THE LIFE HISTORY OF A DRACUNCULOID, PHILONEMA ONCORHYNCHI, IN ONCORHYNCHUS NERKA

Abstract

The life cycle of Philonema oncorhynchi was studied by infecting Cyclops bicuspidatus with larvae obtained from gravid female worms in spawning salmon and by subsequent infection of hatchery-reared salmon fingerlings. Later development of the worms was observed in naturally infected fingerling sockeye salmon maintained in fresh water for 2 years. Development to an infective third larval stage in the hemocoele of copepods was completed in 17 days at 12 C and in 70 days at 8 C. Third-stage larvae were obtained from peritoneal tissues of fish and tunica adventitia of the swim bladder 4 to 10 days after infection. In naturally infected fish, fourth-stage larvae were found in the peritoneal tissues when the fish were 26 months old. They moved into the coelom when the fish were 32 months old and molted to the subadult stage. Visceral adhesions developed in the infected fish at this time. The authors discuss the relationship of the life cycle of the worms to the 4-year life cycle of its anadromous host and advance the hypothesis that reproduction in the worm is correlated with that of the host by the latter's hormones.