Pool frog

The pool frog was first recognised to be different from other frogs by Italian herpetologist Lorenzo Camerano, who published a scientific description of it in 1882. However, Camerano believed that it was a variant of the edible frog (which had the scientific name Rana esculenta at the time) rather than a separate species, and gave it the scientific name Rana esculenta var. lessonae, naming it after Italian zoologist Michele Lessona.^[4] Two years later, Camerano would adopt the concept of subspecies, and in an 1884 publication he refers to the pool frog as a subspecies by the name Rana esculenta lessonae.^[5] Though Camerano studied several pool frog specimens, he did not designated any one of them as a holotype. Therefore, one of these specimens (a female with the specimen number MZUT An 718 kept in the Turin Museum of Natural History) was later designated as a lectotype in 1986.^[6]

The possibility of the pool frog representing a separate species from the edible frog was first recognised in 1966 by Polish biologist Leszek Berger, who proposed this based on ecological and biometric data, though he did not decisively conclude so as he believed doing so was premature.^[7] In 1992, French zoologist Alain Dubois divided the genus Rana into 33 subgenera, with the pool frog being placed in the subgenus Pelophylax under the name Rana (Pelophylax) lessonae.^[8] A study published in 2005 further divided the species placed in the genus Rana at the time into several different genera, with those of the subgenus Pelophylax being assigned to the genus Hylarana, thus the pool frog was considered to be part of this genus under the name Hylarana lessonae.^[9] In 2006, another publication on amphibian taxonomy elevated the rank of Pelophylax from subgenus to genus, a revision which is further supported by a study published the following year, so the scientific name of the pool frog was recombined once again into Pelophylax lessonae.^[10][11]

In addition, some pool frog specimens were formerly thought to represent distinct, separate forms. Hungarian general Géza Fejérváry established the name Rana esculenta var. bolkayi in 1909 for a type of frog found along the Rhône river, but does not designate a holotype specimen.^[12] Similary, Yugoslav biologist Stanko Karaman erected a new subspecies of pool frog given the name Rana lessonae pannonica in 1948 without specifying a type specimen.^[13] Both of these names were declared as junior synonyms of Rana lessonae in 1994.^[14]

The pool frog is a small frog which rarely grows to more than 8 cm (3.1 in) long, although females can grow up to 9 cm (3.5 in). Males are typically around 5 cm (2.0 in) long, while females are around 6–6.5 cm (2.4–2.6 in) long. These frogs are brown or green, with dark blotches along their backs, a pair of ridges running from each eye and a cream or yellow stripe down the middle of the frog's back. The vocal sacs on the male are cream or even white.^[18]

The pool frog is found across most of central Europe from the west coast of northern France to the Western part of Russia. There are also small populations of pool frogs in the United Kingdom, Spain, Sweden and Norway.^[1] Pool frogs were previously thought to be a non-native species in the UK, but studies have shown that English pool frogs are related to the Swedish and Norwegian populations.^[18]

The pool frog is found in damp areas with dense vegetation, or in calm, slow flowing rivers, ponds, bogs or marshes.^[18]

According to Amphibiaweb populations of this frog survive in urban areas and even fisheries. The creation of new ponds and other bodies of water leads to increased dispersal and a growth in the population of these frogs.^[19]

• 
  1897 sciagraph (X-ray photograph) of P. lessonae (then Rana Esculenta), from James Green & James H. Gardiner's "Sciagraphs of British Batrachians and Reptiles"
• 
  Pelophylax lessonae

Pelophylax lessonae was one of only four amphibian species recognised by the UK government as protected under its Biodiversity Action Plan, however on 2007 this was revisited,^[20] adding all of the UK's extant, native herpetofauna to this plan; with a notable exception of the Aesculapian snake do to its complicated re-establishment. The reasons for declining populations are decreased pond habitat from human encroachment and air pollution leading to over-nitrification of pond waters.

The pool frog has not always been recognised as a native British species. Part of the reason for this is that specimens are known to have been introduced from southern Europe (though not from Scandinavia). However research has now shown that the potentially native UK pool frogs are closely related to Scandinavian frogs, not to frogs from further south. A native origin is thus most likely.^[21]

The Herpetological Conservation Trust website states that "The Pool Frog is a European frog and was formerly recorded from two sites in East Anglia although it was lost from one of these in the middle of the 19th century. It was presumed extinct in the wild at the last remaining site by 1995. A single individual known from this population survived in captivity until 1999. Other populations have become established in the UK and it is known that some of these included individuals of British origin in their founding stock."

An English Nature reintroduction project is underway in Breckland, where pool frogs were introduced to a single site in 2005.

There are at least two recognised sub-populations of pool frog, the northern and the southern. The southern clade is more widespread whereas the northern clade is endemic to the British Isles, Scandinavia and possibly Estonia.^[22]

The edible frog Pelophylax kl. esculentus is a hybridogenetic hybrid of the pool frog Pelophylax lessonae and the marsh frog P. ridibundus. Its populations are maintained however through other crossings by hybridogenesis.^[23]