Strange slug like creature?

[Mjb51]

Had my tank for about 4 years now and found the creature in the picture for the first time, I have only added a algae blenny to the tank this year so he must have been in the tank a while and I only spotted it about 3:00am so may be hard to find again!!! Has any one any ideas what it is?? I thought it was a type of flatworm to start, but told it was too big by my lfs, they said it was a nudibranch! But another lfs said it was too big for a nudibranch, and could be a flounder!!!
The rough size of the creature is about 2mm thick, 50mm long and about 35mm wide, it travelled along like a snail any one any ideas?


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[Sniperhank]

That is one large flatworm.

[54seaweed]

Get that thing out of there Asap , I don't think I ever saw a flat worm that big that thing is huge

[Marty.h]

That is a Polyclad flatworm !!! That will eat your snails , clams and decimate them the easiest way to get them out is to simply syphon it out.

I had one once out and in a container tried flatworm exit did not even touch it even after 10 mins in RO water the thing was still alive !!! So only way to deal with it is suck it out and they don't reproduce like the red flatworms.


Will look like this



Just to add they are nocturnal any sign of light and they move like a missile into the rocks so a red light is a must and night hunting to get it out I would if you can get some fresh clam from supermarket and lay it on the sandbed at lights out it will soon sniff that out and be on it and have your syphon tube ready

[mr_X]

Have you had anything die? Do you see evidence of this being a snail predator? I have had these in systems before and have seen no unexplained deaths to attribute to it. There are more flatworms than we know, and they all have different appetites. I wouldn't kill it just yet.

[Marty.h]

Yes I had one absolutely decimate my snail population

[mr_X]

No chance that the snails starved? Did you actually witness it killing them? Meanwhile, the question was for the OP. For such a flatworm to get as big as it did, I would imagine it had plenty of food. I was under the impression these were sponge predators actually-no evidence to back that up except that the large ones in my systems didn't kill anything that I noticed. I really don't keep snails, so they had to eat something else.

[Marty.h]

Yes i caught it completely covering a snail that's how I got it out.

If you google polyclad which is exactly what his picture shows you will see they are very predatory there is even a vid of one eating a crab !!

[mr_X]

Polyclad flatworm is a broad term describing many worms. It's not just that one you have there. They have different diets.
https://en.wikipedia.org/wiki/Polycladida

[Marty.h]

Yes but the one pictured is exactly the same as the one I had the markings and everything is exactly the same

[mr_X]

I'm not saying your wrong, I'm just making sure we have all the facts before we start killing things.

[Marty.h]

I agree I'm all for keeping things alive hence my sump has quite few naughty crabs in. I was simply advising the OP bassed on my own personal experience and sadly it's not a flatworm I would want to keep anywhere near my system.

[bribo12]

I think he big thing here is defining a phylum with a common name. Yes it's a flatworm but there are so many types of flatworms who knows if this one is bad or not. I'd still sump it just in case though


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