Freshwater Triclads (Platyhelminthes, Turbellaria) and Continental Drift

Abstract

THERE is no fossil record of the freshwater Tricladida but they are particularly suitable organisms for zoogeographical studies because of their low vagility. Their dispersal powers have been reviewed by several workers^1–3 and the general absence of resistant eggs or stages, and the fragility of adults have led to the conclusion that the usual agencies of intercontinental dispersal, such as wind, driftwood and birds' feet, have played little or no part in the dispersal of these organisms. Further, the differences in the triclad faunas of the freshwaters on either side of the English Channel, the Menai Straits (North Wales), Skagerrak and Kattegat³, and the Gulf of St Lawrence (unpublished data) may be taken as evidence of the efficacy of even narrow sea passages as barriers to the dispersal of members of the group. In fact, there are only two authenticated cases of passive dispersal in freshwater triclads. These are the introduction of Dugesia tigrina into Europe from North America⁴, and the recent introduction of D. lugubris into North America from Europe⁵. Both can unequivocally be attributed to the activities of modern man. It is also noteworthy that the three temporary water forms which come closest to possessing resistant stages, Fonticola velata, F. vernalis and Hymanella retenuova, all have a fairly restricted distribution in the north-eastern United States⁶. It may therefore be concluded that freshwater triclads are certainly of low vagility and that present day taxonomic and distributional data may be a good guide to their past history.