| Dick, sorry to be argumentative and anal, but....
It is unlikely that the continued use of low salt levels led to the development of salt-resistance in protozoans like Costia. However, it is likely that Costia (Icthyobodo) species or Costia strains which are naturally salt tolerant eventually found their way into farms which continuously use low levels of salt. Some members of this group are marine or estuarine and the taxonomy is poorly understood.
These little protozoans seem small and primitive, but are much more advanced than bacteria. As such, they are not as plastic as bacteria and evolutionary changes are not likely to happen in the time frame we're talking about.
-steve |