Octopuses Have Three Hearts and Blue Blood

Octopuses have three hearts: two pump blood through the gills, and one pumps it through the rest of the body. Their blood is blue because it uses haemocyanin (copper-based) rather than haemoglobin (iron-based) to carry oxygen — an adaptation that works better in cold, low-oxygen water. When an octopus swims, the heart that delivers blood to the body actually stops, which is why they prefer crawling.

Source: Journal of Experimental Biology — various cephalopod biology studies