Many species have evolved to live in groups. Gregarious behavior was believed to be adaptive for whole-population survival and predator evasion until selfish-herd theory was introduced. It proposed that individuals congregate not as a method of protection for the entire population, but instead to better the individual's chance of survival, thereby using the group to benefit itself (i.e., "selfishness"). Selfish behavior is a common part of mutualisms, which are complex, dynamic interactions that often change with biotic or abiotic circumstance. Here, I investigate potential selfish behavior within the mixed-species spawning aggregations hosted by bluehead chub (Nocomis leptocephalus). I hypothesized that the host benefits more directly from the aggregation it supports than previous research suggests – specifically, by using nest associates to decrease its own risk of predation (creating a selfish-herd) and simultaneously forcing associate embryos into marginal nest locations while locating its own embryo in the safer locations within the nest (creating an "embryonic selfish-herd"). In Chapter 1, I investigated the adult spawning aggregation for possible selfishness by monitoring the chub's spatial location within the aggregation and vigilance behavior under varying associate abundances. I found that the host occupied the central location within the aggregation and was less vigilant when associate abundance was high. In Chapter 2, I examined the "embryonic herd" contained within Nocomis nests for possible embryonic selfishness orchestrated by the chub host, leading to increased embryonic survival for chub young and elevated fitness for chub parents. I found that deeper nest sections support higher embryonic survival than shallower sections, and that chub embryo make up a disproportionate percentage of embryos found in those deeper sections. I also conducted a preliminary study investigating embryo-predation by host on associate embryo which produced evidence for embryo-predation by both host and two associates on each other's offspring. Cohesively, my results support the identification of the host as a selfish participant and confirm the presence of both an adult and embryonic selfish-herd in this mutualism. This is the first study to demonstrate selfish behavior on the part of the host in this system and the first to prove the existence of an embryonic selfish-herd. / Master of Science / A fish has three goals: to survive, to grow, and to reproduce. Mutualisms – when all participants experience a net benefit from interacting – arise when two or more species rely on each other to meet these objectives. In many mutualisms, however, not all participants benefit equally. Selfish-herd theory identifies those that maximize their own benefit while minimizing or negating any cost as "selfish" individuals. I examined the mutualistic mixed-species spawning aggregations hosted by a freshwater minnow, bluehead chub (Nocomis leptocephalus), for possible selfish behavior. I found that bluehead chubs disproportionately benefit from the aggregations they host and use the aggregation to improve their own survival, improve their offspring's survival, and obtain food (by eating others' offspring). Thus, I identify bluehead chub as a selfish participant and provide evidence for the existence of two selfish herds in this mutualism – one among the adult fishes and one among their young – both orchestrated by and benefiting the bluehead chub. This study is the first to identify the bluehead chub as such and rewrites our previous understanding of this mutualism, which largely denied bluehead chub as a direct beneficiary. This work contributes to the global discussion of mutualisms by attesting the complexity of these relationships and offers support for re-examination of the classification of many known interspecific interactions, such as those cursorily termed nest parasitisms.