Adrian writes
I once worked with a chap from Hemel Hempsted, who had worked in the
watercress beds there (used to be able to see them from the train).
He told toe-curling stories of the creatures that used to lurk in among
the watercress... starting with leeches

Nothing gruesome about leeches. They're related to earthworms, and the
ones you are most likely to come across feed on detritus and rotting
vegetation at the bottom of the pond.

--
Kay