Host selection and potential predation in the host-parasite interaction between the isopod Tachaea chinensis and freshwater host species.

Tachaea chinensis is an ectoparasite commonly found on diverse ecologically and commercially valuable freshwater shrimps and prawns. Previous studies on this parasite have focused on its distribution and taxonomical identification, while its host preference and/or the potential predation in this host-parasite interaction remained poorly understood. In this study, we investigate the host preference and potential predation of the isopod T. chinensis using manipulative choice and predation experiments under laboratory settings. The preference toward a broad range of host decapods in single-host treatments, indicates a low host specificity, which ultimately aids in the survival of this parasite in the natural environment. Tachaea chinensis responded well to the shrimp Palaemon paucidens when presented with uncommon host species in all three treatments. In host-parasite predation treatments, all the tested P. paucidens shrimp, the prawn Macrobrachium nipponense, and the crayfish Procambarus clarkii were able to consume the isopod-especially the invasive crayfish P. clarkii, which consumed a greater percentage in a considerably shorter time frame (Fisher's exact test, P < 0.01). This study demonstrated for the first time the ability of larger freshwater decapods to prey upon T. chinensis. Despite the large difference in the maximum attainable size of those freshwater species, a high predation pressure by the invasive crayfish on the isopod is anticipated, if they are present in the same environment.

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.

Size Relationship of the Ectoparasite Tachaea chinensis in the Host Shrimp Palaemon paucidens From Shiga Prefecture, Japan.

Monthly sampling of freshwater shrimps infested by the corallanid ectoparasite Tachaea chinensis was conducted at an agricultural canal of the Etsura River system, Shiga Prefecture, Japan, from April 2018 through December 2019. A total of 944 Palaemon paucidens and 279 Neocaridina spp. were collected, and 203 immature T. chinensis were found attached to the carapace of these hosts. Mean prevalence was 23.1% for P. paucidens and 6.4% for Neocaridina spp. Newly hatched T. chinensis ectoparasitizing P. paucidens were detected in August 2018 (minimum body length of parasite 3.2 mm) and appeared continually until June 2019 (maximum body length of 10.3 mm) and were not observed in July. In contrast, T. chinensis on Neocaridina spp. were first detected in August of each year (with minimum body length of 2.0 mm) and were observed until September in 2018 and to November in 2019 (maximum body length of 5.8 mm). Ratios of the body length of T. chinensis to the carapace length of the host shrimp were almost constant (0.8-0.9) with the host species and parasite growth. Although infestation by early-life-stage individuals of T. chinensis was observed on both shrimp species, P. paucidens appeared to be the principal host for individuals larger than 6 mm in body length.