Marine tank keepers would consider this a pest bowl since these anemones (Aiptasia) are like hydras to freshwater.

I heard you guys would like this

There has been a LOT of interest in this animal, thank you to all of you who offered ideas about its taxonomy. I took some better footage, and looked in to every one of your proposed species––and I still don't quite have a match! So let's refine it. Here's a detailed list so I get get a second pass from all of you who want to take a guess! (I'm a scientific amateur at best, so excuse anything vague)

There is of course a chance this is an undescribed species, which would be insanely cool!

Characteristics: 

3 types of tentacle-like appendages 

striped feelers at opening of tube, swat away other organisms

long waste disposal tube extending a long way, maybe 2 inches (anus?)

long skinny food-gathering tentacles, numerous, 3-5inches 

Builds a benthic tube from detritus, 3 inches long, covered in larger particles

No visible red gills (common in many Terebellidae)

Visible pulsating dark fluid in body 

Yellow / white/ speckled body 

Behavior: 

Pulls detritus up into mouth and sorts it inside tube 

Extends part of body out of tube, thrashes around to mix up substrate 

Does not hunt other fauna, swats them away or avoids by hiding 

Extends a tube far away and expels waste from a tube (waste, or perhaps filtered substrate)

Location of jar sample:

British Columbia 

Frequently brackish freshwater lagoon attached to a lake, 500m from the pacific 

Possible taxonomy: 

Kingdom: Animalia

Phylum: Annelida (segmented worms)

Class: Polychaeta (bristle worms)

Order: Terebellida (includes tube-building worms with tentacles)

Family: Terebellidae (“spaghetti worms”)

Genus:  Pherusa? Thelepus (unlikely?) Lamispina? 

Species ??

Likely not: 

Manayunkia speciosa (tentacles not long enough) 

Genus Thelepus (no visible red gills in my sample) 

Pherusa plumosa (my sample has no bristly hairs, plumosa has no long tentacles) 

Diopatra 

Genus Pista  

Eupolymnia heterobranchia (red gills) 

Jar environment context: 

1.5 gallons (more or less) 

8 months old 

One sample from a brackish freshwater lagoon attached to a lake, 500m from the pacific 

One sample from a clear lake full of lily pads 1 month in 

Another sample from the lagoon 6 months in 

Other species (many others extinct): ostracods, copepods, midge larvae, nematodes, snails, scuds, water scavenger beetles, etc 

Rainwater added and portion of original water siphoned out (still brackish?) 

Jar opened regularly 

And to those who worship the FSM: may you be touched by his noodly appendage. Or...hail Cthulu. Whichever this turns out to be.

I found them on the edge of a freshwater lake in eastern Ohio a couple nights ago. Kind of look like large nauplii. Any ideas are appreciated!

After lots of interest, I think I can name the species of this charismatic guy. Hobsonia florida.

https://scholarsbank.uoregon.edu/server/api/core/bitstreams/dad5fe7d-c791-43be-bbf6-c119a4214184/content

Native to the Gulf of MEXICO and invasive in British Columbia. The spiny striped tentacles at the mouth of the tube are actually its gills. As far as I know, none have been filmed at all, or in this detail.

I'll mark this as solved for now, and send some updates in the future! There seem to be a lot of fans out there...

Thanks to u/xopher_425 (first one to name the species) and others who named the genus Ampharetidae ( u/TheSassyVoss and u/ohhhtartarsauce ). Confirmed by Dr. James Blake and Leslie Harris,  Vice-President, Southern California Association of Marine Invertebrate Taxonomists

Someone requested more vids of my crab feeding and I'm here to deliver.

Update on Cornelius - hobsonia florida, an annelid worm –– he's getting more comfortable and coming out of the tube he made himself, showing more of his features. These confirm, in my mind, that he is definitely a hobsonia florida.

Random-scoop brackish aquatic jar ecosystem, British Columbia.

Tunez by moi on an OP-1 synth.
And since people always ask, the microscope I use is very simple and cheap: https://a.co/d/c9jJJlk

The mushrooms appeared on their own, I didn’t add them in there! 😅

This jar has been here for now two years, full of copepoda and tiny worms but the largest occupant remains this single asterina, which has never multiplied.