Gene Transfer Agents (GTAs) are phage-like pthesiss that are produced by many alpha proteobacteria in late stationary growth phase and are capable of transferring chromosomal genes (termed "constitutive transduction"). Examination of alpha proteobacterial genomic sequences indicated widespread occurrence of GTA-like elements. The goal of this study was to investigate gene transfer potential of GTAs of marine alpha proteobacteria in culture as well as in natural marine environments. Another goal was to determine the potential of bacterial symbionts from zooxanthellae and coral to genetically transfer beneficial properties between symbionts. Ruegeria mobilis (ID 45A6) was isolated from cultures of the coral endosymbiotic dinoflagellate, Symbiodinium spp. A goal of the research was to determine if GTAs from this isolate have the capability of transferring genes to environmental recipients and have an impact on settlement of coral larvae. Little is known about coral settlement cues, yet there may be contributions from the extensive symbiotic relationship of coral reef-associated bacteria. Several gene transfer experiments in different environments were performed using transformed isolates of Ruegeria mobilis containing a transposon marker gene. Experiments were also performed using GTAs from the Ruegeria mobilis isolate to observe any impact GTAs have on coral larval settlement, using larvae from the brooding coral, Porites astreoides, and from the reef building coral, Montastraea faveolata. Gene transfer frequencies from statistically significant gene transfer experiments resulted in an average of 2.92 × 10-1 (transfer recipients to total viable population). Coral settlement experiments resulted in a statistically significant increase in larval settlement with the addition of GTAs for 80% of the executed experiments. The entire study has demonstrated that GTA-mediated gene exchange is much higher than any other mode of horizontal gene transfer and it has been established that these genes can be exchanged between bacterial taxa. GTAs can also have an impact on coral larval settlement mechanisms that are not yet completely understood. GTA-mediated beneficial gene exchange may be an important driver in adaptation to an evolving planet.