Abstract
In October 2005, a mass mortality of yolk-sac larvae of yellowfin tuna Thunnus albacares due to an endoparasite infection occurred during the seed production in a semi-closed system tank in Bali, Indonesia. Prevalence of the infection reached 100% maximally. The affected embryo or yolk sac of hatched larvae was filled with numerous parasites by 24 h post spawning. The larvae died due to a burst of the yolk sac. Phylogenetic analysis with 18S rRNA sequence and morphological characteristics of the parasite indicated that the present parasite was identified as a protozoan endoparasite Ichthyodinium chabelardi infecting the yolk-sac larvae of Atlantic sardine Sardina pilchardus or as its closely related species. PCR with a primer set designed from the 18S rRNA sequence detected the genome of the parasite from infected fertilized eggs, yolk-sac larvae and rearing water but not from the gonads of broodstock or feed fish for broodstock. The infection was not observed when the spawned eggs were immediately transferred into sterilized seawater, but the eggs kept in the rearing water for more than 1 h were infected by the parasite. These results suggest that the parasite infects fertilized eggs horizontally through the rearing water.
