The most commonly observed stage is the trophont, or "feeding" stage. The trophont is found on the fish, usually underneath the outer skin layers. Spherical to club- or pear-shaped, with cilia all over its body, the trophont will normally be seen "rolling" or rotating slowly under the epithelium (outer cell layers of the skin or gills) (see figures 3"“6 ). Because the trophont is embedded within the skin, it is relatively protected from any potential treatments. The organism's cytoplasm is more opaque in this phase, which means the lobed macronucleus (and several smaller micronuclei) can be difficult to see in live specimens (Dickerson 2006). Trophonts can range in size from about 48 x 27 µm (~1/20 x 1/40 of a mm) to 452 x 360 µm (~1/2 x 1/3 mm) (1 µm = 1 micron; 1,000 microns = 1 millimeter). The trophont feeds on the body fluids and cells of the fish for about 3"“7 days before leaving the host.
Trophonts will also actively leave fish that have died, but are not immediately infective. They require additional time to develop from protomonts to tomonts, just as they would if leaving a live host.