Nick Upton / Parrot's-feather / Parrot feather milfoil (Myriophyllum aquaticum) growing in a heathland pool, Dorset, UK. August.

Nick Upton / Parrot's-feather / Parrot feather milfoil (Myriophyllum aquaticum) growing in a heathland pool, close up, Dorset, UK. August.

Adrian Davies / Parrot feather (Myriophyllum aquaticum), a highly invasive plant, growing in village pond, East Prawle, Devon, UK. July.

Adrian Davies / Parrot feather (Myriophyllum aquaticum), a highly invasive plant, growing in village pond, East Prawle, Devon, UK. July.

Adrian Davies / Parrot feather (Myriophyllum aquaticum), a highly invasive plant, growing in village pond, East Prawle, Devon, UK. July.

Ugo Mellone / Hooded grebe (Podiceps gallardoi) pair on lake, interchanging Andean milfoil (Myriophyllum quitense) during courtship, Patagonia National Park, Santa Cruz, Argentina. Cropped.

Ugo Mellone / Andean milfoil (Myriophyllum quitense) on lake surface, Strobel plateau, Santa Cruz, Patagonia, Argentina.

Ugo Mellone / Hooded grebe (Podiceps gallardoi) pair on lake, interchanging Andean milfoil (Myriophyllum quitense) during courtship, Patagonia National Park, Santa Cruz, Argentina. Cropped.

Franco Banfi / Tench (Tinca tinca) during spawning period amongst European watermilfoil (Myriophyllum spicatum), Sion, Valais canton, southwestern Switzerland

Franco Banfi / Tench (Tinca tinca) during spawning period amongst European watermilfoil (Myriophyllum spicatum), Sion, Valais canton, southwestern Switzerland

Franco Banfi / Northern pike (Esox lucius) amongst Europe Watermilfoil (Myriophyllum spicatum), Sion, Valais canton, southwestern Switzerland

Franco Banfi / Tench (Tinca tinca) during spawning period amongst European watermilfoil (Myriophyllum spicatum), Sion, Valais canton, southwestern Switzerland