Octopus

Octopuses are commonly known as intelligent — but the specifics of their intelligence are stranger than most people realize. Two-thirds of their neurons are distributed through their arms, which can operate semi-independently. They have three hearts, blue blood, and can edit their own RNA to adapt to temperature changes in real time. They're also the most cognitively capable of all invertebrates by most measures.

Facts you didn't know

• 1

  Two-thirds of an octopus's neurons are in its arms, not its brain. Each arm has its own neural cluster (ganglion) that can process information and control movement semi-independently — when an octopus reaches into a crevice, the arm is partly navigating on its own.

• 2

  Octopuses have three hearts: two branchial hearts pump blood through the gills; one systemic heart pumps oxygenated blood to the body. The systemic heart stops beating when an octopus swims, which is why they prefer crawling — swimming exhausts them.

• 3

  Octopus blood is blue because it uses hemocyanin (copper-based) rather than hemoglobin (iron-based) to carry oxygen. Hemocyanin is less efficient at normal temperatures but works better in cold, low-oxygen water.

• 4

  Octopuses are the only invertebrates confirmed to use tools — they carry coconut shell halves across the seafloor, then reassemble them into a portable shelter when needed. The behavior requires planning: carrying the shells is inconvenient unless you intend to use them.

• 5

  Octopuses have been documented playing — repeatedly releasing bottles into a current and catching them, with no food reward. Play behavior in animals typically indicates cognitive surplus and behavioral flexibility.