Please help ID jelly like hitchhiker

[Suohhen]

This critter arrived on the back of one of the Spiny Astrea I added a little over a week ago. It was tightly closed and looked merely like a lump on top of the shell but yesterday it opened up and overnight it dropped off the snail. The first pic was taken in the evening under poor lighting. The pic from this morning has more detail.

 

[fishguy242]

hi, @KJ any clue?

 

[KJ]

Hard to tell.

 

[F i s h y]

Very interesting. Where did the snail come from? Almost looks like a jellyfish, but thats doubtful. The larval forms of several sea dwellers can have this shape. Im interested to see other people's thoughts

 

[El_Guapo13]

To say that looks like a Jelly is an understatement. It almost looks like it might be an Upside Down Jellyfish, especiallyin that last pic where it is, well, upside down. How big is it?

 

[El_Guapo13]

Also does it move or pulse at all?

 

[vetteguy53081]

Looks like some sort of cnidarian or polyp bailout from a coral that never formed. It may be something to do with the coral itself. Especially given there are 3-4 layers on this structure and hasnt moved since discovery.
This rules out any kind of slug, nudibranch , tunicate or cucumber. Tunicates do come in a variety of shapes, sizes and colors and they don't tend to move, so I wouldnt rule out tunicate as a possibility.

 

[Suohhen]

It seems I have deceived even myself with that second photo. What I felt like I was seeing the first night was something with the shape of a typical clam shell but very delicate. I think the extra dimension is merely a trick of seeing it from only one angle, either that or it swelled up, but right now it looks a lot like what I saw the first night.
The bottom is translucent and shaped like a suction cup and functions like it quite well which is easy to see. The top shell is hard and I don't see any movement but it is probably dead by now, the first night it looked like that cyano like mass on the inside was waving around in the current but whatever that was is gone and for all I know it was algae.

 

[fishguy242]

some sort of stomatella??

 

[Suohhen]

I can definitely see how it could be a snail similar to stomatella or more likely a clam that died long before it got to my tank but took a while to decay and break its foothold. I also wonder if it's possibly a deformity to the astrea's shell that pealed off, but that seems doubtful.
Oh I forgot to respond to the question of origin. The store I got it from usually orders from Sea Dwelling Creatures.