While reading this question about how non-primate mammals could develop a bipedal stance, I became curious about the broader reasons why animals could evolve to walk on two legs.
I found some insights at this question, which inquires about felines in particular. The accepted answer states,
Moving to trees and/or a rocky area would help. Hands and an upright posture are good for reaching and carrying things. They are also good at manipulating things and tool use. An upright posture is also good for spotting predators.
This is a great explanation of why bipedalism might arise on land, but I don't think it holds up underwater.
- There are no fruit-bearing trees underwater, and I can't think of similar plants that creatures would need to be "upright" to reach or climb.
- Even if an alien world has marine trees (big kelp maybe?) I imagine swimming vertically is a more efficient way to reach food. For instance, even though crabs have legs, they can swim in the open ocean. If you can swim vertically, why would evolving to reach vertically be worthwhile?
- While being vertical might help spot predators in a grassy plain, much of the ocean is relatively flat and featureless. I imagine most prey can see predators coming regardless of height.
Given the drastically different environmental factors posed by life underwater, what environmental factors, if any, would cause bipedalism underwater? What would a humanoid-fish-or-crustacean-producing biome look like?