The history of an enigmatic copepod genus Limnoncaea Kokubo, 1914 is reviewed. Copepods of this genus had been recorded from plankton in many fresh/brackish lakes and ponds of Japan. It is recently concluded that Limnoncaea is relegated to a junior synonym of Ergasilus Nordmann, 1832, the species of which are mainly parasitic on fish. Here we explain how lack of interchange between the fields of planktology and parasitology prevented an earlier appreciation of this fact. Recently, postmated adult females are suggested to exhibit a dual life mode: (1) some ergasilids are permanently attached to the host; in some cases, accompanied with highly transformed bodies and appendages to avoid detachment (generally known life cycle in ergasilids); (2) others are loosely associated to the host; like copepodids and adult males, without such transformation. In addition, the latter kind of female carries egg-sacs with a relatively small number of eggs per sac (up to 20), in contrast to larger number of eggs carried by females that are permanently attached to the host. The latter type is known from plankton in the world. Complex life cycles of symbiotic copepods have so far caused several taxonomic confusions not only in fresh/brackish waters but also in marine regimes (e.g., Saphirella problem). Hence planktologists should erase the boundary between planktology and parasitology to understand the natural history of their subject organisms precisely.