Alex Hyde / Green hydra (Hydra viridissima) found in a garden pond. Derbyshire, UK. September. Winner of the Hidden Britain Category of the British Wildlife Photographer of the Year Awards (BWPA) Competition 2017...

Albert Lleal / Green Hydra (Hydra viridissima) and Flatworm (Girardia tigrina), Spain

Jan Hamrsky / Freshwater hydra (Hydra viridis) attached to submerged wood, Europe, February, controlled conditions.

Albert Lleal / Green Hydra (Hydra viridissima) colony

Jan Hamrsky / Brown hydra (Hydra oligactis) polyps attached to the stone, Europe, September, controlled conditions

Jan Hamrsky / Brown hydra (Hydra oligactis) digesting the prey enclosed within the body cavity, Europe, September, controlled conditions

Jan Hamrsky / Brown hydra (Hydra oligactis) with two buds on the body wall, extending its tentacles, Europe, September, controlled conditions

Konrad Wothe / Hydra (Hydra vulgaris), Germany

Albert Lleal / Hydra (Hydra sp), native world wide

Heather Angel / Freshwater green hydra (Hydra viridissima) moving over a petri dish. Green colour comes from the symbiotic relationship with green alga Chlorella vulgaris