Octopuses have a high metabolic rate which requires more oxygen. Their copper-based hemocyanin transports oxygen in low temperatures, due to which their blood appears blue.
Octopuses' two hearts pump blood through the gills and the third circulates it to the rest of the body. Interestingly, the heart that performs circulation stops beating when the octopuses swim, which is why they crawl more often.
Octopuses have a powerful defensive tool to evade predators. They eject a cloud of ink made up of melanin and contain tyrosinase, a substance that irritates predators' eyes and temporarily dulls their sense of smell and taste.
All octopuses are venomous and use their beak to pierce their prey, the venom they inject paralyses the animal, preventing them from causing harm to the octopus during a struggle.
Octopuses have large brains, capable of navigating mazes, solving problems, remembering, predicting, using tools, and disassembling objects. They demonstrate sophisticated hunting behaviours.