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I know the title promised rotifers, we'll get to them. But this was the first thing I spotted and I thought it was a rotifer - I was completely wrong - there's nothing wrong with being wrong - as long as you learn from it and I'm not a professional pond dipper so I have much learning to do.

Music - Aquarium by Kevin MacLeod

But what is it? It's a vorticella - they're single cells. They stick to things with their long stalk and contract any time they sense danger. The thing that looks like a disc spinning around is a crown of tiny hairs - cillia beating up and down creating water currents to drive food towards it.

Vorticella are important - they filter particles out of the water helping keep it clear, By eating tiny bits of food - dead bits of bacteria and rotting stuff that are too small for most things to bother with, they concentrate nutrients into a neat tasty vorticella shaped snack for larger things like crustaceans and small fish.

Mosquito larva

By Rkitko - Own work, CC BY-SA 3.0,

They also - and we're not sure how - stop mosquito larvae growing in water. Vorticella make a glue - a biopolymer to stick their stalks to things, it does something to the larvae. It might block up their pores stopping them from sensing things, it might act a tiny bit like soap - changing the surface tension of the water so the larvae can't breathe at the surface. Either way they could be an important tool in controlling diseases like malaria and dengue.

But why aren't they rotifers?