A hallmark of reproductive proteins that mediate egg-sperm interactions is their rapid evolution. The strong selective pressure to maintain successful fertilization coupled to differences in male/female reproductive strategies often promote arms race dynamics that drive accelerated evolution to maintain high affinity protein-protein interactions. While reproductive isolation often occurs through changes in timing or location of reproduction, the continual co-evolution and molecular refinement of gamete recognition proteins can create boundaries to hybrdization. For the marine gastropod abalone, seven sympatric species with similar breeding seasons live off the coast of California, yet hybrids are rarely observed. During fertilization, abalone sperm secrete a 16 kDa acrosomal protein, lysin, which specifically binds to repeat domains in the egg coat protein VERL. Lysin-VERL interactions are species specific, and molecular evolutionary studies demonstrate strong signatures of positive selection and rapid co-evolution between the two proteins. However, lysin acquires approximately five times as many non-synonymous substitutions as VERL, and the molecular mechanism of how these mutational effects contribute to high-affinity, species specific interactions remains unclear. Using multidimensional NMR, we are characterizing the 3D structures of lysin and VERL from red abalone (Haliotis rufescens). Mutagenesis of key residues that mediate oligomerization state for these proteins strongly affect their functions without altering the tertiary structures. Homology modeling of lysin and VERL from additional abalone species will provide a structural framework to help understand the evolutionary forces driving high affinity conspecific interactions.