Gyrodactylus is and example of a Monogenia from the subclass mono-opisthocotylea. This worm is an ectoparasite attaching itself to the scales on the bodies of fresh water fish. It is unusual compared with other species in that it is larviperous.
The adult worm measures 0.5 to 0.8 mm long. The opisthapter has 16 marginal hooks and 2 large hamuli. This parasite is of particular importance in aquaculture where large populations of fish are confined to relatively small volumes of water, forcing more frequent contact between fish than would normally occur in a natural habitat. Therefore, rapid increases in populations of this worm can occur in this controlled environment. These parasites have a wide host range infecting small tropical fish like guppies through to large commercial fish such as trout.
One of the most interesting features of this parasite, an one which also makes it so devastating, is its reproductive cycle. The entire reproductive cycle can occur on the host. This is because the larvae develop within the uterus of the adult worm. However, in addition to this, within the developing larva there are clusters of embryonic cells within which a second larva is already developing and within it a 3rd and within it a 4th larva is developing. This is known as polyembryony, the result of which produces 4 individuals from a single zygote. As you can imagine this leads to rapid increase in parasite numbers. When fish come close together the adult worms can easily move from one fish to another thus spreading the infection. In addition, the worms can also survive for short periods in the absence of the host.
These parasites have been used in a model system to study the effects of host-parasite population interactions and how parasites impact on host population density. The authors found that in long term experiments, if one fish was placed in a tank with 49 infected fish that there was a gradual increase in the percentage of fish infected followed by a decrease, eventually to zero, resulting in about a 20 percent of the fish surviving but total extinction of the parasite population. If, on the other hand, the experiment was repeated, but this time 2 or more new fish were introduced into the population every two weeks, then the fish population remained relatively stable with oscillations in the parasite population, but the parasite surviving.
These and other experiments led the authors to conclude a number of things:
However, it was also observed that in a closed system, the parasite popultion went to extinction, suggesting that acquired resistance to infection by the host is an important factor determining epidemic behaviour and parasite survival.In addition, it highlighted the importance of immigration of new naive hosts into the population which helped to maintain the parasite numbers.