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Why do active cephalopods need a closed circulatory system?

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A blue-ringed octopus with bright blue rings on its beige body crawls over a rocky, coral-strewn seabed. — cephalopod Blue-ringed octopus (genus *Hapalochlaena*).

Unlike most other mollusks, which possess open circulatory systems, cephalopods are significantly more active and maintain higher metabolic rates. To support these physiological demands, they have evolved a closed circulatory system characterized by a higher order of organization, featuring distinct arteries, veins, and capillaries.1

This closed system ensures that blood remains separate from interstitial fluid, allowing for more efficient transport of oxygen and nutrients to tissues. Because cephalopods rely on hemocyanin to transport oxygen, the closed system—aided by accessory branchial hearts—maintains the necessary blood pressure to force blood through the gill capillaries effectively. This specialized circulation is essential for sustaining the high-energy requirements of their active, predatory lifestyles.1