Host-parasite interactions: A study on the pathogenicity of Mesanophrys sp. and hemocytes-mediated parasitic resistance of swimming crabs (Portunus trituberculatus) at different temperatures

Abstract

Mesanophrys sp. is a ciliated parasite that invades the hemolymph of swimming crabs (Portunus trituberculatus) and reproduces rapidly under low temperatures. We conducted an in vitro and in vivo interaction assays between the P. trituberculatus hemocytes and the Mesanophrys sp. at different temperatures to understand the pathogenicity of Mesanophrys sp. and the antiparasitic responses of crab hemocytes.The results showed that infecting crabs with the Mesanophrys sp. caused a time-dependent decrease in the number of crab hemocytes (including total, granular, semi-granular, and hyaline cells) compared with the two control groups (injecting seawater-treatment and no-Mesanophrys sp. treatment groups).The in vitro interaction between hemocytes and the Mesanophrys sp. resulted in the lysis of the crab hemocytes to varying degrees. The survival rate of the hemocytes decreased with time in both Mesanophrys sp. and non-Mesanophrys sp. treatment (control) groups across all temperatures (24 °C, 20 °C, 16 °C, and 12 °C). However, the survival rate of hemocytes in the Mesanophrys sp. treatment groups (24 °C: 5.3 ± 1.2, 20 °C: 6.0 ± 1.4, 16 °C: 5.7 ± 1.7, and 12 °C: 5.67 ± 0.47) showed a faster and greater declining trend compared to the non- Mesanophrys sp. groups (24 °C: 32.7 ± 2.5, 20 °C: 32.3 ± 1.2, 16 °C: 31.3 ± 1.70, and 12 °C: 36.0 ± 1.63). Additionally, a decrease in temperature (from 24 °C to 12 °C) greatly decreased the cell integrity of hemocytes mixed with the culture medium of Mesanophrys sp. than the groups with culture medium without Mesanophrys sp.The population density of Mesanophrys sp. was significantly higher in the control group (A: with no-hemocytes or no-hemocytes culture medium) than the treatment groups (B: with hemocytes culture medium or C: with hemocytes) at 6–18 h. However, the population densities of Mesanophrys sp. in treatment groups B (13,444 ± 874 ind/mL) and C (52,178 ± 1138 ind/mL) were significantly higher than the control group (A) (1567 ± 258 ind/mL) after 18 h. Conversely, the population density, natural population growth rate, generation time, and length to width ratio of Mesanophrys sp. incubated with hemocytes increased significantly with decreasing temperature than the non-hemocytes group.In summary, the Mesanophrys sp. infects and lyses the hemocytes of the swimming crab and consumes the lysate for its proliferation. Decreasing the culture temperature from 24 °C to 12 °C reduced the resistance of crab hemocytes to the parasite Mesanophrys sp. but enhanced the impact of the Mesanophrys sp. on hemocytes.