How do they know it's Atlantic if the list just said octopus? Does he have any webbing in between his arms? Also do his arms seem extra-long? It sounds to me like you might have A. Aculeatus
, but he would not be from the Atlantic.
I'm don't want to sound like a know-it-all, but the pupil is actually round. Neogonodactylus has a wonderful picture that depicts it: http://www.tonmo.com/gallery/showphoto.php?photo=562
. The eye is a rectangle shape though.
With a set up like that, now I don't want to sound like a know-it-all either, but unless I'm mistaken, the picture shows the rectangular pupil, but that inside the eye there's a circular image, which I think (and Roy's description seems to back this up) is a visible image of something round in the environment, probably a big lamp that a professional photographer used to light the octo, I'm guessing.
Pupil shapes are really interesting, particularly in cephalopods. Despite this nitpick, you're on the right track, because most of the eye parts are round: the retina, lens, and so forth are all round. Humans, and most other animals, and squids all have a round pupil, but a lot of animals have other shapes... cats have the vertical slits, cuttlefish have a strange W shape, octos have the wide, short rectangle, and cetaceans have a pupil that has some two-slit arrangement that uses the lens for focus underwater, while using the slits to make something like a pinhole camera to see in air.
Hanlon & Messenger don't discuss pupil shape, but they do point out that cuttles and octos have a horizontal stripe of longer, thinner receptor cells in the center of the retina, which probably give extra sensitivity in the horizontal plane, which makes sense as they live near the bottom. It's likely that the pupil shapes help utilize this strip better... and the cuttle has both the strip and an area near the posterior part of the eye that's even more optimized for looking forward to aim its tentacle stripes, so maybe the "W" shape has to do with focusing in two different ways-- look all around at bottom level, and look forward for lunch! Of course, the cuttle needs to look up for predators above, and down to look at the texture that it's matching, too.