Neurogenesis, the generation and integration of neurons into brain circuits, occurs throughout the lives of numerous organisms, ranging from mammals to crustaceans. Unlike mammals, the first-generation precursor cells in crayfish, which are housed in a neurogenic niche, are not self-renewing but rather are replenished by the immune system. Previous studies have shown that hemocytes (blood cells) are attracted to the niche and can differentiate into neurons. To better understand the link between the immune and the nervous systems, the influences of the neurotransmitter serotonin and the cytokine astakine on neurogenesis were tested. 1) Increased levels of 5-HT result in an attraction of hemocytes to the neurogenic niche, and also stimulates the expression of astakine in hemocytes. 2) Astakine then encourages the release of semi-granular cells, a particular type of hemocyte thought to be responsible for renewing the niche precursor cells. 3) Adoptive transfer experiments have shown that labeled hemocytes transferred from donor to recipient crayfish are found in recipient neurogenic niches and generate offspring that express appropriate neurotransmitters. 4) Further, exposure of recipient crayfish to increased serotonin levels increases the incorporation of donor hemocytes into recipient niches. With this knowledge, we designed an experiment to test whether serotonin and astakine serve as links between the immune and nervous systems. In adoptive transfer experiments, hemocyte donors are treated with astakine to provide maximal numbers of labeled hemocytes for transfer to recipient crayfish, and recipients are treated with serotonin. We hypothesize that these treatments will result in higher hemocyte counts in donor crayfish and more cells in the neurogenic niche compared to controls.