Cystidicola spp.
Cystidicola are parasitic nematodes infecting many fish species throughout the northern hemisphere. Their life-cycle involves two hosts: an amphipod as first intermediate host and a fish as definitive host. Amphipods become infected by accidentally feeding on the eggs of the parasite. The eggs hatch inside the host and undergoes two molts in the hemocoele before becoming infective to fish. When a fish feeds on an infected amphipod, the parasite migrates from the gut to the swim-bladder where sexual reproduction will take place. The eggs are then produced, enter the gut via the pneumatic duct and ultimately released into the water with the faeces of the host.
Black (1983 Can J Fish Aquat Sci 40: 643-647) discussed the possibility that intensive fishing of the lake trout Salvelinus namaycush in the Great Lakes of North America may have caused a decline in Cystidicola stigmatura infections over the 20th century. In particular, inspection of museum specimens revealed that trouts caught before 1925 were more often infected with the nematode than trouts caught after 1925. This is particularly interesting for stock management as maintaining fish populations below a threshold could limit transmission for some parasite species and improve filet quality. More about this here.
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