Abstract
Tachaea chinensis Thielemann, 1910 is a species of ectoparasitic isopod that infests freshwater shrimps and prawns. Individuals are often found attached to the side of their host shrimp's carapace. Despite the limited space for attachment there, records indicate that several isopods can infest a single host shrimp simultaneously. There are limited data, however, on the prevalence of such occurrences and their potential effect on host survival. Here, we report on the occurrence of multiple infestation by T. chinensis on the freshwater shrimp Palaemon paucidens De Haan, 1844 in Izumo, Shimane Prefecture, Japan, during April and May of 2018 and 2019. The prevalence of infested shrimp was higher (80.0-90.2%) at St. 1, a semi-closed spillway that alternates between continuous water flow and isolated stagnancy, than at St. 2 (45.0-55.0%), an open stream with year-round continuous water flow. The number of parasites per host was zero to nine at St.1 and zero to three at St. 2. A significant positive correlation was observed between the prevalence of isopods on shrimp and the mean number of these parasites per host. In a laboratory experiment in which individual shrimp of three size groups were reared for seven days with different numbers of isopods from three matching size groups, the survival rate of hosts was 100% with one T. chinensis, 90-100% with two, 50-80% with four, and 10-40% with eight. These results suggest that the presence of two or more parasites negatively affects the hardiness of the host, increasing shrimp mortality and potentially reducing their density in nature, especially in semi-closed habitats. This has obvious implications for shrimp farms, which should strive to prevent the entry of this isopod into their water systems.