Cryptocaryon is only known to reproduce asexually, so if that's really the case, inbreeding and lack of genetic diversity isn't the issue. It is certainly possible that conjugation does occur but has just never been observed. Burgess and Matthews believed this was the case and that there was some unknown factor in their setup that kept
Cryptocaryon from getting "in the mood."
b) some acquisition of immunity by available hosts, maybe specifically TO that strain.
They were infecting the fish with 7 different strains of ich, none of which were native to the area the mullet were collected from, so they shouldn't have had any immunity from previous exposures. Indeed, they were easily infected in the early experiments, indicating they had little to no native immunity to most of the strains. Also, newly caught fish were used for each cycle of infection, so they should have never had a chance to develop immunity during the course of the experiments.
If they are dividing, would this erode the telomeres and thus expose this often-replicating parasite to increased genetic damage, ie, the strain 'ages'.
This would pose an insurmountable obstacle for a purely asexual species. There would be no way for them to hit the reset button on senescence and the whole species would go extinct after only a few generations. Most clonal species have telomerase that keeps them from aging as they divide.
but the suggestion that the surviving fish population has to be without new fish for 11 months to achieve this...is at least an improvement in method for somebody whose habit has been to replace every lost fish immediately.
FWIW Burgess and Matthews kept their lines alive for up to 34 cycles. Given that it can take up to 10 weeks for a single cycle, that means if their loss of viability is typical, it could take as long as
6.5 years without any new introductions before the ich already present in a system is nonviable! Even at the average cycle length of 1-2 weeks, you're looking at 9-18 months before the risk is over.
