Molecular characterization of muscle-parasitizing didymozoid from a chub mackerel, Scomber japonicus

Didymozoids found in the muscles of marine fish are almost always damaged because they are usually found after being sliced. Therefore, identifying muscle-parasitizing didymozoids is difficult because of the difficulty in collecting non-damaged worms and observing their organs as key points for morphological identification. Moreover, muscle-parasitizing didymozoids are not easily found because they parasitize at the trunk muscles. Therefore, muscle-parasitizing didymozoid classification has not progressed because there are few opportunities to detect them. Our recent report was the first to describe the usefulness of sequencing analysis for discrimination among muscle-parasitizing didymozoids. Recently, we found a didymozoid in the trunk muscle of a chub mackerel Scomber japonicus. The present study genetically compares the present isolate with other muscle-parasitizing didymozoids. The present isolate differs markedly from the previously unidentified didymozoid from an Atlantic mackerel S. scombrus by phylogenetic analysis of 18S rDNA. It also differs from other muscle-parasitizing didymozoids from other host species based on phylogenetic analyses of 18S, 28S rDNAs, and coxI loci. These results suggest that sequencing analysis is useful for the discrimination of muscle-parasitizing didymozoids. Combining the present data with earlier data for sequencing analysis, muscle-parasitizing didymozoids from seven marine fish species were classified as seven species. We proposed appellations for six distinct muscle-parasitizing didymozoids for future analysis: sweetlips fish type from Diagramma pictum and Plectorhinchus cinctus, red sea bream type from Pagrus major, flying fish type from Cypselurus heterurus, Atlantic mackerel type from Scomber scombrus, chub mackerel type from S. japonicus, and purple rockcod type from Epinephelus cyanopodus.