Identification: There is great morphological variability amongst different populations of N. japonicus. Generally, introduced North American populations have a long body with a subtriangularly-shaped dorsal cephalic shield. The thorax consists of four free segments and a fifth reduced segment, while the abdomen is comprised of three segments. There are recurved and sharp second antennae, as well as rasping two-segmented mandibles. The first antenna consists of six segments and the second antenna of four. The first maxilla displays three spines while the first leg’s endopod displays two spine; there are four setae on the third segment. The first four sets of legs are biramous, while the firth set is uniramous and reduced (Abdelhalim et al. 1993, Baud et al. 2004, Hayden and Rogers 1998, Hudson and Bowen 2002).
Those collected from Saginaw Bay, Lake Huron ranges in size from 0.6– 0.76 mm but in other regions it can reach 1 mm in length (Hudson and Bowen 2002).
Ecology: Neoergasilus japonicus is a freshwater species typical of eutrophic, polluted aquatic habitats. In Lake Huron, it usually attaches to adults more frequently than YOY the fish hosts. It also attaches most frequently to the dorsal fin, followed in frequency by the anal, caudal, pelvic, and pectoral fins, and prefers the region where the fin attaches to the body (Hayden and Rogers 1998, Hudson and Bowen 2002). Host fishes include Largemouth bass, Smallmouth bass, Bluegill, Redear sunfish, Pumpkinseed, Yellow perch, Green sunfish, Rock bass, Channel catfish, Common carp, Goldfish, and Fathead minnows (Hayden and Rogers 1998; Hudson and Bowen II 2002; P. Hudson, pers. comm.).
Neoergasilus japonicus, once established, can survive on many different host fish species. It is capable of swimming and transferring from one individual fish to another. In fact, larvae, males, and immature females do not live as parasites, but are free-living. Only ovigerous females require a host, while those that are non-ovigerous can detach and reattach to hosts. When attached to hosts, it is likely that feeding on fish tissue contributes to the diet of N. japonicus. In contrast, gut analyses indicate that free-living individuals most likely derive their nutrition from blue-green algae and small invertebrates (Baud et al. 2004, Hudson and Bowen 2002).
The reproductive cycle of N. japonicus is rapid and output can be very high. Females and males probably mate once before females attach to their fish hosts. Spermatozoids can likely be retained for a period of diapause, for example over winter, so that egg development can occur later, for example in the spring. In a life time, females can produce a total of 1500–2000 eggs in 14–29 pairs of egg sacs. Sexual maturity is attained more quickly at temperatures of 30ºC than at 20ºC. Population levels are low during the cold winter months, but increase in the spring. After hatching, N. japonicus passes through 6 nauplius stages and around 5 copepodid stages before reaching the adult stage (Abdelhalim et al. 1993; Baud et al. 2004; Hudson and Bowen 2002; Urawa et al. 1980, 1990).
Means of Introduction: Unknown. However, it is most likely that the introduction was associated with fish culture. It could have occurred via the aquarium trade, aquaculture, bait release, or ballast water introduction (Hudson and Bowen 2002).
Emerald shiner (Notropis atherinoides) and Spottail shiner (Notropis hudsonius) are subject to a wild-capture baitfish fishery in MI, and Bluntnose minnow (Pimephales notatus) is approved for capture for personal use by anglers in MI. All three species can host N. japonicus, and capture and transport of them greatly increases the possibility of spreading the parasite to other water bodies, streams, and rivers (Hudson et al. 2011).
References: (click for full references)
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Hudson, P. - United States Geological Survey, Great Lakes Science Center, Ann Arbor, MI.
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Walker, P.G. – Aquatic Animal Health Lab, Colorado Division of Wildlife, Brush, CO.
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