The bite is famous because of us. It evolved for crabs. Blue-ringed octopuses eat small crustaceans: crabs, hermit crabs, shrimp, and now and then a small fish. Several accounts call them primarily crab-eaters, and the hunting descriptions match that. This is not an animal grazing algae or filtering plankton. It is a predator with a short list and a very effective way of ending a struggle.
A crab is a bad meal if you are soft. The shell is the problem, and the claws are the other problem. Venom that paralyzes muscle solves both. Once the prey cannot move, the octopus can work the shell open with its beak, the hard parrot-like mouth at the center of the arms, without being pinched. The same chemistry that can stop a person’s breathing is, in the ordinary case, a tool for eating something the size of a coin.
That scale is worth keeping. The prey is small because the hunter is small. A blue-ringed octopus is not knocking over large crabs the way a big reef octopus might, and it is not interested in a fish too large to hold. Hermit crabs, tucked in a shell that looks like a safe, are very much on the list. The shell is not a guarantee against an arm that can reach and a toxin that does not care about the snail-house.
On a reef flat, the food web consequence is local and ordinary. These octopuses take a share of the small crustaceans, and other animals take a share of the octopuses when they can find them. The rings reduce that second share. They do not remove the octopus from the menu. They raise the price of the bite.