Manipulation of the shore crab Metopograpsus thukuhar by the parasitic barnacle Polyascus planus (Crustacea: Cirripedia: Rhizocephala): Effects of the parasitism on hemolymph parameters and rootlet-infiltrated tissues of the host.

The present study assessed the impact of parasitism of Polyascus planus on the hemolymph parameters and rootlet-infiltrated tissues of the shore crab Metopograpsus thukuhar. First, hemolymph parameters were compared between hosts harboring internal infection (Hit) and non-parasitized host crabs (Hnp). The percentage of Hit with culturable bacteria in the hemolymph was significantly higher than that of Hnp when samples were cultured on Luria-Bertani (LB) agar, but the difference between Hit and Hnp was statistically insignificant when samples were cultured on Vibrio-selective X-VP agar. In addition, hemolymph bacterial load was quantified, revealing that bacterial load of Hit was significantly higher than that of Hnp when samples were cultured on LB agar, but was not on X-VP agar. Moreover, total hemocyte count was significantly lower in Hit than in Hnp. Histological examinations suggested that no cell-mediated immune responses, specifically melanization or encapsulation, were directed towards infiltrating rootlets in the examined tissues. However, large numbers of dead hemocytes exhibiting cytological features of oncotic necrosis accumulated in the rootlet-infiltrated hepatopancreas and hematopoietic tissue, where they were phagocytized by the fixed phagocytes. The mitotic index of hematopoietic stem cells did not differ significantly between Hnp and Hit. In rootlet-infiltrated ovaries, functional follicles were either disrupted (detachment of the follicular envelop) or failed to form. The combined results indicate that internally parasitized M. thukuhar was immunologically suppressed, with oncotic necrosis of hemocytes being a contributing cause of the weakened immunity. Moreover, the significance of the tissue-specific effects on host tissues is discussed in the context of host manipulation by the parasitic barnacle.