YouTube channel Life in Jars made this large natural native saltwater ecosphere in a jar and fastidiously documented the results.
The ecosphere has housed crabs, starfish and a lot more and is currently still housing a lot of crustaceans, paramecium, worms, other invertebrates and even spionid worms.
He also answered some of the most frequently asked questions which I have copied and pasted below:
Q: How can you ‘accidentally’ catch those animals?
A: As you’ve noticed I collected a lot of seaweed and algae. The animals were probably hidden somewhere in that algae. I did not see them.
Q: What does it mean if they ‘disappeared’?
A: In the aquarium hobby, when people say “X has disappeared” It normally means that it was weakened or dead and got consumed by the others in the tank before the aquarist knows. That’s also what happened here.
Q: Why did you not return the crab back to the ocean?
A: The only saltwater I had was the water in the jar. That water was already polluted, which was fine for in the ecosphere but it wouldn’t have been good enough to transport the crab. It would have been a trip of a few hours, too long. If I had tried to take it back it would have died. Instead, it got to live another five months.
Q: If it were in the ocean the crab would have lived a happy long life.
A: If that crab was still in the ocean, the chance of it being eaten at it’s small size or dying for other reasons would have been really high. Only a very small percentage of crab babies ever make it to adulthood. If it makes you feel any better, the crab nor the starfish have consciously suffered. Starfish don’t have brains and crabs aren’t capable of any complex emotions other than physical pain.
For all questions that concern the working of the ecosystem, like: Where do they get air? or You should do water changes please consult this video: https://youtu.be/K0ImENkmIBo
If you want to try this yourself but don’t know how, I made a how to video: https://www.youtube.com/watch?v=2vm8fqqJpZM