It isn't always easy to sex an octopus, but it is fairly simple to tell if an Abdopus aculeatus is male. When the animal climbs onto the front wall of the aquarium (which they usually will do), look at the suckers near the base of the first and second pair or arms. In mature males, they are enlarged. They are smaller in females and juveniles. The hectocotylized arm of the male is also easy to spot. It is the third, right arm. In this shot it looks like the fourth, but it's actually the third. When they look to the side they usually stick their eye over the web between arms I and II- so the left arm I is draped over to the right side, throwing off the count. Note the spermatophoral groove on the trailing edge. It extends the entire length of the arm. Not visible is the small ligula on the tip of the arm.