Let's say that Earth forms having no land, only underwater cave systems, on every level of the ocean. How would evolution unfold? How would fish evolve to live in small to large lightless caves? What would be the dominant species, and what types of technology would be created?
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You can take a look at how other species have evolved to live in near-total darkness under the sea, and how they communicate in water. You'd see bioluminescence, the ability to withstand very cold temperatures, and the ability to use heat (not just sunlight) for photo-synthesis. Sight would be almost entirely replaced by very advanced olfactory systems, pressure and magnetism sensing nodules, excellent hearing. You would even have some animals and plants that could survive being frozen.
"Verbal" communication inside a cave would tendto be higher-pitched and consonant-heavy (whales use low frequencies and vowel sounds to communicate across vast distances, but dolphins 'chatter' in high pitches and lots of clicking type consonant sounds, since their conversation partners are usually nearby and the sound doesn't need to travel as far.)
If the caves are fairly near the surface, then life won't need to evolve to withstand great pressure, so you'd see shapes much like typical marine life. (Think hard, compact bodies like fish, crustaceans, etc.,) If the caves are very far beneath the sea, life will need to evolve to cope with the pressure.
In any depth, animals would almost certainly have some form of echolocation to create a 'picture' of what the cave looks like. The picture could 'look' quite detailed, since echoes would bounce off the cave and give higher resolution to the surroundings. You might have amphibious species that use echolocation above-water, but rely more on smell underwater.
Dominant species would have particularly well-developed senses of hearing, both above and below water. I'm assuming that the caves are connected; a dominant species would be able to travel between the caves and so would be small enough to fit in the tunnels. There would be no whales, or large sharks. The dominant species would be some sort of blind small dolphin. The largest animals would be native to the larger caves, and would probably be large turtles or tortoises.
What might be interesting could be swarms of piranha-like fish that hunt in packs. They could go almost anywhere in a cave system and would be able to corner even large prey in smaller alcoves.
The technology would mainly be used by crustaceans, who are the only species that have evolved dexterity in their 'fingers'. They'd probably make spears to hunt with, and they would learn how to pack cave sand to create barriers against the piranha. Dolphins' technology would be mostly language and communication, which would be highly evolved. Some of the dolphins would also probably figure out how to weave nets using seaweed or other fibers, so they could catch fish and also create spaces that are protected from the piranha.
Why exactly is land an issue? Evolution is driven by plants as primary producers. Phytoplankton evolve to form aquatic plants, whose bodies and roots serve as habitats for primary consumers and their predators. Eventually, these would form large floating islands like water hyacinths. These would eventually form substrates for larger plants, and thus bigger consumers. The substrates formed by dead plants and plant waste would make water shallower in low flow areas, leading to siltation and eventual land reclamation.