chapter 4 



PARASITES, PREDATORS AND SOME PROBLEMS 

 OF ABUNDANCE 



During the work described in the preceding pages, a watch was kept 

 for any signs of disease, for the pathology of wild animals is very httle 

 known, and any information, however slight, may be of value. 

 Although about a thousand tadpoles were examined, only one was 

 found which could definitely be described as moribund from 

 disease. Not one dead tadpole was found. The diseased tadpole had 

 swellings resembling blisters. This may perhaps have been an infesta- 

 tion by a species of the trematode Euryhelmis, for an American species 

 has been described (Ameel, 1938) as having cercariae that penetrate the 

 skin of tadpoles, causing blisters, the cysts being later found in the 

 swollen areas. The defmitive host is the mink, which of course does 

 not occur in the area, but European species infest polecats (Joyeux 

 et al, 1934) and are common in tadpoles as the intermediate hosts. 

 Polecats are almost certainly absent from the area, but ferrets may 

 very possibly have been taken along these banks and ditches. 



Some exceptionally small tadpoles were examined for blood 

 parasites, without result. Whether this short record of disease in 

 tadpoles indicates that they are usually healthy is doubtful, for dead 

 animals in a pond are soon disposed of by the other animals, including 

 other tadpoles. It is quite common to fmd the labial teeth of tadpoles in 

 the gut. I am uncertain whether in some cases these may not be the 

 tadpole's own teeth, swallowed when they become detached, but on 

 one occasion I found a tadpole's beak and these are certainly not 

 detachable. This gap in information is most regrettable, but quite the 

 usual thing in ecology {see Lack, 1954, pp. 152 and 167). 



The tadpole carries many parasites that do not seem to cause disease. 

 Among the Protozoa, large numbers of Opalina are always found in 

 the intestines, and there is usually Nyctotherus as well, but Wenyon 

 (1926) does not consider these as pathogens. 



Metazoan parasites are common. For example, small trematodes 

 occurred in the intestines of tadpoles from Dagger Lane, but not 

 elsewhere, and in 1947 there were nematodes as well. Nematodes 



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