Jellyfish like thing

[Fredfish]

I have seen this critter in my tank on 3 occasions over about 1 year. The first time I observed maybe 6 or 7 individuals, the next, 20+ (remained for 3 days), the last, 1 solitary individual.
- broadly bell shaped: like the liberty bell compressed in height by ½.
- Very small 1 mm or slightly smaller diameter
- Tentacles/cillia MAY be present on the rim of the bell but I cant be shure. When swimming it appeared to have them but when trapped against the sand bottom, I could not tell.
- pulsed 4-5 times per second
- transluscent brown colour
I tried to get a picture, but could not focus on it where it was trapped on the gravel.
Any idea what it might be?

Thanks in advance.
Fred

[rshimek]

Hi Fred,

Sounds like a hydrozoan jelly fish.

Somewhere in your tank you have some hydroids, periodically they bud off jellyfish. In nature the jellies are the sexually reproducing and dispersal stage.

[Fredfish]

Cool! Sort of... My understanding is that some hydroids are bad news.

Do you have any suggestions as to what I should be looking for if I wanted to identify the "parents"?

Thanks again.
Fred

[rshimek]

Hi Fred,

Some hydroids can be bad news, as can be some corals, soft corals, puppies, and politicians. :D

If it hasn't caused any noticeable problems over the last year, I would guess the polyp is pretty benign. Really no way to tell what the polyp might look like with out a good look at the medusa. My suspician would be that it is small, solitary, probably light tan to grey. However, that description would probably cover about 80 percent of hydrozoan polyps.