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On Earth their are thousands of animals with symbiosis with plants such as honey bees and flowers. However for this question I'm focusing on a very specific kind the one that leaf-cutter ants have with fungus. These ants bring all the necessary nutrients to the fungus in the form of leaves. So I'm looking for something similar to happen in the sea. A plant-like organism is in the middle of the ocean with a sea bed thousands of miles under it. It needs nutrients essentially it needs iron which would be lacking near the surface where it needs to stay in order to photosynthesize. So it forms a symbotic relationship with colonial organisms similar to fish that bring nutriets to the plant in return for food and shelter. These organisms would have to find a way back to the plant-like organism which i know some salmon can find their way back to their home river through magnetic feilds and scents. However on such a large scale I'm not quite sure If this would work. I also understand that these colonial organisms would have to be able to withstand pressure changes similar to what a whale has to do while diving. But what kind of adaption in its body would it have to account for this? I'm working on an ecosystem based on a water world planet with very sparse islands. Life has evolved on these islands, but the deep oceans are barren. So this plant-like organism with the help from the colonial organisms move from the shallow waters to the deeper oceanic water. These plant-like organisms will then become the basis of a oceanic ecosystem where they bring up nurtriets from the deep that feed MASSIVE herbivores that feed even bigger predators! I kinda got this idea from subnautica and thinking how the large predatory organisms got their nutriets.

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    $\begingroup$ Ocean thousands of miles deep would run into problems with physics before it runs into problems with biology. $\endgroup$
    – void_ptr
    Jul 18, 2022 at 21:39
  • $\begingroup$ I agree, the physics of an animal diving in an ocean to thousands of miles is implausible. Thousands of feet might work, and I could come up with some plausible scenarios for that. $\endgroup$
    – DWKraus
    Jul 18, 2022 at 21:50
  • $\begingroup$ @void_ptr brings up a good point if reality is that important to you. But, it's worth reminding folks to not get too distracted by the back story. $\endgroup$
    – JBH
    Jul 18, 2022 at 22:27
  • $\begingroup$ Wouldn't the plant already be food for the fish, it would have to be some really special substance they create as well as the fish being reasonably intelligent compared to the average fish. $\endgroup$
    – user96146
    Jul 18, 2022 at 22:31

2 Answers 2

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I'm going to answer your question, but before I do, I need to make a point about how this Stack works:

actually feasible

That's a question that doesn't really make sense here. After all, our purpose is to help you create an imaginary world... But! If you use the tag, we'll create answers that reflect science but only use it as a guide to develop creative new ways of solving the problem. If you use the tag, we'll stick closer to science and give you answers that reflect similar solutions to what we know. If you use the tag, you'll only get answers that really are "feasible," meaning they can happen on Earth because here's the mathematical equations and four scientific paper citations to prove it.

In other words, "feasible" is something you understand. It's not something we understand without reading your mind... or if you delete one of the tags and use one of those I just mentioned.

Now, back to our story.

I give you: the humble mosquito!

enter image description here
Courtesy The Verge. Click to enlarge.

This is the kind of symbiosis I recommend. Drink the fish's blood! It's full of lovely iron, doesn't require your plant (or, less believably, the fish) to travel thousands of miles to the sea floor, and the mosquito sets a precedent for an existing natural solution.

This also allows you the option of developing a more complex biome, if you choose, to rationalize the transfer of iron all the way from the sea floor thousands of miles below to the surface. You'd need a series of hungry critters, each moving the iron a little higher in altitude.

Mosquitos... it's probably their only practical use other than feeding bats. I like bats.

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Monster Filters, Anchor Kelp, Muck Fungus, and Diver Packs:

It takes an ecosystem to make this make any sense. First, thousands of miles is out of the question - the logistics of things living at those depths is tricky, let alone things DIVING up and down those distances. I'll assume a shallower ocean. But ecosystems are complex and take time, so bear with me!

  • PROBLEM 1 - buoyancy: The sea supports a lot of weight. very few things in the sea need to use anything but water to transport materials, so fish are not generally required. Species can reproduce by dumping thousands of offspring into the sea, and survive by a numbers game of having huge volumes of tiny individuals.
  • SOLUTION: Your world has some frightening filter feeders. While small species exist, these beasts are absurdly efficient at cleaning them out. Plant and fungoid species must hook a ride on fish to move their offspring to new areas.
  • PROBLEM 2 - solubility: Water is very good at dissolving iron and minerals like it. The ocean works because these minerals are mostly dissolved in the water and free to whatever species is willing to put in the effort of extracting them.
  • SOLUTION: Your ocean is mineral-poor. Fungoid-like species make their living digesting any organic material that reaches the bottom and holding on to dear life to those minerals.

Those are your worldbuilding issues needed to set up our scenario. Now your species need to evolve the behaviors.

  • PROBLEM 3 - plants need minerals: Your plants need minerals to survive in this mineral-poor ocean, but the minerals are bound up as fungi at the bottom.
  • SOLUTION: The anchor kelp create weighted pods that sink to the bottom. The earliest ones sank, absorbed minerals or adhered to fungi and sought to parasitize. Then they floated to the surface and began a photosynthetic life. The pods need to be fairly big to avoid the filters. They don't drift far, and are vulnerable to being eaten. Eventually, these anchors start to develop symbiotic relationships with the fungus - minerals for calories obtained in photosynthesis.
  • PROBLEM 4 - predation: Your pods are eaten as they sink, and are eaten as they rise. Pods must be fairly caloric going down (like fruit) and nutrient-dense going up. This makes them vulnerable.
  • SOLUTION: Get fish to swim as diver packs to the bottom with the pods, eating little fruiting bodies that ripen just as the fruit sinks. The fish fight off rival species to defend their food. The same or different species are attracted by chemical signals when the pods are ready to come up, and the fish similarly defend the pods on the way up, nibbling them like a salt block.
  • PROBLEM 5 - the filters stop plants & fungus from spreading
  • SOLUTION: Your plants and fungi have become dependent on fish for protection of their pods. With the close relationship of the plants and fish, the plants begin sticking seeds & spores on these fish, and the fish carry the seeds around the filters. Now the fish act like pollinators, but closer.

Now all you need to have a species working like leafcutter ants is for the fish to escort the pods to the bottom and eat fungus instead. The pods get covered in fungus, the fish escort the pods to the surface to eat the fungus there. The fungus is fed, the pods follow their pattern to get minerals, and all species spread together on the backs of the mobile fish.

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