A quantitative study has been made of seven natural epizootics of Polystoma integerrimum in the tadpoles of Rana temporaria. About 4,000 parasites have been counted in about 1,000 tadpoles, in five ponds in one year, and in two in another, besides some additional observations in other ponds.
Wide differences exist between the epizootics, but a general plan can be seen in all. The eggs laid by the adult fluke hatch at two main periods—one at the end of April or beginning of May. and the second about a month later. Some of the larvae which hatch from these eggs (the primary larvae) become neotenic larvae. These lay eggs which hatch in July (secondary larvae).
In some ponds, all tadpoles metamorphose in June. The neotenic cycle in these ponds is therefore abortive, because by the time the secondary larvae hatch, there are no tadpoles. Infestation is secured in these ponds by the presence of primary larvae, particularly those of the second brood. In other ponds, many tadpoles can be found in July or August, and they become infested with secondary larvae, even if they have been relatively free from the parasites up to this date.
There are large losses of parasites throughout the season, and it seems important for the species that some larvae should enter the tadpoles as near to the date of metamorphosis as possible, not only because the gill chamber is not a very safe location, but because there is a high mortality among tadpoles, due to many causes. Late infestation short-circuits tadpole mortality. It is, indeed, suggested that it is late infestation rather than mere multiplication which is the chief function of the neotenic cycle. The life history of the fluke is so efficient that the parasite maintains its existence even though it lays fewer eggs than the host.
The parasites appear to be without deleterious effect on the tadpoles, which are just as large when parasitized as when free from the flukes.
At the early stages of infestation, there is a tendency for the larger tadpoles to have more parasites than the smaller ones, which are often free from the larvae.
Tadpoles are not, in nature, normally infested until well after the stage when they have external gills. Even the rudiments of these have disappeared before infestation begins. Only one tadpole in all the collections was found to be infested while still bearing external gills.
The parasites are not distributed among the tadpoles at random. There is a tendency for a tadpole to be either heavily infested, or else to be without the parasites, intermediate states being relatively rare.
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