One of the first questions a new octopus keeper asks is for help identifying the sex of their new ward.
Usually the easiest way to sex an octopus is by observing the third arm to the right (clockwise) on a sexually mature animal. This arm (hectocotylus) has a suckerless tip (ligula) and a channel for delivering spermataphores.
The ligula is very difficult to identify while looking at a live animal but can be just be seen in the second (hummelincki) photograph.
The channel is easier to see in photographs than trying to view it on a moving animal and is seen as a white line in the third (briareus) photograph.
The easiest way to identifiy it is to see if the octopus keeps it rolled up protectively (all photos). SueNami (the briareus in photos 1 and 3) came with this arm removed up to the webbing. The assumption was that a male would be less likely to loose the arm and we assumed female. As the arm regrew we discovered that we have a boy named Sue and that the hectocotylized arm appears to grow back normally (functionality can only be assumed).