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Mercatoris Questions (was "Hey,D..")


Pygmy Octopus
Oct 15, 2005
Since you have kept quite a few Mercatoris, I would like to pick your brain. If you don't mind of course.

1. There are still fish in my tank, have you had experience with Mercatoris bothering fish in a tank?

2. What is the average temp you try to keep your tank around? Just want to make sure.

I don't want to steal the spotlight but I kept a single Mercatoris for a few months.

The tank was a 75 gallon peacful community, the octopus was kept inside a large plastic critter keeper so it could see outside into the rest of the tank. It would occasionally climb around the walls toward the fish, they would follow it, and my Lionfish was VERY interested in it. I'm pretty sure at one point my Mercatoris was trying to climb onto the Lionfish. Considering its size I would say the fish would probably win if it turned ugly. It never had any trouble conquering fiddler crabs as big as itself though.

My average tank temperature was 76-78.
AM - Take the flashlight anytime!

I have only successfully kept one WC Merc (I had two others that died within two weeks of arival - quite possibly old age - that were obtained after I received Trapper, my first and successfully kept adult) and now her 5 young so I am lucky but not an expert. That being said, we don't have a lot of members who keep them (where is Lev?) so AM and I are the most current (and verbose) keepers.

My tanks also range from 76-78, one usually at the top of the range the other normally closer to the bottom (no chillers needed for these guys). My housing, however, is different. Trapper (and now three of her young) lived in a 45 gallon hex with a sump and LED lighting. IMO, this is too large for a single Merc but Trapper was exceptionally big (and was not thought to be a dwarf species when she was sent, primarily because of her size) and eventually accepted a brooding chamber in the front of the aquarium (I might never have seen her if she had chosen to brood in the back). I keep the other two in a 15 gallon hex (I do think this shape is desirable and gives more swimming room). The 15 gallon may be a bit small for my most active one (Sistrurus knows every nook, cranny and interconnected path in the LR) but is just right for his shy sister, at least at this point since at 6 months, both are much smaller than their mother.

I have not kept fish with any of them with one exception. I raised a couple of broods of sailfin mollys with the intent of providing fry as food. None of the octos had any interest in the fry but one escaped into the main tank and is now about 1" long and impossible to catch. The molly stays at the top of the tank and I never see any kind of interaction. General advice is not to keep fish in with the octos and the arguments have winning logic. The reasoning is more as protection for the octo than the fish (particularly with the dwarfs). These are shy creatures and even if your fish are not agressive, you will be less likely to see your pet if you do not remove the fish. If you have damsels or other agressive fish there is a chance they will pick on the octo and blind it or damage the skin leaving it open to infection. Note that AM kept Einy in a separate tank within the aquarium where he was fully protected.

If you will find AM's theads on Einy, you will note that he felt the octopus was happier with tankmates. Instead of fish (which I realize you already have and will have a hard time catching), I would highly recommend adding one or two serpent stars. There seems to be an interesting relationship between the Mercs (and other octo species) and the serpents. The octopuses will allow the serpents to remain very close and are very gentle with them, most interesting to watch and they serve as a good locator when you can't find your camoflaged buddy. One of my females will move out of her den just long enough to have her "maid" come and clean it out, returning after the serpent has done the house cleaning.
OK, another question. I have a 20 tall and a 30 long. Do all of you think that it would be best to set one of these up and put the octo in it, or leave it in the 100 with the other fish?

I have a 55, but would rather not go through the chore of setting it up.
OU_JDub;102577 said:
OK, another question. I have a 20 tall and a 30 long. Do all of you think that it would be best to set one of these up and put the octo in it, or leave it in the 100 with the other fish?

I have a 55, but would rather not go through the chore of setting it up.

I can say that when I put Einy in a tank by himself he was very inactive. I would leave it in with the other fish... you are housing it inside another container though right?
Well, I think it would be best to catch her hiding in a small enough piece of rock that you can just pick up the rock and place it inside a smaller holding area, like a large plastic critter cage with netting over the top. That way you don't have to handle her and stress her out by chasing her around.
When I had to catch my cuttles (of course, they aren't as squirmy as an octopus :roll:), I used a small plastic cup and gently scooped them up. That way they were still in water when I transferred them. Maybe you could put some food in a cup or critter cage and entice the octopus to explore...
I use dome lights (24 LED's) to light the 45 gallon tank durnig the day, 3 white and 2 blue, and one red that I have on 24/7. In the 15 gallon I use an outdoor flourex light with a red filter over the lense (24/7) and no other lighting.

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