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The world in question is an Earth-sized planet with a deep global ocean and core of water-ice.
Given the lack of an accessible sea floor to root oneself to, how would a complex free-floating organism function? How might nutrients enter the system? Is this sort of system plausible?
On the moon titan, beyond the initial ice layer, there is an entire ocean layer. If you continue further down you hit ice VI (which is always ice, just with a denser crystalline structure). This deeper ice layer is caused by sheer pressure.
Presumably your ocean world will have a point deep enough that the pressure will be too great for water to remain liquid and will turn to ice. It matters not that the core of titan is solid. Your planet could conceivably be entirely made of ice through and through.
The problem would be the rest, which is to say, all organic life is built from carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. You can obtain only two of these from the surrounding water. As far as we know, a water planet with no other elements than hydrogen and oxygen could not sustain life. You'd need to have to allow the water to be rich with other minerals in order to justify a thriving ecosystem . The fewer the minerals, the fewer lifeforms you will see grow on such a planet.
My idea is that minerals come from meteorites which have crashed into the ocean planet over the course of millennia. These meteorites would float like underwater islands, being too heavy to stay on the surface but too light to sink below the crushing ice VI layer.
Plantlife can grow on these islands, and like a coral reef, you'd find most life would grow exclusively from these islands. This isn't to say life couldn't exist between islands, but they would be larger creatures and they could probably swim quickly enough to reach other islands.
While life would live near the bottom, I don't think life would ever physically touch the ice layer, as it would likely quickly freeze over anything that touches it, plantlife or otherwise.
Well, let's look at the producers first; they're at the base of the food pyramid, so once they're there, a full ecosystem can form. Plants need four basic things to grow: air, water, nutrients and sunlight. You've got air, you've got sunlight and I'm preeeettty sure you might have some water around there. The problem is nutrients.
Plants evolved on Earth from algae and other simple flora - which are aquatic. They get their nutrients from carbon, oxygen, nitrogen and phosphorus. So, there's oxygen in the water, but the rest are not present. So, you need to obtain the other minerals from something external to the planet.
This could be two things - minerals present in meteorites or artificially introduced minerals. I presume you want the evolution to be all natural, so the idea of a sapient race inserting carbon, nitrogen and phosphorus into the planet's seas may not be satisfactory.
I haven't read the other answers yet, but I wouldn't be surprised if someone else hasn't followed the same intellectual yarn until reaching the conclusion that meteorites are probably the most likely option. The bolides could be laced with nodes of these crucial minerals, which are expulsed into the surrounding ocean, and thus, space-age aliens.
Well, not really. That gives us the building blocks for simple, algal life-forms, but there's still a way to go before fish-people can invade the Earth. We call that way evolution.
So, my original taxonomy of evolution is that it is made up of 3 very closely linked processes - improvement, complication and selection. Note: I am not an evolutionary biologist and do not claim that this is an accurate allocation, it's more of a simplified way of breaking it down.
For plants, improvement usually means more efficient reproductive systems, allowing them to multiply at a higher rate and proliferate. However, plants will also improve their systems for remaining buoyant, consuming nutrients, supporting their structure, transporting nutrients and photosynthesising. You may know that algae are unicellular, so such systems are less tangible, or sometimes non-existent. I recommend you look at how algae evolved into multi-cellular plants to get a better idea of how these systems would come about.
So, the next process in my categorization of evolution is complication. In looking at this picture by the venerable speculative biologist C.M Kosemen (You might know him as Nemo Ramjet.), you can see the vast difference between the first generation of species and the eighth one.
Similarly, if we look to the real world, the anatomy of a Haikuicthyan are vastly simpler than that of a modern Sockeye Salmon. This is very much linked to the last process - improvement - because in improving, the systems of an organism must become more complex. So, to summarize it, over time your plants would evolve into more complex forms.
Now to the third process, selection (again closely connected to the previous two mechanisms.). Natural selection is of course the way nature deals with the incompetent for survival. These include genetic aberrations whose anomalous characteristics hinder their survival ability. For example, there's a reason you don't see many wild albino animals - they simply don't live as long in the wild. However, genetic anomalies can be favoured by selection as well. For example, look at Hemeroplanes triptolemus, the famous snake caterpillar. It mimics a poisonous viper to deter predators. They probably evolved from few caterpillars with an unusual pattern of spots on its head that looked vaguely like a snake. These caterpillars would be eaten less often, and the more spotty caterpillars, the higher chance of them breeding with eachother, and the spots get more and more refined with each generation until you've got a new species.
Then there's another aspect of selection, extinction. In the natural world, success comes in two forms - adaptability and specialization. For example, the simple bivalve genera haven't changed since the time of the dinosaurs. They don't find themselves on the top of the food chain because they're not specialized, but they still survive for dozens of millions of years. Then when you look to the higher layers of the trophic pyramid, you've got the ubiquitous carnivorans, and below them, the ungulates. You could call them successful as well, for they are perfectly adapted for their own environment. But the average carnivore/ungulate genus lasts for only 6-8 million years. They do not deal well with climate change, and the world is an ever-changing place. So, the successful organisms on your water-world will either be simple but adaptable, or advanced but inflexible.
So, you may be wondering, why am I teaching you so much general evolution? Well, Stack Exchange can give you a basic idea of a plan, but refining and continuing your evolution will be up to you. With a synopsis of evolution frequently compared with your planet, this gives you a head-start in the solo territory of worldbuilding.
The basic plants on your world, I think, would look like lilypads, with a buoyant structure to keep them afloat and the other organs submarine. Then you could have plants that grow on their stems, or epiphytes, rather like this real-life tree and the plants it carries:
In one exobiology project of mine, I postulated that floating sponge-like organisms could evolve, and provide a solid surface for "terrestrial" life to evolve on. If something like this might happen on your planet, there could be life like seaweed and coral etc. Also, plants can grow on slow-moving organisms, like the moss on this three-toed sloth:
So, once you've got plants, you can eventually evolve organisms to feed on them. By this, of course, I mean animals. A lot of early animals were benthic, but that's not going to be possible in a seabed-less world. But, all those animals mainly evolved because that's where all the plants were - on the seabed. On your planet, the plants are nearer the surface, so the first animals would probably be simple, buoyant creatures that fed on the plants. The most efficient way of doing that with a worm-like animal (The majority of animals today are worms like Annelids, Platyhelminthes and Nematodes, and they were probably some of the first.) is to have one orifice at the front for consuming food, and another at the back for expelling the waste. Over the course of evolution, worms could turn into bell-shaped animals, giving you cnidarians, or they could become sturdier and form fish-like creatures. The most simple fish today are lampreys, and they do look pretty worm-like, so it's possible that worms could evolve into similar creatures on another world.
Then, you know the drill, the lampreys evolve into cartilage-boned fish and bony fish and all that (I'm not suggesting you rip off Earth's fish, but the basic streamlined design is pretty successful.)
There are some things fish could evolve into that haven't happened on Earth. For example, what if the fish learn to breathe air? And don't think this requires land - just look at lungfish. They evolved "lungs" from aquatic ancestors.
This one was dug up by a guy, alive, encased in a cocoon on dry land.
There are a many possibilities that come with air-breathing. One is that fish could evolve to colonize the topsides of the "lily-pad" plants, to breed or evade predation. This could essentially give you fish that looked like pinnipeds...
Wonderful. The other possibility I thought of is that breathing oxygen supplies more energy to the brain than using gills. This could see a sapient species evolve (I have a personal project centered around a race called the Vendrikk, serpentine aquatic animals that eventually created an industrial civilization. ) To become sapient, they would need intelligence, provided by oxygen, manipulators, which could be claws, tentacles, even prehensile tongues, and collaboration. They could do this to hunt in packs, or for protection.
Well, I've got to go now, and I know it's been a massive answer, sorry if it's too long. I hope it's helped you, and I wish you well on your exobiology project. Happy worldbuilding!
Planets are not always homogenous.
Although the core is ice as you say, various tidal forces from its moons, to its own orbit and rotation, and irregular heating and cooling of its surface and atmosphere, cause different stresses in the core, meaning it is possible to have quite violent localised or 'hot' eddies, vents, or bursts, even though the majority of the core is still ice.
If the water is not pure and contains a certain concentration of hydrocarbons, it is possible that these could form free floating clumps of hydrocarbons that then form the threads of life.
These chains would probably be in the form of floating bacterial chains first - forming a 'glue' on which other organisms can evolve. Have a look into bacterial microbial mats. This will form where hydrocarbons exist, and are simply a loose assemblage of bacteria that multiply to form a mat.
Then the mats will rise to the surface to gain heat, or float at a certain depth, ideal to absorb nutrients from below.
It would be ideal if the bacteria could grow around hydrothermal venting rather than be blown away, so I imagine that this bacterial structure would be very 'thread-like' and low density, almost like a very dispersed soup, where links and water resistance would keep the bacteria centred where they need to be.
On the surface of these mats, they may over time grow quite thick. Such as in Shark Bay, Western Australia, where bacteria grows layer on layer over billions of years, to form stromatolites.
Eventually life may become multicellular, and the earliest forms could be algae like mobile forms, that exist amongst bacteria saturated matts, such as seaweed.
Then all you need is sexual reproduction, an oxygenation event, and presto, you might get diverse animal and plant life.
The problem with all-ocean worlds is nutrient transport. At least some will get trapped in the sediment at the bottom of the ocean.
If Earth is a normal planet, then the geologic cycle is a significant part of nutrient cycling, especially for Ca, Mg, S, and P You can either handwave around this, or come up with a different mechanism. e.g. there are bottom feeders that feed on the ooze, but have inefficient digestive systems, so their farts bring clouds of silt up to the surface. Or the long term temperature cycles favour the collection and sudden release of dissolved gasses.
Earth has an average ocean depth of 12,000 feet. This would give about 9000 feet if we leveled everything out. Try this: Put a moon close to the planet. Tide forces go with the inverse cube of distance so if you put the earths moon at 50,000 km (1/8 of present distance) the tides would be 500 times as strong. Several hundred foot high tides would both make the coast of continents uninhabitable, and would erode continents as fast as they formed. This would give your planet a mix of shallow and deep oceans.
If you want to see a classic example of a story with a floating ecology read Stan Schmidt's novella "The Demon Breed" That particular planet has continents too, sidestepping the nutrient cycle thing.
Also see Alan Foster's novel Cachalot, which is on an all-water world.
There was something similar to this in the movie Interstellar, that might be something to check out. Like some previous posts, I agree that it would have to be a very nutrient rich ocean to provide the start for life forms.
Another thing I would wonder is, does this planet have a moon or multiple moons? Our moon affects us by causing the tides, so I expect that a moon might cause rougher seas, and life forms would be forced to adapt.
It might start out from bacteria, and then evolve into a fish eat fish society. Since the it is majorly deep water, gills would be the most common breathing apparatus. Fish might evolve into being able to glide to escape predators, and might even evolve into a species similar to waterfowl. Different types would evolve to suit different levels of the ocean. The deep sea fish are probably very blubbery, since it's cold down there. Most likely everything has a air sac to act as a life preserver. Things likely bunch together when they go to sleep.
I'm not sure anything would live on the ice core. The pressure would be tremendous, although something might evolve form bacteria, but look totally different from the life-forms above. This is the only place where plants with roots could possibly live, unless they had humorously long roots that reach all the way down to the core.
If you wanted a sentient life-form, probably a mer-race would be the best idea. They would be a little different then the typical idea,though, evolving from fish instead of humans. They probably have some sort of natural weapon to defend themselves from predators with. Their hands, evolved from fins, will probably look similar to a sea otter or lions paw at first. They could use materials like fungi mats and plants, and maybe bones to build shelters.
Plants might come a little later, and marine fungi might also evolve. In free-floating plants, fungi, and coral, they wouldn't be able to really anchor themselves to anything except other lifeforms. They might take up a parasitic life-style, anchoring themselves to fish and even other plants. Another theory is that they might glob together in big schools, making huge floating mats, and use air to create a sort of ballast to keep them from either sinking or staying on the surface.
How would a free floating ecosystem on an ocean world function?
I'll go ahead and say it wouldn't.
As far we know, life requires 3 components in combination. Nutrients to form into complex, self-replicating molecules (RNA and DNA on Earth), energy to drive that process, and a solvent of some sort (water on Earth) to allow the nutrients to mix together. Theoretically, it might be possible for some form of life to evolve in an organic solvent, like benzene or whatever else, but we can't say for sure.
Your water-world would have an abundance of one component, but a distinct lack of at least one of the other two in the same place. These nutrients that form the building blocks of life tend to be and/or are delivered in forms that are heavier than water. So they'd sink. Which brings you to the big problem you're going to have, nutrients. Even if you did find a way to deliver them in sufficient quantities to allow life to form, (asteroids, whatever), you'll have a big issue with concentration. A gigatonne of phosphorous (or whatever) won't do you much good if it's diluted to 3 parts per million across your ocean world, nor if it sinks to the icy core, where there's no energy to drive the chemical reactions required to create complex molecules.
Looking at Earth, life developed in places where the water and land meet - the land provided nutrients and a convenient anchor to keep the building blocks of life from diluting away, the water provided a solvent for them to mix in, and the energy was provided by either EM radiation or geothermal vents. I don't see a way for the three components to exist in a world that's all water - where you might get concentration with mineral-rich asteroids sinking to the core, you won't have energy, and where you have energy (near the surface), you won't have sufficient concentrations of the building blocks of life.
You'll need plant life floating on the surface, a la C. S. Lewis' Perelandra.
For minerals, you either need really long roots on the floating plants, or something that draws the deep, mineral-laden waters to the surface where the floating plant life can exploit them.
Simply. They build the floating platform. Plant a forrest. Release all kinds of insects. Release birds. Release a fox. Just wait a milion year and then watch how it evolved.
