Parasitic pathways and evaluation of non-specific parasitism of ciliate infected captive corals

Coral disease is a serious challenge for thesurvival of both wild and farmed corals. Ciliate infection causes severe damage on the coral health. The present study is the first report on the non-specific infection potential of ciliate parasites on four kinds of corals. The ciliates were isolated from an infected coral, Euphyllia glabrescens and re-infected to various coral species (Acropora muricata, Goniopora columna, Lemnalia fiava, Heteroxenia pinnata). This study aims to bring insights on ciliate infection rate, specific characteristics of infections, and infection pathway of ciliate in corals. In addition, the study also assessed the nature of parasitization in multiple corals, which could be helpful for a significant risk assessment on large-scale coral aquaculture. The 18S rRNA ciliate gene identification results showed about 99% sequence similarity with the Philaster lucinda found in Acropora coral. The purified ciliate was fed with 5 × 106 cell mL−1 of zooxanthellae and could achieve good proliferation. The specific growth rate of the ciliates was increased after 18 h of infection in the E. glabrescens, reaching 828.33 ± 26.10 cell mL−1. Subsequently, the atrophy of coral polyps was observed after 12 h. After 24 h of infection, the coral began to die as the amount/number of zooxanthellae and chlorophyll a were significantly decreased. The coral suffered with 100% mortality within 72 h of infection. The histological study revealed that ciliate parasitized to coral's endoderm while entering the coral coelenteron via its mouth. Tests of ciliate infection revealed that four types of cultured corals were infected by this ciliate and rapidly rot and die, which suggests its non-specific parasitism potential. Furthermore, this study confirmed their onset of infection on different corals simultaneously, which is an important aspect for future coral research, ensuring coral disease management and designing drugs for ciliate prevention.