Histology
The histological technique was as follows: whole clam soft tissue
was removed from the shell, transverse section was cut in
the middle of the body to allow fixative to penetrate tissues
and tissue was preserved in Davidson’s fixative solution for
24 h, dehydrated with graded ethanol, cleared with xylene,
embedded in paraffin wax and sliced to 5 lm transverse sections
at different organs using KD-2258 rotary microtome
and stained with Harris’ Hematoxylin–eosin and examined under
light microscope. Photos were taken using microscopeequipped
digital camera.
Prevalence and infection intensity of trematode larvae
The infection prevalence (% proportion of infected individuals
to the total number examined) was calculated by dividing
number of clams with parasite infection or pathology prevalence
by the number of clam population examined and multiplying
the product by 100.
The infection intensities (% area of transverse tissue section
occupied by parasites, n = 60+ slides, at least 2 slides were
examined for each of 30 samples examined per site) was assigned
semi quantitative scales based on the extensiveness of
the affected area according to Kim and Powell (2007) by giving
rank numbers of 0–4 according to the area occupied by parasites
as follows: 0 = no infection; 1 = light infection intensity