| Studies show that several species of parasitic flatworms that infect the marine snail, Cerithidea californica, have a division of labor where individuals are specialized for either reproduction (reproductives) or defense of the colony (soldiers). Research has focused on understanding the morphology, distribution, and behavior of parasites in the more common, single infections. Studies have shown that these parasites will kill each other when a snail is simultaneously infected with two species, but little is known about these interactions. Here, we provide insight into these interactions by examining the morphology and distribution of these organisms in doubly infected snails. These double infections are a snapshot of a transition that is occurring, since two different parasite species typically don't share their snail host for an extended period of time. Consistent with our predictions, the subordinate species is displaced; fewer soldiers are present and the reproductives look unhealthy. The incoming, dominant species has many immature parasites and is beginning to establish an infection. These seemingly insignificant creatures are important when it comes to the make up of an ecosystem; they take up resources and affect the health and survival of hosts in estuarine communities.
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