Some Experiments on the Chemotactile Sense of Octopuses

1. A method of training octopuses to make chemotactile discriminations is described. 2. The animals can readily be taught to distinguish between porous objects soaked in sea water and similar objects soaked in sea water with M/10 KCl added. 3. Using pairs of objects soaked in solutions with different amounts of KCl added, minimum distinguishable concentrations were determined over the range M/10M/100. Effective discrimination broke down when the ratio of the two test concentrations was of the order of 1.3. This approximates to the values found in chemical discrimination experiments with mammals and insects. 4. Attempts were made to determine the minimum detectable concentration of added KCl by progressive dilution of the M/10 KCl solution used in the initial training. Additions to sea water down to M/105 were distinguished and there was some indication that octopuses can, in fact, detect added KCl at much smaller concentrations; the addition of M/1010 KCl produced a significant decline in the ability of animals under training to discriminate between M/106 KCl and sea water. 5. Experiments on acid discrimination indicated that acids were distinguished on a basis of their pH, with acetic tasting noticeably more acid than HCl or H2SO4 at the same pH. Again, the result closely parallel those that have been obtained with mammals and insects.