| Summary: | The relation between the monogenetic fluke Gyrodactylus alexanderi Mizelle and Kritsky, 1967, and its fish host Gasterosteus aculeatus was examined. Factors influencing the number of flukes on isolated fish at 15°C weremeasured, and the data combined to produce a computer model simulating a Gyrodactylus population. When tested in a multiple-host situation, the model predicted results that were similar
to those observed experimentally.
Experimental populations of 20 flukes on individual fish increased for two weeks to a mean of 61 then decreased over two further weeks to a mean of 9. Fish that lost their fluke infestations were refractory to further infestation for about three weeks. A humoral immune response
was not thought to be involved because of the relatively rapid rise and decay of the resistance, and because the course of the infestation
on previously exposed fish did not differ from that on fish that had never been exposed. Handling of the fish was also shown not to affect the pattern.
Control of the numbers of Gyrodactylus appeared to be a mechanism by which fish shed the flukes on sheets of 'cuticle' formed by the epidermis.
Hooklets of the flukes were attached to the cuticle and their purchase on the fish was lost when the cuticle was shed. Although all fish, whether infested or uninfested, shed cuticle every 1-2 days, that shed by the heavily infested fish \vas more opaque and more electron-dense, perhaps in response to the feeding activities of the flukes. The change is suggested to be the be the basis for the increased 'shedding efficiency' of the fish which in turn was shown to be related to the number of flukes present on the fish five days earlier.
The mean life expectancy of flukes on fish was 16 days at 15°C. Flukes reproduced by giving birth to two daughters, one after 1.6 days and the other after 6.9 days. The rate of reproduction in fluke populations
that were increasing was similar to that in decreasing populations
on fish recovering from an infestation. The mean life expectancy of flukes away from fish was 1.8 days. It was estimated that of the flukes shed, less than half of the survivors reattached during any 24 hour period.
Other factors found capable of influencing population size were kept constant or minimised during the experiments and were not incorporated
in the model. These included: predation on flukes by fish 15-30 mm in length; mortality of fish due to Gyrodactylus; stress on the fish; density of the fish population; and temperature. === Science, Faculty of === Zoology, Department of === Graduate
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