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Sources

Where the facts come from

Every factual claim on this site links to its source. This is the full list, and every place where the sources disagree.

How we checked

Each fact was written alongside its source. Our build script stores the source text and, for every fact, checks that the key words and figures really appear in it. 118 of 118 checks passed when this site was last built. All sources were read on 28 Oct 2026. Facts that can change (species counts, sizes, depths and records) carry “checked 28 Oct 2026” next to their source link.

Main sources: Smithsonian Ocean, the Monterey Bay Aquarium, the Natural History Museum, London, Shedd Aquarium, NOAA (Kodiak lab, Ocean Exploration), MBARI, the Australian Museum, AIMS, the American Museum of Natural History, peer-reviewed papers in PLOS ONE and Marine Biology, Britannica (its main article text only) and Merriam-Webster.

When sources disagree

Where trusted sources give different figures, we don’t pick one: we show them all, each with its source.

How many octopus species are there?

Nobody knows exactly. New species keep turning up, especially in the deep sea, and some have no scientific name yet.

How big does a giant Pacific octopus get?

A soft, stretchy animal is hard to weigh and measure, and old records are hard to check. These figures are far apart, so we show them all rather than pick one.

What is the smallest octopus?

The two sources name different species, and only one gives a measurement.

How deep do octopuses live?

The 2019 Java Trench sighting is newer, and much deeper, than the figures on NOAA’s and the Smithsonian’s pages.

Which octopus lives longest?

NOAA calls the giant Pacific octopus the longest-lived species, but the deep-sea mother spent 53 months brooding alone. How old she was in total, nobody knows: deep-sea octopus lifespans are a mystery.

How many people have blue-ringed octopuses killed?

“At least three” and “several” are both rough counts. Neither page gives a full list.

Can octopuses see colour?

  • Cephalopods have only one type of light-receptor cell, so they are most likely colour-blind. But a recent study suggests their oddly shaped pupils might let some of them tell colours apart anyway. Nobody knows for sure yet. Smithsonian Ocean: Octopuses, squids and relatives

Still an open question.

How many brains does an octopus have?

One central brain, or nine? It depends whether you count each arm’s nerve centre as a brain.

Octopuses, octopi or octopodes?

  • Merriam-Webster lists three plurals: octopuses, octopi and octopodes. Octopuses is now the most common. Merriam-Webster: octopus
  • ‘Octopi’ is the oldest plural, made on the mistaken idea that the word was Latin. It is really Greek, which is where ‘octopodes’ comes from. Merriam-Webster: octopus

All three are in the dictionary. Octopuses is the one most people use.

All sources (23)