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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.

Here are some very simple pictures (apologies for the overal quality; these were made in a rush.) enter image description here

Their evolution goes something like this:

enter image description here

here's a (slightly) more detailed look at spider molds; Those clumps where additional hyphae branch out are previous victims, most likely small fish.

enter image description here

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?

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    $\begingroup$ I think it might be possible it wouldn't differ significantly, but why wouldn't animals also evolve to fill these very favorable roles? Or did animal filter feeders go extinct? A lot of the animal filter feeders have moving parts to improve efficiency/move food, so how do fungi compete? The earliest multicellular animals were likely filter feeders (we're more closely related to sponges than octopi) so you might severely affect the evolution of animals and thus radically alter the ecosystem. $\endgroup$
    – DWKraus
    Jan 13, 2021 at 22:49
  • $\begingroup$ It's a seeded world, so a small selection of animals were placed on an empty planet. Near as I could tell, Fungi were in the best position to take the role, as the animals present are all chordates or arthropods (and maybe a couple worms) $\endgroup$
    – Globin347
    Jan 14, 2021 at 0:00
  • $\begingroup$ That makes sense, I'll give it some thought. $\endgroup$
    – DWKraus
    Jan 14, 2021 at 0:19

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Mass Extinction of Filter Feeders/Lack of Evolution of Filter Feeders

This happened with the introduction of zebra mussels into the Great Lakes. The lakes used to have a number of filter-feeding species, such as paddlefish, but the introduction of zebra and later quagga mussels resulted in the lake being filtered so heavily that they became stripped of plankton and now paddlefish can only survive in rivers and smaller lakes where zebra mussels can't get a foothold.

The big difference between your world and IRL Earth is that the dominant reef-building organisms are filter-feeders, rather than photosynthesizing symbionts like corals, stromatoporoids, kelp, etc. That said, there are some examples that could help you get some idea of what it would be like, such as crinoid, rudists, and inoceramid clams.

Problems when plants get a foothold on land

You might end up with a mass extinction of filter-feeding fungi when plant life gets established. Something kind of similar happened with the evolution of plants in the Devonian. There used to be a very diverse community of coastal oceanic filter feeders in Earth's oceans, like crinoids and stromatoporoids. Then, in the Devonian, when large plants with deep taproots emerged, it sped up erosion and all of a sudden huge amounts of sediment started to flow out of the rivers into the ocean. Animal communities at the estuaries of rivers, which were formally very diverse, were smothered by sediment, leading to the modern pattern where reefs and other complex ecosystems tend to not grow around the immediate area where a river spills out into the sea.

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  • $\begingroup$ It's worth noting that due to the unusual circumstances here, plant life will probably already be established on land by the time these fungi evolve. $\endgroup$
    – Globin347
    Jan 14, 2021 at 2:10

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