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No octopus hatchlings are "easy" to raise but mercs are the easiest of the ones we have journaled (bimacs seem to have more livinging offspring but we only have tank bred from tank raised with O. mercatoris). Unfortunately, we have not seen many mercs for about 4 years. Fortunately, we are seeing quite a few all of a sudden this year (Yours, @sedna, Mote Labs).
It has been awhile since I have had hatchlings but here are a couple of old mercatoris threads that may be helpful: Varys babies (@gholland - Varys was a wild caught female. This is the log for her hatchlings with a backward link to the mother and forward link to the offspring of this batch) Trapper's Babies (@DWhatley - Trapper was a wild caught female. This is my log of her offspring with backward links to Trapper and forward links to their offspring).
Additionally, @TMoct just had O. briareus hatchlings and you may want to read and follow Iris' thread. @sedna just started experimenting with Mercatoris Manor so we are hoping for offspring there but it will be awhile
For some additional reading, look at the sticky in Octopus care titled Raising Octopus Hatchlings. There are both internal and external links there that should give you some confidence.
Awesome! If you haven't already done this, make sure there is lots of very tiny food available.
My recent experience in a nutshell:
1. They don't do very well unless they're in the main tank.
2. Keep the overflow for your filtration system covered with a fine mesh. I use "tulle" from the craft store for the mesh. Be sure to keep an eye on it, since it will cause a flood if it gets clogged up and blocks the overflow!
3. Their growth appears to be limited to how much food they find and eat, so keep lots of little tiny shrimp and/or crustaceans available.
4. They eventually eat each other (at least the Briareuses did), and I'm not sure there's a good way to stop this when there are so many babies.
5. When you get down to a more manageable population, you can separate them out into individual critter-keepers (if you want) with mesh over the top to keep them isolated and safe from each other. (The critter-keepers sitting within the main tank to keep the water quality high).
when I turned on the lights to the main tank I saw a baby crawling on the glass, that made my day, had to go run errands so was excited to find them hatching like mad in the critter keeper, the one I noticed this morning has made up ressidence in an empty shell, have seen a few more crawling around the reef tank but I will just let them be, I have to pull the fish out due to iche going around, so it will be a fishless tank for a few weeks, pefect for my babies to grow some, there is a grass shrimp population in the main tank plush plenty of amphipods as well so they should have plenty to eat.
Unlike O. briareus, O. mercatoris don't tend toward cannibalism of animals of the same size. From my own experience, the mom will not be a problem but I had some questionable incidences of adult males getting into the breeder nets (they may have only been after the food but it was unclear if they snacked on the babies). I found I could keep two or three in a breeder net after about a month and 5 while they were smaller. Put plenty of snail shells in the nursery. They will find them. Hand feeding is recommended (using a pipette) as they are not likely to catch fast food for some time yet.
I recommend keeping two or three together as they age. Mercs seem to be much more active,unlike any of the larger species we keep (the males more than the females), when kept in small groups. We have seen no fatal issues keeping well fed siblings together throughout their lives (the jury is still out on if unfamiliar juveniles and/or adults or animals of extreme differences in size have problems living together). Post #2 in this thread has links to two families of mercs that started with a wild caught female and end with their grandchildren. It is a lot of reading but should be helpful for the next few months.
I will mention, because always I lament when I don't, that I always highly recommend no fish of any kind in with octopuses. Since your female was and adult AND brooded right away, you may not have experienced problems but it simply will not work that way for the hatchlings or juveniles, one or the other will not survive a normal lifespan.