The sea creature known for having three hearts and blue blood is the octopus. Octopuses have a unique circulatory system with two branchial hearts that pump blood to the gills to pick up oxygen, and one systemic heart that pumps oxygenated blood to the rest of the body. Their blood is blue because it contains hemocyanin, a copper-based protein that is more efficient than hemoglobin (found in humans) at transporting oxygen in cold, low-oxygen environments like the deep sea. This adaptation helps octopuses survive in their challenging underwater habitats. The systemic heart actually stops beating when the octopus swims, which is why they prefer crawling as it's less tiring.