TURN THE CAPS LOCK OFF ITS REALLY EASY!!!!!!!!!
: an all caps message with a missing apostrophe and 9 exclamation marks joining in to complain about someone's overzealous use of capital letters.
Although I found the original message is somewhat annoying to read, I wasn't terribly impressed with the style in all of the responses, either... and I'd prefer not to make people with legitimate octopus questions feel attacked too much around here. It's worth noting that there are punctuation, spelling, or grammar errors in every post in the thread
except for nickcoletti's second post (and maybe this one, if I'm lucky.)
(edit: and Cuttlegirl's post, who snuck in as I was typing this one.)
To address the original question: octopuses hunt both visually and by using a their suckers to taste, smell, and touch. They certainly recognize prey visually when it's in sight, but they often reach into nooks and crannies seeking food, even if they can't see it. I'm not sure if many studies have been done about long-distance chemical sensing like smell... Hanlon & Messenger suggest that the research that has been done shows that smell is an important factor in finding food for octopuses in particular. Octopus suckers have about 100 times as many smell/taste sensing cells as squids and cuttles, who hunt mostly visually. Cephs also have olfactory organs near their eyes, but very little is known about their use relative to the suckers' sense of smell.
I don't know why your octo would be ignoring the crab, though... usually, they're very good at being aware of their prey by both smell and sight, and if it was showing hunting patterning, it seems likely it was aware of where the crab was. Maybe it's either "playing cat and mouse" with the crab, or it wants to eat the crab but thinks it's too big to handle?