Hmmm... I will have to try this with a lid although I'm a bit averse to smearing it with anything that will pollute my already kind of dirty water.Hi Dave,
I haven't seen the shell-chewing/dissolving thing you mentioned, sorry. I fed mine mostly live grass shrimp and the occasional hermit crab. They typically went for the grass shrimps immediately by enveloping them in the web. It's beautiful behavior - the whole body goes transparent and the mantle become extremely elongate and narrow, presumably to minimise the shadowing over the prey as the web comes over. As soon as the shrimp would tail-flip into the web the octopus would immediately flash its dark stripes. It was very cool to watch.
The other question you asked was about their vision - is this in relation to your animal's lack of interest in the confined crab? My impression is that their eyesight is comparable to any other species', but I also recall that most successful crab-in-jar trainings involve smearing the entire outer jar surface with fish or shrimp paste, which encourages exploration and often leads to incidental lid opening, and the animal learns the task by continued exploration driven by olfactory and not visual cues. I don't doubt the animal can see into the jar, but I suspect the correct combination of cues to prompt exploration are not there.
On a side note, did you ever attempt to trigger mimicking behaviors by showing photos of different predators?