In laboratory experiments, the swimming behaviour of the ectoparasite
Argulus foliaceus and its infection rates on juvenile
perch (Perca fluviatilis) and roach (Rutilus rutilus)
were examined. The highest infection rate and a preference for perch
juveniles were obtained in darkness, the lowest infection rate and a lack
of
preference in the light, when aquaria with glass
walls (high reflectivity) were used. In the light, when aquaria were lined
with black plastic (low reflectivity) an intermediate
level of infection for perch and the highest for roach was recorded. Under
such conditions roach were significantly more
heavily infected than perch; an attack rate 4 times greater was recorded
for
brighter (more reflective) roach juveniles than
for perch. Within the aquaria with a low reflective interior parasites
swam
4·4 times slower and were observed predominantly in the central
area, while in the highly reflective aquaria fast swimming
A. foliaceus were recorded mainly near
the walls. The primary role of visual stimuli for the host search behaviour
of
A. foliaceus in the light is suggested. Parasites
can effectively use such stimuli only in the low reflective surroundings.
Highly reflective glass aquarium walls produce
numerous secondary local light sources, which cause fast, erratic parasite
movements and prevent the efficient location of potential hosts.