Ichthyodinium Infection in the Embryos and Yolk Sac Larvae of Pacific Bluefin Tuna Thunnus orientalis

Abstract

In July 2003, we observed abnormal opaqueness of both the yolk in the embryos and recently hatched larvae of Pacific bluefin tuna Thunnus orientalis spawned at the Ohshima Experimental Station in Wakayama, Japan. The yolks were infected with a number of a unicellular parasite. All the infected yolk sacs burst one day post-hatch. Consequently, all the host larvae died and the parasites were released into seawater. The infection rate of this disease was less than 16% in 2003 but spiked up to 98% in 2004. However, the disease has not been observed since 2006. The pathogen has been identified as Ichthyodinium sp. belonging to Alveolata.