Goldfish were experimentally infected with different population densities of the acanthocephalanPomphorhynchus laevis, and maintained at different temperatures. The density of infection had no effect upon the establishment of the parasite, but a 12°C rise in water temperature reduced the recovery after one week by 30%. Thereafter there was a continuous loss of parasites during the course of infection. The rate of this loss was independent of worm burden and temperature, but increased under conditions of host starvation. Both male and female parasites attached themselves in the same region of the alimentary tract, with a mean position about 19% along its length. They remained in that region throughout the course of infection, and the population did not migrate down the intestine with increasing age. In starved fish, the region of attachment was nearer the oesophagus, at low temperatures nearer the rectum, and when crowded the occupied region extended anteriorly. Male worms were unable to establish as easily as females, but once established survived better, and the sex ratio after two weeks was in favour of males. The results are compared with the results of studies on other acanthocephalans and on fish-cestode systems. It is recognized that although the establishment of fish acanthocephala is affected by temperature to a lesser extent than that of some fish cestodes, temperature in conjunction with changes in fish diet is a major control upon the flow of parasites through acanthocephalan-fish parasite systems. The significance of the results to the interpretation of data based on field observations is discussed.
