Hydroids?

[CnidaChris]

I have these all over the back of one or two rocks. I think they are hydroids but wanted to see if anyone had a better ID. Some of them have started to develop cups that pulse like a jellyfish.

 

[Mikedawg]

Looks like it to me

 

[KJ]

Yes.

 

[mikimoto01]

What did you do about these? I think I have the same. I think one of them produced a tiny jellyfish though.

 

[Mikedawg]

They feed on particulate matter in water and often seem to go away on their own, especially if you increase filtration. Related to jellies and when you research their life cycle you'll find a free swimming form.

Search colonial hydroids on R2R for other information/opinions.