Otocinclus behavior is one of the neatest things about keeping these fish. Sure, they don't pair up and act like the Flintstones as Cichlids do, but they're still fun to watch.
Otos are spunky little workers... most of the time they can be seen busily going over plant leaves and the sides of the tank, sucking away at any scrap of algae they can find. You can't help but laugh at the way they move their sucker mouths on the side of the glass, in full view of those who look at the tank.
They also stay in one position for long periods of time. It's almost as if they're real cats, not just catfish, and they like to nap for part of the day and then go about cleaning in a frenzied way.
I have seen groups of otocinclus school (see picture above), but I've also seen a dozen otocinclus scattered in a tank seeming to ignore each other. I think the schooling instinct may be brought on by certain environmental conditions. Certainly when first purchased and put in the tank, a group of otocinclus school in response to a stressful situation.
Another time where otocinclus will school is if they are looking for food. They will tend to follow each other on the move in hopes that they will both find something good to eat. They can all start moving together in a frenzied search for a place to eat and then disperse onto separate plants for feeding.
I have also seen them in groups while feeding, although not as much as when on the move. Sometimes a couple of otocinclus will share the same plant, going over different parts of the leaf. They don't seem to notice each other as other fish do (either warding off or trying to be attractive) but they just go about their business.
Fishkeepers will also notice that sometimes otocinclus will swim rapidly up to the surface of the tank and take a gulp of air. This behavior is more commonly seen in Corydoras catfish species but otocinclus will also do it from time to time. It's just a way for them to get more oxygen. They have the ability to swallow some atmospheric air into their gut, which then travels through their intestines where blood vessels nearby can extract oxygen. It's a natural thing for them to do.
I've never seen anything written about this behavior but I've seen my otocinclus do it many times so I must mention it. Sometimes, while stuck to the glass, the otocinclus will move it's body away from the glass then closer to it, almost as if it's doing a Rocky Balboa pushup where it flexes the lower 80% of its body. It will do this three or four times then rest, and then do it a little more. Very strange. I don't really know what it's doing, but my current theory is perhaps it is pushing food down the digestive track??? If anybody has another idea or knows what it is doing, let me know!
(Jan 20th, 2005 update regarding Ab crunches) After the most recent spawning report of otocinclus, I now believe that the Ab crunch behavior mimics the act of spawning. Now this might not mean the fish is ready to spawn, but that for some reason they are practicing the behavior or just trying to stay "in shape".
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