Octopuses have three hearts; the systemic heart stops during jet-propelled swimming; blood is blue from copper-based hemocyanin
Octopuses have three hearts: two branchial hearts push blood through the gills, and one systemic heart pumps it through the body. When an octopus uses jet propulsion to swim, the muscle contractions physically squeeze the systemic heart shut, pausing its beat. Their blood carries oxygen not with iron (vertebrate hemoglobin) but with copper (hemocyanin), which turns the blood blue when oxygenated.
Sources
A. Octopuses have three hearts: two branchial (gill) hearts and one systemic heart.
ScienceABC, "Why Do Octopuses Have Three Hearts?" (scienceabc.com/nature/animals/why-do-octopuses-have-three-hearts, fetched 2026-07-27) — "two branchial hearts pump blood through the gills" and "one systemic heart circulates it to the body"; systemic heart identified as "the largest of the three and the one closest in job to our own heart, driving oxygen-rich blood out into the arms."
OctoNation, "How Many Hearts Does an Octopus Have?" (octonation.com/how-many-hearts-does-an-octopus-have/, fetched 2026-07-27) — "two branchial hearts that pump blood through the gills, and one systemic heart that circulates oxygenated blood throughout the body."
Scientific American "Octopus Chronicles," Katherine Harmon, July 13 2013 (scientificamerican.com/blog/octopus-chronicles/octopuses-survive-sub-zero-temps-thanks-to-specialized-blue-blood/, fetched 2026-07-27) — "the three hearts" that pump hemocyanin through octopus bodies; independent institutional source corroborating structure.
B. The systemic heart stops beating when the octopus uses jet propulsion.
ScienceABC (fetched 2026-07-27) — "When an octopus uses jet propulsion to swim — typically to escape a predator — its systemic heart actually stops beating. The muscular contractions required for jet-propelled swimming physically compress the heart, preventing it from pumping."
OctoNation (fetched 2026-07-27) — "that systemic heart stops beating every time the octopus swims! When the mantle contracts hard to fire that jet, it pinches those drainage channels (called venous sinuses) shut from the outside." Independent corroboration of both the fact and the mechanism.
C. Octopus blood uses hemocyanin (copper-based) instead of hemoglobin (iron-based), turning it blue when oxygenated.
Wikipedia, "Hemocyanin" (en.wikipedia.org/wiki/Hemocyanin, fetched 2026-07-27) — "Hemocyanin was first identified in 1878 by Leon Fredericq in Octopus vulgaris"; "colorless Cu(I) deoxygenated form and the blue Cu(II) oxygenated form"; mechanism: "Each hemocyanin monomer holds a pair of copper(I) cations in place via interactions with the imidazole rings of six histidine residues."
PDBe/EMBL-EBI article by Romana Gáborová (ebi.ac.uk/pdbe/articles/what-do-snails-spiders-octopods-and-queen-england-have-common, fetched 2026-07-27) — "Octopuses and horseshoe crabs have blue blood because the protein transporting oxygen in their blood, hemocyanin, contains copper, instead of iron"; "Hemocyanin is a copper-containing protein which gives hemolymph a blue colour when oxygenated, yet is colourless when deoxygenated"; "The oxygen-binding site is composed of a pair of copper cations (oxidation state +I when deoxygenated, +II upon oxygenation)."
Scientific American (Katherine Harmon, 2013, fetched 2026-07-27) — "The copper in hemocyanin makes their blood run blue." Independent third source.
D. Most octopuses crawl rather than swim long distances (consequence of the stopping heart).
spacedaily.com summary (fetched via search 2026-07-27) — "many bottom-dwelling octopuses tend to crawl along the seafloor instead of swimming for long stretches" attributed to the heart stopping during jet propulsion.
ScienceABC (fetched 2026-07-27) — describes swimming as "extremely exhausting" because the systemic heart cannot beat during the contractions, consistent with crawling preference.
What we refused to claim
"Only 25% as effective as hemoglobin" — cited by ScienceABC but not independently corroborated in this fact-check. Exact efficiency ratio not spoken in the script.
Antarctic hemocyanin cold-adaptation — verified (Scientific American 2013), but tangential. Not in scope for these beats.
"Blue blood even when deoxygenated" nuance — technically hemocyanin is colorless/clear when deoxygenated, blue only when oxygenated. The script says "uses copper, which turns it blue" — accurate (standard shorthand used by Smithsonian, Scientific American, NHM) but taken to mean the oxygenated state. Not misleading; no correction needed.
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