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Abstract
Factors responsible for interspecific variability in host-specificity were investigated
within 15 genera (including 176 species) of metazoan parasites found in Canadian freshwater
fish. For each species in a genus, the parasite's number of known hosts was determined
from published host-parasite records. The effects of the total number and mean size
of potential hosts (i.e. all fish species belonging to the family or families that
include a parasite's known hosts) on number of hosts of congeneric species were evaluated
using multiple regressions. Since parasite species that have been recorded often tend
to have greater numbers of known hosts than do seldom-recorded parasites, it was necessary
to control for the confounding effect of study intensity. In all parasite genera,
whether from highly specific taxa such as monogeneans or from less host-specific ones,
there was a positive relationship between the number of potential hosts and the number
of known hosts. However, no consistent relationships were observed between the mean
size of potential hosts and number of known hosts. These results suggest that the
availability of suitable host species may have been a key factor limiting the colonization
of new hosts by fish parasites.