Strange horsetail.

Strange horsetail.

Observation - Strange horsetail. - UK and Ireland. Description: Found this horse tail is a pond. Cannot find a similar thing in my books. Could possibly be introduced. The leaves are unbranched, flat and dark green. The stem is solid, with small holes running through. Any ideas please? Thanks

Found this horse tail is a pond. Cannot find a similar thing in my books. Could possibly be introduced. The leaves are unbranched, flat and dark green. The stem is solid, with small holes running through.

Any ideas please?

Thanks