High genetic diversities between isolates of the fish parasite Cryptocaryon irritans (Ciliophora) suggest multiple cryptic species

The ciliate protozoan Cryptocaryon irritans parasitizes marine fish and causes lethal white spot disease. Sporadic infections as well as large-scale outbreaks have been reported globally and the parasite's broad host range poses particular threat to the aquaculture and ornamental fish markets. In order to better understand C. irritans' population structure, we sequenced and compared mitochondrial cox-1, SSU rRNA, and ITS-1 sequences from 8 new isolates of C. irritans collected in China, Japan, and Taiwan. We detected two SSU rRNA haplotypes, which differ at three positions, separating the isolates into two main groups (I and II). Cox-1 sequences also support the division into two groups, and the cox-1 divergence between these two groups is unexpectedly high (9.28% for 1582 nucleotide positions). The divergence is much greater than that detected in Ichthyophthirius multifiliis, the ciliate protozoan causing freshwater white spot disease in fish, where intraspecies divergence on cox-1 sequence is only 1.95%. ITS-1 sequences derived from these eight isolates and from all other C. irritans isolates (deposited in the GenBank) not only support the two groups, but further suggest the presence of a third group with even greater sequence divergence. Finally, a small Ka/Ks ratio estimated from cox-1 sequences suggests that this gene in C. irritans remains under strong purifying selection. Taken together, the C. irritans species may consists of many subspecies and/or syngens. Further work is needed to determine if there is reproductive isolation between the groups we have defined.