O. mercatoris is not my favorite kind of octopus to keep. I've always found them to be a bit boring, but Tom stopped shipping the larger ones after so many losses with this cold winter. I've really been wanting a shot at raising large egged octos, and Tom hadn't had one merc die in shipping, so I got 4. Yes, I got 4 O. mercatoris from Tom to put into my 20 gal tank. Yes, I was worried that this was a stupid move, while we know mercs can live together, I'm not sure 4 in 20 gals is the best move. BUT, I have other tanks and can re-home anyone not behaving… They came on Tuesday, and the first problem was acclimating that many. At first, I just slit holes in their bags and dripped that way until they were full. Then I put them into small glasses for the duration so they'd be easy to move. They dripped for 3 1/2 to 4 hrs. They pretty much all found a spot to hide as soon as they were put into the tank. They were boring. Until the lights went out and the nocturnal shenanigans began… One went right into a hole and covered itself with a shell. It has stayed there ever since, I've only seen her come part of the way out of her den, so I called her "Scaredy"- I wasn't sure what sex she was then. She keeps the classic merc pose, peeking out of her den, keeping a shell for a door near by. "Bully" started getting into fights with anyone the minute the white lights turned off. He and "3 of 4" (I know, real creative- like a Borg designation) are the same size. Bully tried to pull Scaredy out of her den, but she wouldn't budge. Then Bully got into it with "3 of 4." They were beak to beak and I was freaked out. They looked for all the world like they were having a slap fight, arms flailing all around and mantles huffing. It was over as quickly as it began. As they were getting into it, I noticed the smallest, Midgie, climbing on the glass toward me. I was worried for the tiny one. Bully disengaged and then went back to Scaredy, when I realized that he wasn't trying to pull her out, he was trying to slide his third right arm into her den… All that time, Bully is going back and forth between Scaredy's den and 'fighting' with 3, and Midgie is inching closer to me. Then Bully gave up on the other two, and was on the same wall coming toward me. I was worried for little Midgie, until he crawled right up to Bully and tried to start something! I was even more surprised that Bully ignored him! The next night, Midgie proved his masculinity by making his own rounds to Scaredy's den, and trying to slide that third right arm through her shell "door." So it has gone every night. They all have their own dens, and I can find them every morning. There is a time at night right after the red lights come on and I can see all three of the others out, but then on of the bigger ones hides. I can always ID Midgie, but if Bully and 3 of 4 are both out, I can't tell them apart. I'm not even sure it's fair to say that Bully really is a bully, half the time it could be 3. Midgie is actually the most brazen, comes right out onto the front glass every night to see me, and seems to be the one most likely to start fights. I don't really know what to say about the fighting. It never lasts long, it never seems serious, and I can never tell what it's about. They aren't fighting over dens or food. After four days now, they are stick feeding (except for Scaredy who blew her food away last night), they have their own dens that they return to… I'm not worried yet, they are all in fine shape and they haven't hurt each other at all. Finally, I am finding mercs to be kind of fun!