What are these creatures?

[canadianeh]

Saw this one on glass. It looks like a worm with a fan like thing on the back.

Are the above the same as these ones on my rock?

 

[P-Dub]

Yes, they are!

 

[canadianeh]

Yes, they are!

Ok. But what are they?? That’s what I really want to know.

 

[P-Dub]

Annelidia. Tiny feather dusters. Beneficial filter feeders.

 

[canadianeh]

Annelidia. Tiny feather dusters. Beneficial filter feeders.

Thank you. Are there any downside of having too many of them? Any animals that eat them?

 

[P-Dub]

Welcome! No downsides. They will ebb and flow in numbers depending on the nutrient levels. More food - more worms, less food - fewer worms.