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FAQs about Flatworm Identification 5

Related Articles: Flatworms (incl. Planaria), Pest Flatworm Control by Anthony Calfo, Worms, Featherduster Worms,

Related FAQs: Flatworm Identification, Flatworm ID 2, Flatworms/Planaria 1, Flatworms 2, Flatworms 3, Flatworm ID 3, Flatworm ID 4, Flatworm ID 6, & FAQs on: Flatworm Behavior, Flatworm Compatibility, Flatworm Control, Predator Control, Chemical Control, Flatworm Selection, Flatworm Systems, Flatworm Feeding, Flatworm Disease, Flatworm Reproduction, & Worms, FAQs: Worm Diversity FAQs, FAQs 2, FAQs 3, & Worm IDs 1, Worm IDs 2, Worm IDs 3, Worm IDs 4, Worm IDs 5, Worm IDs 6, Worm IDs 7, Worm IDs 8, & Worm ID FAQs by Group/Phylum: Flatworm Identification ID, Nemertean, Proboscis, Ribbon Worm ID, Nematode, Roundworm ID, Nematomorpha, Horsehair Worm ID, Acanthocephalans, Thorny-headed Worm ID, Polychaete Identification, Polychaete ID 2, Tubeworm ID, Hirudineans, Leech ID, Echiuran Worm ID, Invertebrate Identification,

Flatworm or something else?       4/11/18
Hi WWM Crew,
Was hoping to get an ID on the mysterious creatures in the attached photos.
<Appear to be Acoel flatworms... Likely no big deal. See WWM re>
Thanks for your help!
Adam Clayton
<Welcome. Bob Fenner>


Re: Flatworm or something else?       4/12/18
Thanks, Bob!
<Welcome Adam. BobF>

Flatworms?     2/27/16
Hi, Could you tell me what kind of flatworms these are?
<Not from these pix; no. Likely Acoels...
I don’t have many but there are a few on my glass. I have a Hoeven’s Wrasse in my quarantine but he still has a few weeks before I put him in my tank. It’s a new tank 2 months old and I only have 2 clowns in the tank. I’m wondering if I should wait for the wrasse or does with flatworm exit?
<I'd wait. You should read: http://www.wetwebmedia.com/pestflatwrmanthony.htm
and the linked files above. May be that having "some" of these are better than possibly poisoning, polluting your system.>
Was hoping to get your thoughts.
<Please learn to/use the search tool, indices on WWM. Bob Fenner>

Tiny worm?        1/30/16
I was introducing a peppermint shrimp into my tank and noticed this little guy on the sand next to it. I was able to snag it out before it disappeared. It's about 1mm and seems to have a circular mouth. It also moves fairly fast for something this size. Could you identify it for me?
<Tis a Acoel flatworm of some sort... Go ahead and search, read on WWM re... some folks over-react... Me? Meh.>
Thanks a million!
<As many welcomes. Bob Fenner>

flat worm id?      12/2/14
what kind of flat worm is this ? it was maybe 4-5 cm long..
<Mmm; don't know other than that it IS a flatworm. Bob Fenner>


Worm & Crab ID      8/18/13
I have spent hours on your site and Google.com trying to ID these creatures.  The worm(s) are dark then a small white section then a more greyish color.  Very tiny worm like creatures.  Those are Nerite eggs on the glass and they are about the size of one or two eggs. 
 <Looks to be some sort of flatworm; "Planaria" sort... not likely a worry>
The two crabs are two separate creatures I find at night roaming my tank. 
I think they are both the same species, but not absolutely sure.  I have found the molt from crab, but it was a bit bigger than either of these creatures.
I'm not sure if I have anything to worry about.
<General opportunistic omnivores; judging from their claws... are consuming some life in your system; more as time goes by... I'd be reading re Decapod compatibility on WWM>
 Our tank has not been at its best due to our special needs two year old. 
She is doing much better now giving us much more time for our water critters. 
 Thanks a Ton,
<Welcome. Bob Fenner> 

full size pic.

flatworms in reef tank?     8/17/13
I've noticed what appears to be white flatworms with little antennas
<Mmm, Platyhelminths don't generally have "antennae">

moving across the sides of my tank. I have not seen them on any of the live rock just the glass, so I have been taking them out when I can.
Today I noticed a one larger one and included a picture. From what I can gather on-line some of these things are VERY BAD and some or so-so (won't destroy your tank). I have a 165 gallon with Fiji live rock going through the cycling process (been about 4 weeks now). They don't look like the rust brown flatworm but then again I am only seeing the underside of them which is white. I'm waiting for the bad news  ;-(  
Thanks for you help!
<I wouldn't over-react. Likely this/these are of little to no matter; and will likely "pass" in time on their own. If it were me/mine, I would do nothing (other than read on WWM re Flatworms period) for now. Bob Fenner>
<Oh! In enhancing your pic; this appears to be a Gastropod... See WWM re Stomatella. BobF>     8/17/13

Nudibranch? ID     5/22/13
Last night I found this little critter on my torch coral, I have noticed it has been losing a tip every week or so but nothing drastic.
I touch the approach that if it is on a coral it most likely eats it but wondered if you knew more. It's very light averse and almost completely flat. It's about one inch long.
<This appears to be a flatworm. Can you describe its movement/locomotion?
Bob Fenner>

Re: Nudibranch? ID     5/22/13
Hi Bob,
Thanks for the quick reply,
The end to the left of the photo (with the line clear line down the centre of the animal) appears to be the "head" end and is the direction of travel. It appears to move very smoothly and quite quickly in the direction of travel with contraction.
<Ahh, a Platyhelminth>
On close inspection there is a very very thin orange line around the edge of the work and the coloration is made up of brown spots and lines surrounding the clear pattering through the centre.
<And notably; I'll note... a bold gross color and marking appearance to a Chiton... perhaps camouflage. Not likely damaging anything here. Bob Fenner>

Help Help!      4/23/13
Please help me ID these little guys. I found them today as I scraped the glass. They are tiny, black, slimy, & they move fast!
<Flatworms. Read more here--  http://www.wetwebmedia.com/flatworms.htm >
Sent from my iPhone
<Sent from Jordan's laptop.>

Brown Specs on Live Rock    2/9/13
Hi - I have a 110 SPS tank with a few soft corals that bee up and running for 8 months. I recently noticed brown specs appearing on my live rock in the last 2 week. They almost give the appearance small flake food.( see attached photos). What do you think it is?
<Impossible to tell from these pix>
They smear when you touch them.
<A good clue... still could be many things... algal, Cnidarian, Poriferan...>
I am assuming that they are some type of brown algae. I should mention that about 6 weeks ago I added 3 EcoTech Marine Radions LED to this tank but I have been slowly increasing the intensity to acclimate the tank. I am only running the max intensity of these light at 48%. Coral are responding well.
However this Brown type algae is showing up all over my live rock.. I can certainly back down my lighting intensity but just want to make sure I don't have a bigger problem that I think. Thanks -Ron
<I'd get a look/see through a microscope... one that can make images... as detailed on WWM... Send along these pix. Bob Fenner>

Re: Brown Specs on Live Rock     2/10/13
Thanks for the response! Just to let you know, I took a piece of rock to my LFS. They said it was red flatworms.
 I never had flatworms and I've been in the hobby for 20 years. I added a six line wrasse to the tank today. How long do you think it will take the wrasse to eradicate the problem?
<Mmm, depends... mainly on the species of Flatworm... some are unpalatable to most all fishes. See WWM re>
 I was reading on your site it said to add carbon which I ready have carbon reactor running. is there anything else I can do?
<In terms of what? See WWM...>
Also what would happen if the problem of the flatworms went unaddressed and what should I do if I do see any progress from the wrasse?
<All covered... on... WWM. Too many folks panic re... not usually a real issue. BobF>

Mystery guest discovery    9/16/12
<Good morning Mike>
 Would you guys watch my clip of a guest in my 7yr old tank that I discovered during a water change? I'd love to know what it is.
<Appears to be a Polyclad flatworm. I would remove it.>

WWM Video/IMG_1429.MOV
Re: Mystery guest discovery 9/16/12
What danger does it pose?
<They're predators. Diet consist of a wide variety of sessile organisms and invertebrates.>

Need help identifying this critter....      6/25/12
Hello again.
I have noticed a few of these recently, one in my main tank and one in my refugium. I need some help identifying, have searched the site and not found or have missed a match.
I have had a few egg layings by my Nassarius snails, but I would think that even in the larval stage this guy would have some type of shell by now. He is about 3/16" long, has grown quite a bit in the last 2 weeks.
Thanks for any help and thanks again for what you do. Your knowledge is invaluable.
Bob Colley
Thornton, Colorado
<Look to be a flatworm of some sort... Search on WWM, the Net w/ the genus Waminoa... and as Doug Adams might say, "Don't Panic". Bob Fenner>

Flatworm id, and Montipora hlth.    4/26/12
Mr. Fenner,
<Hey Jeff>
I have some new frags in a quarantine tank and I recently noticed what appears to be a few small flatworms.  The first one I saw was on the glass and a few came off the corals during a Coral Rx dip.  I attached a few pics, one through a microscope at 100x, with the hopes that you could id them and advise how to move forward with them.
<Do appear to be flatworms; can't tell much more>
 If they are a potential pest I would like to keep them out of my main reef.
<I would do so>
 I read all over you site and everyone seems to have issues with their main display but none dealing with flatworms in quarantine.  I am assuming the approach would be different while in qt.
<I would use a commercial de-wormer... Flatworm EXit likely>
 I am also attaching a picture of an encrusting Montipora that has a white patch that has developed over the past two days.  My initial thought was Nudibranchs so inspected it and a Montipora cap but didn't see any or any eggs.
<Very hard to see... but may not be due to Nudibranch predation>
  These frags are also in quarantine and all frags look great except for the one Monti.
<I'd remove it to elsewhere>
 I understand that there could be numerous reasons for this dead spot but any advice you could give would be much appreciated.
<The usual recommendation to visit/read on WWM re Acroporid disease/pests/predator (FAQs) and pulsing iodide-ate additions>
 Thanks for you help.
<Welcome. Bob Fenner>

Strange worms on glass -- 12/19/11
Good afternoon. Thank you for having this great site. I'm hoping that you can help me.
This morning I noticed about 100 of these strange yellow worms crawling all over the glass in my saltwater fish only tank. Should I be worried. What do you think can be. Thank you.
<Mmm, appear to be some sort of flatworm, "Planaria"... not really a worry, and will very likely disappear on their own in time (due to predation, lack of food...). Bob Fenner>

Identification Help (Polyclad) New Species ?? 11/29/11
Hey Guys/Gals at WWM;
<Howsit Noel?>
Thanks for the time, dedication and the services you provide to us enthusiasts as I often find myself constantly referencing back to your site for more info & research. So cheers for that!
<Thank you>
Now for the issue at hand. Recently on 11/24/2011 upon viewing & inspection of a friends LPS reef tank. I discovered these interesting polyclads (pics attached 827X687) on the underlying branches of his Euphyllia ancora.
Upon further inspection, I found two more and quickly placed them into a specimen container. These particular polyclads ranged from 1/4 inch to a full 1 inch long and say about 1/8 inch in width. I've researched and posted some info on my local forum and club forum for some assistance in indentifying this Polyclad and seem to come up empty handed. I have researched your 17 pages of worms and 7 pages of Polyclad identification along with the FAQs and came across this thread.
http://www.wetwebmedia.com/fltwmidfaq3.htm (Same Polyclad)
This raises a concern to me as this might be of a new species and a future problem for LPS corals. Since the capture of these polyclads on the above mentioned date, on 11/27/2011 (3 days later) one of the three has laid eggs (pic enclosed). So now we(reefing community) will have a reference point as to an incubation period before the eggs hatch (guess we can all benefit from knowing that).
Before I forget... here's a little information about his tank: 155Bowfront
72" LED Fixture, NO SPS AT ALL. Strictly an LPS (large polyp stony) coral Fanatic(LOL).. and mostly, if not all his corals originate from Australia.
Param.s are excellent and irrelevant to this issue.
Hence I ask WWM for some assistance. If you have any more info regarding the identity of this type of Polyclad, or where I can check for more info regarding the OP thread I linked. I would greatly appreciate it. With over 4,500 species of polyclads, I'm sure we can find one that matches and where it originates from!! LOL!
Thank You & Warm Wishes,
Noel. NY Reef Club
<Mmm, I don't... but do want to encourage you to seek help w/ this ID (an old roommate did his grad. work on the Turbellarian Paravortex, the causative organism of "black spot disease"...). By sending these pix and specimens to folks who deal w/ Flatworm taxonomy. You can find them via a quick search on the Net... my own preference is to deal w/ local folks if possible. Do you live near a Natural History Museum? University w/ a Bio./Zoology dept.? Bob Fenner>

Re: Identification Help (Polyclad) New Species ?? 11/29/11
Hello Bob.
<Howdy again Noel>
It is an honor.... and whoa, that was a fast and timely response.. LOL.
Actually, I do have a very good friend of mine, who's daughter is studying Marine Biology at a College located in Purchase NY who can possibly assist.
It hadn't dawned on me to ask, and I'm sure this might serve her as extra credit. Great idea! Also, there is The American Museum of Natural History here, however I'll doubt I'll receive any assistance in the helping with the identify of this Polyclad, but it is worth a shot. l will keep you posted as to the progress made so that we may have a better understanding and hopefully determine if this will be of a great concern to us all.
<Appreciate this>
Warm Regards to You and the WWM Crew,
Noel. NY Reef Club
<Thank you. BobF>

Identification 10/23/11
Hope you can help identify this thing growing on my overflow pipe. Thanks.
<... looks to be a flatworm of some sort. See WWM re. B>

Unknown parasite/ organism found in filter 5/23/11
WWM crew,
I have been searching the site for a while, and can not find anything to help me out. I am tearing down a 55 gallon, and was cleaning the filter and this weird little critter came out.
<Looks to be some sort of flatworm... these are often only mildly damaging... more like "space parasites" then actual; some are "free living">
The critter DID not come out of the intake or outtake holes, but I am still interested to know what it is.
The 55 gallon was a holding tank for a couple of my brackish water red and green scats, that have since been moved over to the 90 they came from (I moved 800 miles, and could not part with fish). I did an extensive look at the fish, and I do not see anything that would hint it is a parasite. They are eating like pigs, and they all have there color. No red streaks, or visible irritations of the scales on any of them.
<Not at all likely parasitic or environmentally damaging to fishes>
System set-up was a 55 gallon, with a Jager heater (set to 80), Rena XP2 canister filter, Whisper 300 air pump, sand substrate (CaribSea, to be specific), and the SG ranged anywhere from 1.008-1.0015. I fluctuate the SG, for better health of the Scats. Levels of Nitrates, Nitrites, and Ammonia, all read 0.
The critter is about a centimeter, by a centimeter, and has veins that are running the entire surface of the body. In the center, there is a mass that looks like blood. To me it almost looks like a tick you would pull off a furry animal. I looked in the books I had available to me, and did not see anything like this. I also have NOT punctured the center blood looking mass
out of fear of what could happen. Any information you could tell me would be greatly appreciated.
Until then, back to cleaning!
<Thank you for your message. Bob Fenner>

Enlarged, but blurrier

Planaria id 4/25/11
Hello wwm,
You were very helpful last time I found one of these. Perhaps you are able to id this one.
<Is a flatworm of some sort. Read here: http://wetwebmedia.com/flatworms.htm
and the linked files above>
Should it stay or should it go?
<... as you'll read...>
I can say that its been in the tank for at least six months. Found it in a filter sock. Keeping it in the sump for now.
Thank you,
<And you, Bob Fenner>

Saltwater Question - Live Rock Creature ID 4/6/11
Hello WWM crew,
Thank you all for taking the time to help answer the many perplexing questions this hobby provides.
I've read the site for years but due to the abundance of information I have not had to ask any questions. Today is the day I am stumped. Despite searching whatever terms I could think of I cannot ID this creature. It appeared as a flat *thing*,
<Ahh! Indeed>
maybe 3/4 of an inch in length, and a half inch across. It appears rectangular but the edges are slightly rounded, it almost looks like if you rolled out cookie dough. It is an orangish/tan color. It presented itself on a rock after I rearranged my tank. Attached is a photo showing this creature in a variety of phases, I apologize for the poor quality in advance. The creature began
flat looking, then became somewhat curled up, then became in a "U" shape.
It moved from the rock to the substrate eventually, it was fascinating to watch. I researched various worms but I did not have any luck finding a positive identification. Any help or guidance towards the right path would
be greatly appreciated.
Thank you for your time,
<This is a flatworm of some sort/species. If singular and in a good-sized volume (tens of gallons), I'd leave, enjoy. If there come to be many... Do read here: http://wetwebmedia.com/fltwmcont.htm
and as much of the linked files above as gives you sufficient input to move forward, stay put. Bob Fenner>

Worm ID -- Likely Acoel Flatworms -- 3/10/11
Greetings crew!
<Hello there, Lynn here today!>
Was walking by my tank earlier than usual this morning, before lights went on, and noticed that there were these tiny worms crawling all over the rocks and my yellow polyps. Never seen them before as the polyps are normally open and cover them.
<It happens that way sometimes.>
Any idea on what they are? Are these the dreaded flatworms? Have not found any similar pics to them.
<Well, I'd need a more close-up shot to confirm, but they do look a whole lot like Acoel flatworms, aka 'Rust-Brown Planaria'. For more photos and information regarding control, please see the following links:
Good close-up photos of Convolutriloba spp. for comparison: http://sites.google.com/site/macropyga/identification
Enter 'Acoel + Planaria' into our Google search engine for many related FAQ's: http://www.wetwebmedia.com/Googlesearch.htm >
They are about 3-4mm long, reddish color, smooth body.
<Fairly typical>
Pic attached. Best I could get of them.
<Thanks. Do let us know if what you have is something other than the Planaria/flatworms listed above and we'll give it another shot. Be sure though, to include a close-up photo so we can see a bit more detail.>
<You're very welcome. Take care, Lynn Z>

Frustration setting in! Reading... flatworm ID, hair algae contr. 11/21/10
Hey there crew, Jordan here. So I'm asking for a little help with an ID of sorts and am also getting to the point of frustration. My tank has been up and running since May 2010. All has been great so far until recently. I did loose several fish to an Ich outbreak. I realize the problem is probably still there but there is no way of getting the remaining fish out of the tank or finding somewhere to put them as I only have a 10g QT. All the fish I have purchased have been watched well at LFS and have been there for at least 4 weeks before I touch them. I am down to 1 flame angel, 1 hippo tang, 1 false percula, 6 Chromis, 2 diamond gobies, and 1 cleaner wrasse. All these fish appear ok as of right now. My question is not
really with the Ich issue as much as with the sudden (last few months) constant growth of hair algae and what appears to be little brown/orange flakes covering a lot of the algae and rock. This is the ID question I have and pictures to follow.
<Mmm, these are flatworms...>
All the corals that I do have, 1 frogspawn, 1 torch, 1 green Acro frag, 1 open brain, 2 x Rhodactis are doing awesome.
Tank parameters
125g w/ 25g sump
3 x150w HQI canopy from AquaticLife 72"
All water parameters have always been perfect. I use AquaVitro salt and do 10-15% water changes weekly. PH- 8.3 Sg 1.025 Nitrate/ite 0, Mg 1440, Cal 420ppm, Alk 3.5meq/l
I run 2 Vortech mp20 powerheads one on each side wall and a Koralia 2 in one corner. If you have any suggestions re algae problem or ID of flakes I would appreciate it.
<Read here: http://wetwebmedia.com/fltwmid.htm
and the linked files above... "control"...>
Feed x2 a day, New life spectrum pellets/ PE Mysis and Fish & Reef #2 - one or the other
Clean up crew is roughly - 15 Nassarius snails, 6 turbo, 30 blue leg hermit, 15 Cerith
Do I need more cleaners/algae eaters?
<Mmm, not the approach I would take... nutrient exclusion, use by/through a DSB... remoted best...>
Is amount of flow possibly the problem?
<Possibly a contributing cause>
I really do not thing phosphates are the problem, I also have a lrg bag of Chemipure elite in sump, 1 month old
Please any thoughts would be greatly appreciated. I know this sounds like a lot to ask for.
<Mmm, not... Please read here:
and the linked files where you lead yourself. Enjoy the process... a measure of all things is what you become by them. Become yourself. Bob Fenner>

Polyclad flatworm?? 11/15/10
Hi everyone! I've been playing midnight peek-a-boo the last couple of weeks and ran across this guy. After researching at your sight I think he might be a Polyclad flatworm?
<Appears to be so>
I read that some might go after Tridacnid clams at http://www.wetwebmedia.com/fltwmid.htm. I'm a little worried, because I've had a couple of T. Croceas for a year and a half and just purchased a T. Maxima and a T. Derasa. in the past couple of weeks. I've attached the pics and hope you guys(or gals) can help me out.
Thanks again!!
<Mmm, well... if you're too/that concerned, I would vacuum this flatworm out. Bob Fenner>

Polyclad Flatworm ID - 8/9/10
<Hello, Lynn here today.>
I found this huge 4" (Polyclad?) worm for the first time after the lights went out after having my tank set up for over two years!
<Yikes, that is a seriously big flatworm!>
Clearly, it has been able to survive off something in the tank and thrive. The only specific ID on your website from something that looked similar (a brown Polyclad as well) to this said it was a clam eater (noting the clam-like mantle pattern on the worms body)- but I don't see how this is possible since there's no clams in my tank.
<Some Polyclads with that brown, blotchy appearance are indeed clam predators but that sort of pattern is common to many species.>
I have no intention of trying to trap it (even if I could) as it's done no harm to my reef inhabitants for over two years.
<I imagine it's done harm to something, but whatever it is may be able to regenerate or multiply quickly enough to withstand the Polyclads level of predation (at least so far).>
I am curious though, any idea what it is or what it eats?
<Unfortunately, I can't tell you much other than it does appear to be a Polyclad flatworm of some sort. Identifying these guys would be so much easier if there were only five species worldwide! As for its diet, I'm afraid it's a 'same goes', situation. All I can relay is that from what I've read, most seem to be carnivorous predators that prey on anything from Gastropods to small Crustaceans, Bivalves, tunicates, Bryozoans, sponges or other sessile organisms, while others may well be scavengers. I've even read that some might be herbivores, but I can't confirm that. The problem is that there are an awful lot of flatworms out there, and not all have been studied.>
Sorry for the very poor picture quality but my camera couldn't seem to get a good shot with all the Cyano on the glass and bristle worms!
<No worries, I can still see what a neat not-so-little guy you have there! I'm sorry I wasn't able to give you more concrete answers. For more information on flatworms, please see Bob's section re: http://www.wetwebmedia.com/flatworms.htm Take care, Lynn Z>

ID On An Unknown Creature: Likely Polyclad Flatworm -- 7/4/10
Hello WWM,
<Hello Brandon, Lynn here today.>
My email is in regards to a creature that I cannot ID and have no idea if it is safe for my reef.
<Fire away>
I have a 20L that I have had for going on 8 years. I have had the same live rock since I started, up until about 6 months ago I had the opportunity to buy some (an additional ten pounds or so) very nice Marshall Island live rock off a reefer who was tearing down their superb 125 reef. I absolutely love the rock and have had no problems ever with known hitchhikers, or I may be oblivious because I know there are a lot of them! Until last night I walked by the tank and noticed a black blob coming out of one of the holes that my Tailspot blenny is almost always perching out of. It is about an inch and a half long, half an inch wide and extremely flat. It also has a lot of tiny white dots all over its back.
<I'm guessing that you didn't see any sort of antennae or other protrusions on the back, or around the head?>
I know that some of these kinds of creatures have a shell, but I could not notice one.
<Yep, some Scutus spp. look like black sea slugs with sometimes covered, elongate, whitish shells on their backs. What you have doesn't look like a Scutus though; the markings are off. I've seen black/dark individuals with a marbled or blotchy appearance but never one with actual spots. What you have could be a Nudibranch of some sort but I think it's more than likely a Polyclad flatworm. Polyclads are mostly nocturnal predators that feed on small invertebrates and can move at a surprisingly fast clip. I would remove this little fellow.>
I noticed that it doesn't like the light, when I turned back on all my lights from going to dusk it began to go back further into this little crevice
<Typical behavior>
....which is the reasoning for the crappy pictures.
<Hey, they're good enough to tell that it's not a Scutus! If you'd described a black slug-like animal to me, that would have been the first thing that I'd have suggesting ruling out.>
I am not sure what other information you need, but I have gone ahead and attached three photos to hopefully help ID the unknown creature.
<Thank you>
I just need to know if it is reef safe (hard, soft and clam safe),
<Thank you for specifying! 'Reef safe' means different things to different people. In this case, if indeed what you have is a Polyclad flatworm then it should be safe around the corals but iffy with the clam. I know of at least one species (Stylochus matatasi) that's been implicated in Tridacnid deaths. Thankfully, that's not what you have. I just used it as an example.>
..how long it lives
<That I'm not sure of.>
..and if it is poisonous to my system if it dies?
<Well, from what I can tell, there are indeed toxins in at least some Polyclads, particularly in the genus Planocerid. I'm not sure if, or to what degree, they're present in other genera/species. My thinking is that considering 20L (5.29 gallons) is such a small amount of water, it wouldn't take much to cause a problem, whether ammonia or toxin related. I'd err on the side of caution and remove the flatworm. Please see the following links for more information and comparison:
See Pseudobiceros sp. '#8' (click for larger image): http://www.wetwebmedia.com/WormPIX/FlatwormPIX/Flatworms2.htm
See Pseudobiceros #3: http://www.marinelifephotography.com/marine/worms/flatworms.htm
Scutus: http://www.seaslugforum.net/factsheet/scutus >
I greatly appreciate your help!
<You're very welcome!>
<Take care, Lynn Z>

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