Noticed tonight (in one of my long still-cycling staring contests with rocks) that there were a few strange "holes" in the substrate by a well-populated piece of LR. I looked into one of the caves (about the width of a pencil) and something that had been blocking the light suddenly moved out of the way. Scared the bejeepers out of me. So I grabbed my red flashlight (okay, the app on my phone for a red flashlight...) and my turkey baster and shut out the lights.

I used the baster to blow some substrate over the "mouths" of the three caves where I'd seen movement, and watched as, lo and behold, within minutes there was more movement, and a bunch of the substrate collapsed into the hole, and was pushed out of another. I saw a glimpse of what appeared to be a tiny claw (white) that quickly vanished. I repeated this "experiment" three times with the same results, seeing something definitely digging and pushing this stuff out of its caves! Whatever it is, it seems quite small but certainly dedicated to clearing out these sub-rock tunnels (my apologies to Creature X for ruining his or her night).

As a side note, over the span of the last week, I've heard about five loud snaps from the tank. I thought it was the end of the thermometer getting caught in the flow and tapping something but it seemed loud - like someone throwing pebbles at a window. I'm not saying (or hoping, really) that what I have is a mantis shrimp though I know that so far it could fit the bill. I'd much rather it be something less skilled...

Anyone have any ideas what it could be, and what I can do to find out what it is? I'm a bit concerned only because my clean-up crew will be here the middle of next week, and I'd rather them not all be eaten by Creature X!

Sounds like a Pistol Shrimp to me ...

Sound like a pistol since you saw a large clow. look it up in google images to get a better pic to sewe if you can id

The claw type is definitely similar, and the MO of cave-digging seems to be in keeping as well. Will continue to read up on it and keep an eye out for a better look, but for my own peace of mind, is this a hitchhiker I should (at this point, anyway) be terribly concerned about?

Try food to coax them out to see what it is.

shut off all pumps and drop a piece or 2 of shrimp away from the holes a couple inches and wait.

it should show it self since it probably has not eaten since your rock was boxed up atleast.

Living in substrate, big claw, snapping sounds? Definitely sounds like a free pistol came for a ride... I'd be happy, but that's me

Definitely try to lure it out and snap some pics.

My pistol shrimp does the very same thing, he has little caves all over the place....doesent like it if you fill them in tho lol very entertaining id be very pleased to get 1 free but mine did eat all & i mean all of my stomatellas which i had hundreds of!

Checked again this morning and there's nice big cave/tunnel that definitely wasn't there last night when I went to bed. I promise not to fill it in again this time! Didn't mean to ruin the little guy's night, haha.

I am actually super-psyched about this! A bit concerned for my incoming snails but pistol shrimp are fascinating, and I'm glad to have the chance to watch one in action! Will try to lure it out with a bit of shrimp later to make sure that's what I have, and get a picture if I can! It also makes sense that it would be a pistol, which I read to be quite territorial, since the first time I heard the snaps (four of them, scared the heck out of me) was when I was chasing the ex-xanthid around and it clambered into a hole inside the rock. I put the rock back and snap snap snap snap! Poor crab probably didn't know what hit him...

Thanks again for the tips, y'all - time to research my new little buddy!