As you’ll recall if you read my recent article on Yellow-bellied toads Bombina variegata you'll know that I recently wandered about the Romanian countryside, hunting for frogs. You can never have too many frogs and, these days – what with the global amphibian crisis and all – you typically don’t. The good news is that Bombina was far from the only anuran* I got to observe and photograph in Transylvania. While exploring tributaries of the Valea Cheii River and camping by the water in the Sibisel Valley (this was a palaeontological expedition, so we were actually looking for other stuff), surely I encountered numerous Western Palaearctic water frogs, right? Yes, surely indeed.

* Anura = frogs and toads.

Water frogs (aka ‘green frogs’ or Western Palaearctic water frogs: the species of Pelophylax) were heard calling at just about all of the rivers and river-side sites we visited. Initially I only got a few brief glimpses. These frogs were so good at hiding that I never got a prolonged close look, nor was I ever able to catch or photograph one. The qualities of their often noisy calls make them annoyingly difficult to pinpoint. One of them was muddy brown, fairly plain (without obvious marbling or striping on its legs) and with a light brown vertebral stripe. That’s kinda weird, since it doesn’t really match any of the water frog species of the region: the vertebral stripe suggests that it was a Pool frog P. lessonae.

Leg length compared in (left to right) Marsh frog, Edible frog and Pool frog. Illustration by Bas Teunis, from Beebee & Griffiths (2000).

Water frogs/green frogs are ranids – that is, ‘true frogs’ of the sort familiar to many people (especially those of the Northern Hemisphere). Like so many other ranid clades, they’ve long been included within an enormous, super-inclusive version of Rana. It’s increasingly thought that Rana of tradition is (like Bufo) such an unwieldy, paraphyletic monster that it should be split up into numerous separate ‘genera’, though of course not everyone agrees with this approach. The task of splitting Rana sensu lato into bits was adopted enthusiastically by Frost et al. (2006), a study that has upset and angered at least a few people who work on amphibian systematics and phylogeny: for responses see Hillis (2007) and Wiens (2007).

I don’t have time right now to discuss this topic (I will do so at some stage), but the water frog clade that includes the European species I’m discussing here groups well away from Rana and its close kin (Lithobates, Babina, Odorrana) in most recent phylogenies (e.g., Chen et al. 2005, Frost et al. 2006, Stuart 2008, Kurabayashi et al. 2010). The name Pelophylax Fitzinger, 1843 – long recognised as a ‘subgenus’ within the super-inclusive version of Rana – applies to the water frogs and is the name I’ll use for them here.

Left lateral view of a Romanian water frog that I'm cautiously identifying as a Marsh frog. Note the attractive 'marbling' on the thigh.

Anyway, I did eventually succeed in capturing and photographing several water frogs. Water frogs are often highly individualistic – that is, individuals frequently differ very obviously from one another in colour and markings. Lacking vertebral stripes, with whitish (rather than yellowish) thighs, and proportionally long legs, I reckon that the individuals shown here were Marsh frogs P. ridibundus but I’m not entirely sure. It is possible to differentiate Marsh frogs from Pool frogs by way of the proportionally smaller metatarsal tubercles in the former, but I didn’t know this at the time.

This frog lacks a pale vertebral stripe: this and several other features led to provisional identification as Marsh frog. However, the yellow on the posterior part of the thigh suggests Pool frog!

The Pool frog and Marsh frog (despite their names they’re not restricted to pools or marshes) occur sympatrically across much of Europe. The Marsh frog – so named as it was first associated with Romney Marsh in south-east England – is the larger one, and in fact with an SVL of 15 and even 17 cm it’s Europe’s largest (native*) frog. It’s not only long-legged and large, but also robust. Warm lowlands with large, slow-flowing rivers are typical habitats. The Pool frog is not only (usually) smaller than the Marsh frog, it’s also (usually) greener, and it also (usually) has a pale vertebral stripe and (usually) has paler dorsolateral folds. But populations are highly variable, and some Pool frogs (especially so-called ‘northern clade’ populations) are often very brown. Both Marsh and Pool frogs seem to have increased in average size across at least part of their range, apparently as a direct response to climatic warming (Tryjanowski et al. 2006).

* The American bullfrog Lithobates catesbeianus occurs as an alien in parts of Italy (Lombardy, Mantua and Pavia).

Confusing things further is that Marsh and Pool frogs hybridise, producing an animal termed the Edible frog. Long regarded as a distinct species and known as P. esculentus, it’s a hybridogenetic hybrid: that is, if Edible frogs backcross with either of their parent species, they make more Edible frogs. And when Edible frogs mate with other Edible frogs, they also (usually) produce more Edible frogs. Edible frogs exist across much of continental Europe (sometimes accounting for a huge proportion of the local water frog population), especially in disturbed areas like gravel pits.

A phylogeny for the Pelophylax frogs, from Lymberakis et al. (2007). Note the distance between the Marsh frog (P. ridibundus) and Pool frog (P. lessonae).

Genetic studies have recently shown that numerous water frogs found throughout Europe, northern Africa and the Middle East – often assumed to be Edible frogs – are actually highly distinct relative to Pool, Marsh and Edible frogs, and about 12 species are now recognised for this region. Some of these have only been recently named (e.g., Epirus water frog P. epeirotica Schneider et al., 1984, Albanian water frog P. shqiperica Hotz et al., 1987, Cretan frog P. cretensis Beerli et al., 1994, Karpathos frog P. cerigensis Beerli et al., 1994) while others have been resurrected from synonymy (e.g., Perez’s frog P. perezi Seoane, 1885). Phylogenies indicate that Marsh and Pool frogs aren’t especially close relatives within the clade: an assemblage of Aegean and Middle Eastern species are apparently closer to the Marsh frog than it is to the Pool frog (Lymberakiset al. 2007).

Extinction and reintroduction

Partial right ilium of Pelophylax cf. lessonae from Middle Saxon site at Gosberton, Lincolnshire, UK. From Gleed-Owen (2000).

The identification and history of English water frog populations has always been an area of particular interest. The Marsh frogs long established in south-east England are fairly securely identified as introductions (they have what’s known as a trail of introduction), but the same isn’t true of all English Pool frog populations. It turned out that a population from Norfolk were very similar genetically, morphologically, acoustically and in habitat choice to the Pool frogs of Norway and Sweden. They weren’t aliens introduced from southern Europe, as long assumed, but – most likely – ‘neglected natives’ belonging to a cold-adapted, so-called ‘northern clade’ of the species (Zeisset & Beebee 2001, Snell et al. 2005). What appears to be a Pool frog ilium, discovered in Middle Saxon deposits (c. 600-950 AD) in Lincolnshire, also seems to confirm native status for the species (Gleed-Owen 2000).

Unfortunately, this was all realised far too late. Without protection, the population dwindled into extinction, with the last known individual dying in captivity in 1999. A reintroduction programme – involving ‘northern clade’ animals from Sweden – has been underway since 2005, however, and the animals involved have now bred.

Hmm, I seem to have written rather more than originally intended. Oh well. More soon. For previous Tet Zoo articles on European anurans, see…

Refs - -

Beebee, T. & Griffiths, R. 2000. Amphibians and Reptiles. HarperCollins, London.

Chen, L., Murphy, R. W., Lathrop, A., Ngo, A., Orlov, N. L., Ho, C. T. & Somorhai, L. L. M. 2005. Taxonomic chaos in Asian ranid frogs: an initial phylogenetic resolution. Herpetological Journal 15, 231-243.

Frost, D. R., Grant, T., Faivovich, J., Bain, R. H., Haas, A., Haddad, C. F. B., De S, R. O., Channing, A., Wilkinson, M., Donnellan, S. C., Raxworthy, C. J., Campbell, J. A., Blotto, B. L., Moler, P., Drewes, R. C., Nussbaum, R. A., Lynch, J. D., Green, D. M. & Wheeler, W. C. 2006. The amphibian tree of life. Bulletin of the American Museum of Natural History 297, 1-370.

Gleed-Owen, C. P. 2000. Subfossil records of Rana cf. lessonae, Rana arvalis and Rana cf. dalmatina from Middle Saxon (c. 600-950 AD) deposits in eastern England: evidence for native status. Amphibia-Reptilia 21, 57-65.

Hillis, D. M. 2007. Constraints in naming parts of the Tree of Life. Molecular Phylogenetics and Evolution 42, 331-338.

Lymberakis, P., Poulakakis, N., Manthalou, G., Tsigenopoulos, C. S., Magoulas, A. & Mylonas, M. 2007. Mitochondrial phylogeography of Rana (Pelophylax) populations in the Eastern Mediterranean region. Molecular Phylogenetics and Evolution 44, 115-125.

Kurabayashi, A., Yoshikawa, N., Sato, N., Hayashi, Y., Oumi, S., Fujii, T. & Sumida, M. 2010. Complete mitochondrial DNA sequence of the endangered frog Odorrana ishikawae (family Ranidae) and unexpected diversity of mt gene arrangements in ranids. Molecular Phylogenetics and Evolution 56, 543-553.

Snell, C., Tetteh, J. & Evans, I. H. 2005. Phylogeography of the Pool frog (Rana lessonae Camerano) in Europe: evidence for native status in Great Britain and for an unusual postglacial colonization route. Biological Journal of the Linnean Society 85, 41-51.

Stuart, B. L. 2008. The phylogenetic problem of Huia (Amphibia: Ranidae). Molecular Phylogenetics and Evolution 46, 49-60.

Tryjanowski, P., Sparks, T., Rybacki, M. & Berger, L. 2006. Is body size of the water frog Rana esculenta complex responding to climate change? Naturwissenschaften 93, 110-113.

Wiens, J. J. 2007. Book review: the amphibian tree of life. Quarterly Review of Biology 82, 55-56.

Zeisset, I. & Beebee, T. J. C. 2001. Determination of biogeographical range: an application of molecular phylogeography to the European pool frog Rana lessonae. Proceedings of the Royal Society of London B 268, 933-938.