Preliminary in vitro functional evidence for reflex responses to noxious stimuli in the arms of Octopus vulgaris

The arms of Octopus vulgaris perform a number of functions (e.g. prey capture, exploration) putting them at risk of damage. Nociceptive reflexes provide one defence and as there is a paucity of evidence for such reflexes in cephalopods we have investigated this in isolated arms. The arms were removed immediately post mortem from O. vulgaris, suspended vertically or mounted horizontally and the responses to a pinch of the distal arm and application of acetic acid (0.1%–5.0%) or tap water to the tip recorded (video). Isolated arms rapidly (~ 1 s) withdrew in response to a pinch, tap water and acetic acid (threshold 1%) applied to the tip region. A “quasi-joint” formed in the proximal arm during the withdrawal response in horizontally mounted preparations. No response was evoked by sea water or gentle compression. Withdrawal responses were abolished by axial nerve cord section proximal to the site of stimulation.The results demonstrate that the arms are capable of reflex withdrawal to a “noxious” stimulus without reference to the brain. Neurophysiological studies in a more technically refined preparation are required to define the temporal characteristics of the reflex and to characterise the putative nociceptors (or other receptor types) and to determine if information from them reaches the brain.