These leeches grow to approximately 20 centimeters in length and can be distinguished by the prominent dorsal organ, the function of which is unknown. On their own, they are more of a nuisance than a threat, although they tend to congregate in large numbers. They appear to spawn from large egg clusters, some of which can be seen aboard the Ecliptic Express. Progenitor leeches infest an area after the destruction of a Mimicry Marcus, from which they are composed. Like Mimicry Marcus, they are vulnerable to fire, although individually they have little health.