On the face of it, this should pose a big problem for crucian carp. They live in ponds and lakes in northern Europe and Asia that freeze over in winter, so the fish have to survive for months without oxygen until the ice melts in spring.

But the carp – and their close relative the goldfish – have developed a workaround. When they metabolise carbohydrates anaerobically, the end product is not lactic acid but alcohol, which is easier to remove from their bodies.