In Hydrichthys, the polyp has no tentacles but develops a root-like stolon which it thrusts through the skin of its host, usually a fish, to suck the blood and body fluids.

During the medusa stage, Hydrichthys lives independently in the ocean.

In one species, Hydrichthys sarcotretis, parasitism is taken a stage further when the hydrozoan attaches itself to the copepod Cardiodectes medusaeus.

Both male and female fish do not reproduce and seem to grow faster when attacked by the copepod and it seems to have a negligible energy demand from them.

[3] The hydrozoan parasite castrates the copepod, a process called hypercastration.