Host Size Selection in the Ectoparasite Tachaea chinensis (Isopoda: Corallanidae) under Laboratory Conditions

Tachaea chinensis is a temporary ectoparasite infesting freshwater shrimps and prawns in eastern Asia. This study investigated the host size selection by T. chinensis across common freshwater shrimps under laboratory conditions. A total of 70 isopods were allowed to select between host shrimps of different size and different species in pairwise selection experiments. In treatments involving different sizes of the same host species, T. chinensis tended to select the medium host option in all of the four treatments. Similarly, T. chinensis demonstrated greater preference towards medium host (90%) when provided with mixed host options (medium Palaemon paucidens vs small Neocaridina spp.). However, despite the increase in the infestation proportion on medium Neocaridina spp., the isopod significantly selected the small P. paucidens when provided with a choice between medium Neocaridina spp. and small P. paucidens. In manca stage (1 day after hatch) T. chinensis treatment, the isopods showed no specific preference between large and medium Neocaridina spp. These results suggest that T. chinensis is likely to show size specificity according to the developmental stage, a size specificity that ultimately ensures adequate space for isopods' growth while maintaining a minimum risk of predation.