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Common octopus

Octopus vulgaris

Most of its neurons are in its arms. A 2021 review found very strong evidence that it's sentient.

Neurons

500 million estimate · whole nervous system: about 300 million in the arms, 200 million in the central brain

Young JZ (1963) The number and sizes of nerve cells in Octopus. Proc Zool Soc Lond 140(2):229-254. doi:10.1111/j.1469-7998.1963.tb01862.x; Young JZ (1971) The Anatomy of the Nervous System of Octopus vulgaris. Oxford: Clarendon Press.

Evidence

• A review of the behavioural evidence for consciousness in cephalopods.
  Mather JA (2008) Cephalopod consciousness: Behavioural evidence. Conscious Cogn 17(1):37-48. doi:10.1016/j.concog.2006.11.006.

The 2021 sentience review

Confidence that each of the eight sentience criteria is met, for octopods:

1. very high The animal possesses receptors sensitive to noxious stimuli (nociceptors)
2. very high The animal possesses integrative brain regions capable of integrating information from different sensory sources
3. high The animal possesses neural pathways connecting the nociceptors to the integrative brain regions
4. high The animal's behavioural response to a noxious stimulus is modulated by chemical compounds affecting the nervous system (e.g. local anaesthetics, analgesics, opioid-type systems)
5. medium The animal shows motivational trade-offs, in which the disvalue of a noxious or threatening stimulus is weighed against the value of an opportunity for reward, leading to flexible decision-making
6. very high The animal shows flexible self-protective behaviour (e.g. wound-tending, guarding, grooming, rubbing) of a type likely to involve representing the bodily location of an injury or noxious stimulus
7. very high The animal shows associative learning in which noxious stimuli become associated with neutral stimuli, and/or novel ways of avoiding noxious stimuli are learned through reinforcement (habituation and sensitisation do not count)
8. high The animal shows that it values a putative analgesic or anaesthetic when injured (e.g. self-administers it, or learns to prefer a location where it is available)

The New York Declaration on Animal Consciousness (2024) finds at least a realistic possibility of conscious experience in all vertebrates and in many invertebrates, including cephalopods, decapod crustaceans and insects.

Assess it against the published criteria