So, I'm asking this on behalf of a seeded world project. There are no corals, sponges, kelp, or anemones in this world's oceans (although there are seagrasses); instead, all those roles are taken up by aquatic fungi.
I've taken to calling them "sea molds" for lack of a better name.
The most basal variant is called a "hair mold", because it looks like a wad of hair underwater. It consists of the normal, underground hyphae network that most mushrooms are made of, a small, puffball-like body on the surface, and a different type of hyphae that grow out of the body into the water. These are the filter-feeding hyphae nets that catch plankton.
Some of these have evolved into more derived variants:
Rock molds have much larger bodies, covered in either calcium carbonate armor or chitin. (Apparently, fungi can make chitin. I just learned this recently.) There are holes in the bodies through which hyphae reach into the open water; these can presumably be retracted.
The other group is stalk molds, which basically place the round "pseudo-body" on a stalk. Some of these branch out into multiple stalks, which may have rocky armor themselves or may be toxic.
The most derived variant is the spider mold, which catches fish like an anemone.
Their evolution goes something like this:
here's a (slightly) more detailed look at spider molds; Those clumps where additional hyphae branch out are previous victims, most likely small fish.
So, given the lack of corals, kelp, sponges, and anemones, and the presence of these derived fungi, how will this world's oceans differ from those on earth?