Parasites of Fish from the Great Lakes - Great Lakes Fishery ...

that have direct life cycles. Cestodes infecting salmonids use copepods as intermediate hosts, and
these copepods are common in the cold-water habitat. Acanthocephalans use isopods, amphipods,
ostracods, and maybe copepods as intermediate hosts. Most of the nematode species have indirect
life cycles using a variety of intermediate and paratenic hosts (invertebrates and vertebrates) that
salmonids feed on, thereby becoming infected with these parasites. Many of these parasite species
may use paratenic hosts. A paratenic host (also called a transport host) is an animal that the
parasite occurs in but does not undergo any development that is necessary for it to infect the next
organism in its life cycle. Furthermore, the parasite does not mature in the paratenic host. A
paratenic host can be considered a “bridge” from the intermediate host to the definitive host. The
number of fish-parasite species in the Great Lakes that utilize paratenic hosts is unknown, but this
means of transmission is believed to be common.
The number of monogenean parasite species reported for the Great Lakes fish were: Lake
Michigan (2), Lake Superior (26), Lake Huron (69), Lake Erie (55), Lake Ontario (79). The
number of monogenean species increases as one moves downstream in the Great Lakes basin.
However, the small number and nature of studies performed on Lake Michigan fish could play a
role in this small number of monogenean species. Extensive surveys on the monogenean species
have been done in the other Great Lakes but not Lake Michigan.
One aspidobothrean species, Cotylogaster occidentalis, was reported from the large intestine of
one fish species, Aplodinotus grunniens, collected in 1961-1975 from Lakes Erie and Ontario.
Freshwater mussels are the normal hosts of C. occidentalis where it matures and undergoes egg
production. It also has been reported from turtles and A. grunniens that are believed to be infected
when they eat infected mussels. The infrequency of C. occidentalis in Lake Erie and Lake
Ontario fishes and its absence in the other lakes can be explained by the fact that C. occidentalis
normally infects mussels and only a few fish such as A. grunniens prey on mussels, and that A.
grunniens was not examined from Lakes Superior and Huron. Also, the infrequency or absence of
the mussel host in these lakes probably plays a role in the infrequency or absence of C.
occidentalis in the Great Lakes. Aplodinotus grunniens was only examined in one study (Pearse
1924a) in Lake Michigan and C. occidentalis was not reported.
It is not known whether this lack of or infrequent reports of glochidia (larval stages of freshwater
mussels) on Great Lakes fishes is due to a low number of unionid mussel species or to the low
number of parasite species in the Great Lakes. Furthermore, some species of glochidia are hostspecific
and it is possible that these fish species have not been examined for parasites.
Parasite Host Specificity
Host specificity plays a key role in the presence of parasite species in animals. The occurrence of
a parasite species in an environment is made in association with the host species necessary for the
completion and continuation of its life cycle. Specificity exists not only for the parasite to its host
but also for the fish host to its environment. A parasite is dependent on the range of hosts in
which it can occur and reproduce. The complexity of the life cycle of a parasite, the number of
intermediate hosts required, and the specificity of the parasite at each stage of development will
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