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Male octopuses usually carry their 3rd right arm curled, the tip is modified (not always obvious and does vary within species from descriptions I've seen), over-sized suckers on certain arms is the easiest way to tell in my opinion. My O. briareus for example has obviously larger suckers closer to the mouth, about 3 or 4 inches down there is a dramatic decrease in sucker size. Females seem to have a more uniform appearance to their suckers.
On OhToo (hummelincki), I am seeing a very distinct difference in the third arm (not as obvious on Octane -also hummelincki - though). He does not keep still on the glass long enough for me to get a good look and I am going to concentrate on getting some pictures this week but the arm is shorter and fatter than the others and about an inch of the end has a tan colored row (possibly two rows) of some kind of "bumps". It is going to require a good photo to see the details and I still can't identify the spermataphore channel but this is the first octo I have had that the hectocotylus (modified arm) is very visible.
very interesting so mostly based on there arm's is the easiest way defining a male from a female once I acquire my first specimen of a Cephalopod I hope I can with you guy's help identify it as male or female.