The Acanthocephalan Parasites of Eider Ducks

Because of their wide geographical distribution through arctic and subarctic regions of the northern hemisphere, the eider ducks offer some unusual opportunities for the study .of distribution and hostrelations of their parasites. When the Acanthocephala from eider ducks were first studied, the fauna of each continent was regarded as distinct, because it had been reasonably well established that the majority of species of these parasites encounter~d in the United States were distinct from those found in central Europe. This belief failed to take into account the fact that in the arctic the usual distributional bartl:ers to both the definitive and the intermediate hosts are lacking. Recent studies -in northern Europe, especially by Lundstrom (1941, 1942), and Dew collections. on the American continent have added significantly to an understanding, of .:this fauna. The new evidencesmake it seem apparent that for water\u27 birds of the arctic region the acanthocephalan parasites are very widely distrilmted -geographically. This statement does not contradict the generalization that migratory birds have little influence on inter-continental distribution of acanthocephalan parasites. It seems more probable that suitable intermediate hosts for Acanthocephala are widely dispersed in the arctic and subarctic regions and that local bird populations, even though they may have undergone subspecific or specific differentiation, are by food habits and habitat exposed to infection by identical species of parasites on the two continents. This condition of birds under ecologically similar conditions is a direct parallel to that described for fishes of the same regions where numerous species on the two continents serve as definitive hosts for Neoechinorhynchus rutili (Mueller) as demonstrated by Van Cleave and Lynch (1950)