Wednesday, March 16, 2011

Death of the Tadpole

Common Frog Tadpoles, Rana temporaria
It is the time of year for spawning by the Common Frog, Rana temporaria. Through out Ireland eggs are being laid that will become frogs in about 16 weeks time. However, making its way there is a tough journey for R. temporaria. Many are taken, as food by birds and large fish while some, like a large number of tadpoles I spotted recently, may not even make it that far. I discovered a large number of recently hatched (only about 7 days old) R. temporaria tadpoles in a large amount of mucus that must have been the remains of the egg casings alongside a marsh pond.
Dead Tadpole and Egg-Mass
The tadpoles were a good two feet away from the water and may have been laid during recent heavy rains. However there has been little such rain in the past few days that must have lead to water levels dropping, stranding the egg masses. When I discovered them, ice was present on their surface from the previous night, probably speeding their death.
Common Frog Tadpoles, Rana temporaria
Tadpoles of R. temporaria are quite resilient, so it was a shame to find so many of them ending up like this. Loman (1) has shown that stress on tadpoles, such as drying of habitats leads to an early metamorphosis. Such an adaptive response as this means the stresses on the tadpoles must have been severe.
Common Frog Tadpoles, Rana temporaria
In Ireland R. temporaria was considered a recent introduction, possible from Norman time c. 1000 years ago (2). This was due to the lack of archaeological evidence to support its arrival by natural colonisation. However, a phylogeographic study of R. temporaria in Europe published in 2009 (3) provided evidence that the Common Frog is native to Ireland, having survived the most recent glaciation periods to affect the country, possibly in a refuge in the South West of the country. Many of the frog haplotypes from Ireland were monophyletic groups that are unique to the country. Other groups were found that are more closely related to those found in the west of continental Europe and it is thought that these were introduced by human actions (or possibly across a land bridge with Britain). In fact Smith (4) states that in 1696 a Fellow of Trinity College Dublin moving common frogs from England to Dublin.

  1.   Loman 1999, Amphibia-Reptilia 20 pp. 421-430
  2.   Sterry 2004, Collins Complete Guide to Irish Wildlife p. 94
  3.   Teacher et al. 2009, Heredity 102 pp. 490–496
  4.   Smith 1964, The British Amphibians and Reptiles 3rd edn.

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