Signal crayfish – invader, cannibal, survivor

Appletreewick, Yorkshire Dales Its body is as dark as the river at its deepest, where the peat-stained water turns as black as molasses

The heatwave hits its stride before breakfast, building to a dog day intensity that will relent only with the last red moments of the sunset. For the long hours between, an endless afternoon, the light ceases to move, training its intensity on the elderflower, oxeye daisies and buttercups of Wharfedale until their colours take on the bleach-brightness that signals high summer in England.

The weather brings people out of hibernation, and into encounters with unfamiliar forms of life. "Look at the size of that crayfish!" The woman paddling in the untypically warm river Wharfe near Appletreewick points near her feet, causing half a dozen swimmers to coalesce around the spectacle. Children express something between amazement and open-mouthed horror.