Octopuses have three hearts, and two stop when they swim

An octopus has three hearts: one systemic heart pumps oxygenated blood around the body, while two branchial hearts pump blood through each gill. When an octopus swims by jet propulsion, the two gill hearts stop beating — which is why octopuses prefer crawling; swimming exhausts them quickly. Their blood is blue because it uses copper-based haemocyanin rather than iron-based haemoglobin to carry oxygen.

Source: Wells, M.J., 'Octopus: Physiology and Behaviour of an Advanced Invertebrate', Chapman & Hall (1978)