We’re all familiar with Pavlov’s conditioning experiments with dogs. Dogs were treated to an unconditioned stimulus — something to which they would normally respond with a specific behavior, in this case, meat juice which would cause them to drool. Then they were simultaneously exposed to the unconditioned stimulus and a new stimulus, the conditioned stimulus, that they would learn to associate with the tasty, drool-worthy stimulus — a bell. Afterwards, ringing a bell alone would cause the dogs to make the drooling response. The ability to make such an association is a measure of the learning ability of the animal.

Now…how do we carry out such an experiment on a cephalopod? And can it be done on a cephalopod with a reputation (perhaps undeserved, as we shall see) as a more primitive, less intelligent member of the clade?

The nautilus, Nautilus pompilius despite being a beautiful animal in its own right, is generally regarded as the simplest of the cephalopods, with a small brain lacking the more specialized areas associated with learning and memory. It’s a relatively slow moving beast, drifting up and down through the water column to forage for food. It has primitive eyes, which to visual animals like ourselves seems to be a mark of less sophisticated sensory processing, but it has an elaborate array of tentacles and rhinophores which it uses to probe for food by touch and smell/taste. Compared to big-eyed, swift squid, a nautilus just seems a little sluggish and slow.

So let’s look and see how good a nautilus’s memory might be. First, we need a response to stimuli that we can recognize and measure, equivalent to the drooling of Pavlov’s dogs. While they don’t measurably salivate, the nautilus does have a reaction to the hint of something tasty in the water — it will extend its tentacles and rhinophores, as seen below, in a quantifiable metric called the tentacle extension response, or TER.

The scoring system for tentacle extension response (TER) in chambered nautilus. TER was graded every 5 s from a minimum score of 0 to a
maximum score of 3. Each level corresponds to a range of percentage extension relative to the length of the animalʼs hood. Zero is recorded when all
tentacles are retracted into their sheaths. A score of 1 corresponds to an extension of <33% of the hood length. A score of 2 corresponds to extension
between 34% and 66%, and 3 is given when tentacles are extended beyond 67% of hood length.

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