The role of eye-like spots of parasitic leeches (Hirudinea, Piscicolidae) in searching for fish-hosts

Piscicola geometra (L.), Piscicola respirans Troschel, 1850, and Caspiobdella fadejewi (Epshtein, 1968) find their hosts (fishes) by reacting to their shadow. Eye-like spots whose number ranges from 8 to 16 depending on a species, located on the posterior sucker are the main receptors perceiving the decrease in light intensity and thus enabling an immedaite reaction to the host's shadow. They are distributed on the perimeter of the sucker, are absent only from the section that is constantly shaded by the leech body. Experiments with a simulated shadow reveled considerable differences in the threshold light doses between the studied species: P. geometra - 12 lux, P. resprans - 4 lux and C. fadejewi - 2.5 lux. Below these values the leeches do not perceive the decrease in light intensity and do not make host-searching movements. The smallest decreases in light intensity, at which the leeches still make host-searching movement, are: P. geometra - 5 lux, P. resprans - 2 lux, and C. fadejewi- 1.5 lux. In case of P. respirans and C. fadejewi such small decreases in light intensity (2 - 1.5 lux) may be caused only by small fishes which, according to the results of the experiment, should not be infested. This is confirmed by field observations - small fishes are mostly free from parasites. Large fishes (casting a large shadow) have much more numerous leeches.