<p>Most scientific models have an invariant number and relatively stable state of model elements. This feature is important for ensuring that a model represents the target system in an invariant way, such that the same explanation or prediction can be generated each time the model is used. Abrupt changes in model elements may render a model useless. In this paper, I highlight the prevalence of fluctuating model elements in model organisms. A model organism with fluctuating model elements does not have a fixed number or stable state of model elements during the modeling process. I distinguish between representational, representation-supporting, homeostatic, and R model elements in a model organism, all of which fluctuate in terms of quantity and state. I will discuss the roles and implications of these fluctuating model elements in the modeling activity involving model organisms. I aim to show, in contrast to the received wisdom in the literature on scientific modeling, that fluctuating model elements in model organisms: (1) do not necessarily hinder the representation of the phenomenon of interest; (2) can play a representational role in scientific representation.</p>