Gills from
each host were also obtained and placed in tap water
to search for monogeneans. Monogeneans were fixed
in glycerin ammonium-picrate (GAP) in order to study
their sclerotized structures. After the morphological
evaluation, specimens fixed with GAP were remounted in
Canada balsam following Ergens (1969); some specimens
were fixed in hot formaline 4 %, stained with Gomori’s
trichrome and mounted in Canada balsam to study their
internal organs. Endoparasites were removed to a Petri
dish with saline 0.65 %, prior to fixation. Digeneans and
cestodes were fixed with hot (steaming) 4 % formalin.
Acanthocephalans were maintained at 4 °C for 12 hours in
distilled water, and then fixed in 70 % ethanol. Nematodes
were fixed with hot (steaming) 4 % formalin or 70 %
ethanol. All helminths were processed following standard
procedures. Identification was made using specialized
literature, and representative specimens of all helminth
species were deposited at the Colección Nacional de
Helmintos, Instituto de Biología, Universidad Nacional
Autónoma de México (UNAM), Mexico City (CNHE).