The pool frog (Pelophylax lessonae) is a species of frog in the family Ranidae. It is found in and around shallow, stagnant bodies of water across much of Europe, from France in the west to Russia in the east. Active in such areas from March to October, it disperses from the water to hibernate in woodland soil during the winter, and can tolerate being frozen overnight while hibernating. A close relative of the marsh frog, the two species can interbreed to produce a hybrid called the edible frog, which maintains its own population by breeding with one of its parental species. The pool frog is very similar in appearance to these related species, but can be distinguished based on vocal sac color, snout shape, leg length, the shape of a hardened lump on the foot, and skeletal features.
Active during both day and night, the adult pool frog is a predator that feeds mainly on invertebrates as an adult, but begins its life as a tadpole with a herbivorous diet. The breeding season is in early summer, when males use vocalisations to ward one another off and attract females. The frogs are sensitive to changes in temperature, which determines their reproductive behaviour and calls. Mating and egg-laying occur in water, where the tadpoles develop until they metamorphose and emerge onto land in late summer. At all life stages, pool frogs are hunted by a wide range of predators such as fish, birds, snakes and otters, and are parasitized by trematode and nematode worms. Tadpole development can vary based on temperature, pH, chemical exposure and the types of predators present.
Furthermore, P. l. lessonae can be split into two genetically distinct lineages that form a northern and southern clade respectively. While pool frogs as a whole are not considered to be endangered globally, populations in some countries have declined very severely. Northern clade populations are now considered critically endangered in Norway and were once wiped out from the UK, though they have since been reintroduced to the latter country.
