1: Shrimp compete with fish.
Here is an example of an arthropod body plan evolved to be a match with fish. Maybe not esoteric enough for your needs but the real deal.
2: Swimming bug x armored fish.
But let us get more awesome. We will start with the most water adapted insect I can ...
Dipygus is a fundamental flaw in a very early embryonic stage. It involves a split in the back bone, yielding two pelvises. The tail merge you require would involve merging the two inner legs, that would be one leg from each pelvis.. Maybe you've seen post-operation images, but at first, the child will have to separate lower bodies,
Start with the right scaffolding
As illustrated in these images, barnacles such as Sacculina carcini can become parasites, creating new structures within a crab host. Your barnacles were adapted by bioweapons specialists to present appropriate HLA antigens and signals such as OX40 to suppress the human immune system. Infected swimmers become reclusive, ...
Food is the limiting factor, so we can see how we can improve this. First of all, we can check where nutrients go. Most of the nutrition is found along coastal waters. This is because the light can still reach most layers, while biological debris used as nutrients can't float out of reach. This seems most important. Maybe food can be gained ...
They're smarter than you think...
Single dinoflagellate cells (plankton) actually produce an eye with a lens and retina. Having a retina is a good hint that they may keep mental track of the positions of objects, though I haven't seen much work on this.
Now you may wonder what a single cell can do without a brain. But remember, cells have many thousands ...
If they can link together to accomplish complex tasks then maybe.
Taking ants for reference they can't accomplish much on their own but are an unstoppable force in great numbers. They don't have a civilisation per se but they can build large nests, wage wars between colonies, farm mushrooms and herd aphids. They do this thanks to two things: mandibles to ...
Photosymbiotic great barnacle.
For reference: regular barnacles.
The barnacles are the only sessile members of crustacea, and the giant barnacle leverages the robust crustacean body plan to outcompete other sessile marine organisms with photosymbionts. As with other photosymbiotic marine organisms (...
Increased intelligence isn't something that magically appears in a species; it has to be selected for... and physically possible.
First of all, a sapient brain is necessarily bigger than a plankton's body. It takes a minimum amount of physical hardware to compute anything.
Second, if plankton can't even control their movement, they don't ...
Could a species of planktonic beings, with sufficient intelligence and creativity... While being unable to control their motion is a massive detriment
No, no, no, no, no. Nope. How did they develop "intelligence and creativity" if they cannot even control their motion?
While it is true that form follows function, the lack of a form will disable ...
Look to sponges and corals:
Physically, something that doesn't move has a very different set of evolutionary pressures on it. You chief factors will be food supply, predation, and parasitism.
Mega-barnacles will need a lot more food passing through their filters. Their hunting style may come to look more like a spider or fishing trawler sending out nets to ...
Creatures as big as humpback whales can feed on small food like krill, as long as it is available in sufficiently large amount.
However, considering that you want it fixed on the seafloor, it will not be able to go after its own food. This limit its habitat to area with constant, nutrient rich water current.
My gut feeling is that it could not get much more ...
The closest real mammalian counterpart from a creature like yours in terms of overall bodyplan (arm flippers, feet acting like fins) are seals and sea lions, and sea lions don't quite fit the bill since their 2 legs are still very much capable of acting like decently standard legs, unlike the legs of a seal which are essentially locked in a perpetual ...
Here are some of my ideas:
The vascular system would be made in such a way that the body could stay warm without the need of an overly thick layer of blubber (marine mammal fat).
The inside of the lungs would have two layers of oxygen-carrying capillaries and twice the number of alveoli found in those of humans, permitting long diving sessions of fifteen to ...
The question describes a multi-stage lifecycle with a crucial missing step:
Pre-natal - fish-people embryos developing in uterus of large "fish-like" animals
Transition 1: Birth
Brood-ling - very young fish-people stay with fish-like parent for protection
Juvenile - childhood phase with young fish-people free swimming the ocean
Transition 2: ...
Do it like insects but with multiple metamorphosis stages. Metarmorph from fish to human within a cocoon or sac. Then return to the water and metamorph to the other fish form when the human body ages.
So there'd be two pupa stages instead of one.
So you life cycle would be
Human Adult (terrestrial)
Breeding Stage Adult (aquatic)
Specifically, what are the steps of reproduction/growth that allow new individuals of the stages detailed above to appear?
I would go with the age of sexual maturity/procreation before the age of metamorphosis into humanoid shape, the latter being sterile.
transforming back from humanoid into a fish to procreate is one transformation too many
The easiest way such a complex series of metamorphoses are created would be through Genetic Engineering. If you have humans who are indistinguishable from us today, and they have smart brains and all intelligence required to be like us, then it stands to reason they are us, and at some point in the future we Genetically Engineer ...
I feel an unaccustomed humility, thinking of how to improve the jellyfish. The jellyfish! Oldest of us, how can my little schemings improve on 500 million years of your trials, your survival of the fittest?
Let us consider the squid.