However, after a particularly hearty meal, the web is likely to contract around it entirely, such that the cell-clumps are actually touching each other. This becomes a fruiting body, in which the outer layer of cells becomes a tough shell, and the inner cells burst open, and the inside becomes a soup of nutrients and genetic material. This fruiting body will then eject spores. The spores have changed slightly in form so as to be highly buoyant, as the bottom of the ocean is of no use to them. They float across the ocean surface until coming in contact with a matleaf, at which point the spore will release its cell-clumps.
In some ways, subwebs are a parasite on matleaves, as although they don't actually eat matleaves, they do compete with them for food and weigh them down a little, though they are quite light. On the plus side, though, they could snag and devour something that would otherwise have munched on a matleaf, and semi-digested remnants of their prey may include fragments that are small enough to be picked up by matleaves' filter baits.
This new lifestyle has allowed the subweb to spread to everywhere that matleaves inhabit.