Descriptions of caligiform copepods in plankton samples collected from East Asia: Accidental occurrences or a new mode of life cycle?

抄録

Parasitic copepods usually have one or more free-swimming larval/juvenile stages for dispersal or infection. However the present study has revealed that adults and chalimus stages of caligiform copepods have often been discovered in plankton samples collected from East Asia. This is the first report on free-swimming adults of caligiform copepods from Japanese and Korean waters. Adults of the following species have been discovered in this study. In Japanese waters: Caligus coryphaenae (4♂♂), Caligus sp. (1♂), Pandarus sp. (4♂♂, 16 chalimi) off Nansei Islands in May 2003 and 2006; C. sclerotinosus (1♀) near a fish farm, Ehime Prefecture in December 2006; C. undulatus (1♀) off Ube, the Seto Inland Sea in July 2006; and C. undulatus (1♂) in the Ariake Sea in June 2007. In Korean waters: C. orientalis (1♀, 1♂) from brackish waters of the Mankyong River and C. undulatus (1♂) from the Seomjin River in October 2006. It is interesting to point out that C. undulatus has never been recorded as a parasite infecting any host, but has been collected from plankton samples in East Asia, India and Brazil. It is also noteworthy to mention that males (80%) were found more frequently than females (20%) in this study. We considered the following possibilities for the occurrences of caligiform adults originally infecting fish: (1) escaping from irritation or diseases in the host, (2) looking for an opportunity to switch hosts, (3) change in their life mode, and (4) accidental detachment. In the case of chalimi, it could be accidental, because they would be tightly attached to the host using a special organ called the “frontal filament”, and cannot grow up to the adult stage without nutrient supply from the host.