Thyonicola americana is a species of parasitic sea snail, a marine gastropod mollusc in the family Eulimidae.

[2] The adult female Thyonicola americana bears little resemblance to a mollusc, having no shell and a coiled worm-like form.

[2] After taking about six months to mature, the parasite can reproduce continually, reaching peak reproduction in late summer.

Not all individuals eviscerate, but when this occurs, the parasite is expelled (due to its attachment to the gut wall) and dies, and any remaining egg capsules are liberated into the open sea.

However evisceration of the host results in the death of the parasite, and when this happens many individuals will not have reached full maturity and will fail to complete their life cycles.