The development of Cryptocaryon irritans in a less susceptible host rabbitfish, Siganus oramin.

Cryptocaryon irritans is a ciliated protozoan parasite infecting the gills and skin of almost all marine teleosts, resulting in severe disease. Notably, C. irritans is unable to cause significant pathogenesis in rabbitfish, Siganus oramin, a well-known less susceptible host. However, little is known about the development of C. irritans in rabbitfish. Thus, in the present study, rabbitfish were artificially infected with C. irritans at a 1/2 maximal tolerance dose (MTD), using the susceptible host, large yellow croaker, Larimichthys crocea, as a control. The daily food consumption (DFC), survival, and relative infection intensity (RII) of the fish were observed, and the relative number of tomonts (RNT), reproductive rate, and characteristics of C. irritans tomonts were measured. The lethal concentration 50 (LC50) of C. irritans on rabbitfish and large yellow croaker was 2236, and 264 theronts/g fish, respectively. The DFC of rabbitfish decreased by 14.6% at day 1 post-C. irritans infection (PI), but immediately returned to normal DFC levels by day 2 PI, and was not affected by secondary infection. However, large yellow croaker stopped feeding at day 3 PI, and all fish died following secondary infection with C. irritans. The RII of rabbitfish exhibited a significant downward trend at 6 h PI, and the reproductive rate of C. irritans was 8%; in contrast, the RII of large yellow croaker was not significantly different from 0 to 72 h PI, and the reproductive rate of C. irritans reached to 59.05-fold. Over the same time frame, the diameter of tomonts from rabbitfish was smaller compared with those from large yellow croaker. These results indicated that the rabbitfish were somehow able to limit the development of C. irritans, such that most trophonts left the host during early developmental stages, whereas those that survive could only develop into small tomonts.