Though their antennules retain their sensory function, the main second antennae of the adult females are adapted to clinging to the gill filaments of host fishes.

In many Ergasilus species it is not clear that mature females are able to release their grip once attached, but when forcibly detached from the host's gills they swim without difficulty.

Another adaptation in parasitic females is that their first legs are armed with heavy, blade-like spines, and in some species the joints also are fused, stiffening them and increasing their effectiveness for harvesting host tissue.

[3] Since the Ergasilus females attack the gills of fish, a heavy infestation can cause severe damage and secondary infections, interfere with respiration, and sometimes kill the host.

In some fisheries and aquacultural enterprises the mortality and morbidity among fish stocks can present serious economic and ecological problems.