What Ate my Blue Leg Hermits?

SteveMM62Reef

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I have two Clown Fish, Neon Dotty Back, Flameback Angel, and Four Green Chromis. Also an Emerald Crab, Three Nassarius Snails, Cleaner Shrimp, and Six Bubble-tip Anemones. Noticed all six of my Hermits are gone. Iodine level is fine.
 
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KingLucy1997

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I just discovered this new 'crab/snail' ambush predator in my tank eating vulnerable shells tiny blue legs due to a massive overnight drop in pH into the 7's, and then my dummy confused the pH up and pH down solutions and grabbed and used the pH down once by mistake, putting the entire aquarium into danger for 5-10 minutes before I was able to get back normal parameters. It seems like some of the shell-less hermits died from the fluctuation, but I saw other snails and hermits that looked dead begin to recover. I tried to help one of them by moving a shell closer to him that I thought was empty. When it got close, this black amorphous blob popped out of the shell, and started to devour my pour baby hermy whole. I disrupted the feeding, but it was too late for the hermit crab. It seems that I have multiple of these things and they are incredibly camafloughed. They either go shelless in pebbles and I noticed one's beady black eyes and twin black feelers sticking out just barely. The more sinister type of ambush I have witnessed is that they pick up a shell like a hermit and crawl slowly towards their prey like an unassuming harmless snail. It seems to move with some sort of tentacles that are snail like but definitely not a snails foot. They aren't that parasite that takes over the entire crab because they were going after dead/mostly dead ones and I could see its mouth open to the size of its entire body to start trying to swallow its prey whole. Maybe they are the larvae of a pebble crab? I can't ID this species anywhere. I can't even put it into a general category because it can morph. They are incredibly fast (I couldn't even get close to catching them with my tweezers), so kind of odd they go ambush style and scavenge.
Does anyone know what this is and how to get rid of it, especially if it will grow into a full blown black crab or something? Its seems highly prolific and makes me nervous for how many of these I have, how big they will get, and what they will start targeting once they are at their full size unless they are already.
Tank description:
-5 gallons
-1 correline macroalge (penicillium sp., the tallest of this species)
-3 (maybe only 2 less now after the pH fiasco) of white shelled blue-legged hermit
-1-2 Mexican Red Hermit crabs
-10ish Cerinth snails
-I also inroduced 2 peppermint shrimp into the tank after a water change and they died in the resulting pH fluctuations required to bring the tank back into the shrimps habitable range.
-used to have countless blue legged hermit crabs by their population is taking a huge dip bit from my inexperience (this is my first tank) and the difficulty to maintain constant water parameters in a small tank for a newbie.
Is this some sort of terror isopod? Please don't tell me it gets huge, I can't catch them and they can pretend to be snails so I would have a really hard time getting rid of them. If I supply enough food (ex. brine shrimp colony, roids and phytos, copepods, store bought food) wlll they leave my hermits alone? Can I starve them out? I don't even know what these monsters are so no idea if I should just chuck my tank out the window and give up on this hobby for a few years.
 
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I have no idea what that pest is but if your tanks only 5 gallons you could probably just empty the tank and catch them pretty easily. Do they look like this ?
 

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KingLucy1997

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I don’t recall alien shape shifting much, really
AC5CF4C5-7ACF-4504-8891-A9623D16BECA.jpeg
but I was sleep deprived. I took some pictures though, I ended up nuking the tank and all equipment to be safe. It scared me and I’ve never seen any thing that can be just as strong in salt water and fresh water. Out of an abundance of caution i Nuked the nearest terrestrial pot plant too it because it had fallen recently and I noticed some similar structures to the monster. I do think that was overkill because I was going on 48 hours without sleep at the time, so maybe I I was just hallucinating that last part. I know the infection itself was real because I have pictures and videos. I’ll attach the pics, but if you want a more detailed look/description from me let you know and I’ll post the videos. (I do swear In it, mostly because I am realizing hundred of hours and dollars are going down the drain along with my brain melting. The upside to this is that it was incredibly hard to to with my aquariums and terrariums by my desk and I spent way too much time maintaining/growing them. Do you think there’s realistically a chance the terrariums got contaminated too? They have no pools of water but are very humid and primarily contain terrestrial isopod crustaceans. I a corrupted blue legged hermit (whatever this thing is it turned it into a frankensteins Zombie two eye on one side head, it’s had to asymmetrical long algae grass looking arms. Looked like it was still wearing its host’s lesser bladder pond snail shell.didn’t notice it until water level dropped below it, which prevented its ability to move.In the line of Alien, the Cerinth snails I bought had variously sized spots I thought was natural until the at least 2 monster ripples out of these red spts

moots ?
(in a shrmp like form yeah but I’d have maicby ripping itself out of the
 

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KingLucy1997

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Here’s proof this was the aquatic version of the thing. I swear it would become completely translucent at some angles of shining light on it.
 

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KingLucy1997

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What are we looking at?
Here is a better picture of one of the blue legs that appears to have been assimilated. Or maybe it used to be a Cerinth. It has been so heavily modified by the creature its hard to even tell what it used to be. It almost looks cobbled together by other things in the tank. That arm looks like seaweed but it is also claw (maybe?) the rear appendage looks like it’s reaching towards to attached to another infected snail or crab below which is the blue blob. I think this is a very scary, rare flatworm. I know flatworms can take over the bodies of crustaceans, and it’s plausible that a flatworm could change its shape, but I’ve never heard or seen of any of this dramatic camaflauge/shapshifting ability In any species, but if it’s not an alien some exotic flatworm is my explanation.
 

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KingLucy1997

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What are we looking at?
Here is a better picture of one of the blue legs that appears to have been assimilated. Or maybe it used to be a Cerinth. It has been so heavily modified by the creature its hard to even tell what it used to be. It almost looks cobbled
You didn’t by chance use any pieces of meteorites as live rock did you?
I wish it was a meteor rock. I actually used regular Carrabian live rock that was shattered and boxed up. It was the most mass produced box of live rock. The infection happened after I added a second batch of animals and macroalgae to the tank, so the live rock in the tank was about 6 months old so I don’t think it’s a live rock issue.
 

KingLucy1997

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The clear looking thing behind the frag plug?
The thing is literally everywhere, I wish the water wasn’t as cloudy so we could see it better. To orient yourself, look at the white hole/circle at the bottom center left of the tank that has little blue crab legs around it. That used to be a blue legged crab, but the creature seems to have bored through it and is maybe using it as a mouth to filter feed or abuse. Didn’t notice any movement. Look behind the ‘mouth’. There is a crab looking head with tenticles that go in and out of the substrate spreading around it. These were all over the tank.
 

KingLucy1997

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What are we looking at?
Now simultaneously on the other side of the tank, this creature appears to be grabbing the blue legged snail’s astra snail from both sides. The right side looks more crab like, and the left side had these little frills on them to kind of like a mini sea fan or something. Those white things on its ‘head’ that you can see on the right upper part of the shell would turn to follow my flashlight, so I think it was at least photosensitive, and not at all scared of me if had full vision. The giant pouch coming out of the astra shell started inflating after the thing grabbed it. I think it’s injecting something into it which is causing the hermits crabs tissue/guts to swell absurdly. I could see it the pouch enlarging over time.
this whole thing freaked me out. I saved a sample of the detritus contained within the original bag of the 4 inch Penicullus macroalgae and it did appear there were some flatworms in their, but I only saw them once. I also noticed the first couple days if I turned the vial on its side the particles would self assemble into a crab head like mass, but it doesn’t do that anymore. It was coexisting with copepods in the vial. Do you know of any lab I could send these samples to get them genetically tested to try an ID the monster’s species, or if it’s even ever been described by science yet?
 
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TnFishwater98

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Here is a better picture of one of the blue legs that appears to have been assimilated. Or maybe it used to be a Cerinth. It has been so heavily modified by the creature its hard to even tell what it used to be. It almost looks cobbled

I wish it was a meteor rock. I actually used regular Carrabian live rock that was shattered and boxed up. It was the most mass produced box of live rock. The infection happened after I added a second batch of animals and macroalgae to the tank, so the live rock in the tank was about 6 months old so I don’t think it’s a live rock issue.
What 2nd batch of animals ?
 

KingLucy1997

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In that I had about 8 blue legged hermit crabs in the tank, which was the first time I ever set up a salt water tank a couple months ago. I ordered blue legged hermit crabs with a yellow whip Gordian. When I say batch I mean a shipment of new animals to put in the tank. It wasn't actually my second batch.
 
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