O. bimaculatus sexing
I forget where I found the information, but when I first got my bimac, Andy, I read that for this particular species, bimaculatus, males do not possess the modified tentacle or hectocotylus, but instead have obviously enlarged suckers on the 2,3 and 6,7 arms. This feature is only noticeable in mature animals. Species where there is a hectocotylus, it is usually on the third arm. The pacific octopus at the Long Beach Aquarium is very easy to sex as a male when he comes out because of this feature.