Why does an octopus prefer crawling to swimming?

Because its main heart stops when it swims. An octopus has three - two pushing blood through the gills and one, the systemic heart, sending it to the rest of the body - and that third one ceases beating during jet propulsion. Swimming is therefore not merely tiring but oxygen-starving, and an octopus that jets away from something arrives exhausted.

Their blood is poorly suited to the job in the first place. It uses haemocyanin, which carries oxygen on copper rather than iron, works better in cold low-oxygen water and carries less than haemoglobin - hence three hearts to move it. That is a good design for a cold deep sea and a poor one for exertion.

So the animal solves its problems by other means. Crawling costs little; camouflage means not being seen; and a great deal of octopus behaviour - the tool use, the squeezing through gaps, the mimicry - is what an intelligent soft animal does instead of being fast. Two thirds of its neurons are in the arms, which taste what they touch and keep working when separated, so the body is closer to a committee than to a machine driven from the front.